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Dig Foundation


My foundation design

Designing foundations needs lots of thought.  It took me a while to get to the foundation design that is shown here...

Foundation Exterior

 Foundation Exterior (Cropped)

I did a lot of reading of building science papers and a lot of thinking.  I then talked through the proposed foundation design with every knowledgeable person I could find.  I talked it through with a civil engineering friend, the guy at the local county building department, my local friendly building inspector, my structural engineer, and various building product suppliers.  Eventually after lots of iterations I arrived at a design I am happy with.


Here is the drawing version for the internal concrete walls...

Foundation Interior walls 

Foundation Interior Wall (Cropped) 

Accurately mark out house footprint

You should now after the excavation have a completely level flat site.  During the excavation you will have uprooted all the stakes you previously put in to show the house outline.  Anyway they were probably not very accurate if the land was not flat.  You can now mark out the house accurately given the flat surface.

North-South reference

Start by re-doing your "North South" reference string.  This should be from the northern perimeter of the excavated flat surface to either due south or to a stake as far south as you can get such that the string is accurately pointing at the object of interest (eg a specific mountain).  Note that this initial north-south reference string does not represent anything on the building (eg a wall or whatever).  It is purely used to establish the direction you want the building to face.

North south reference line 

You may also need a copy of the string nearer the bottom of the excavation.  You can set this lower string by hanging plumb-bobs from the upper string.

East-West reference

Next re-establish your "East West" reference.  This will be placed at the northern edge of the footing for the main house rectangle.  It needs to be exactly at 90 degrees to the "North South" reference string.  I find that using a 90 degree dual laser tool is useful for establishing a right angle.  You can align one edge with the string at the required point along the string.

Laser 90 degrees 

Laser setting right angle 

Setting right angle laser along string 

East West reference string 

On your CAD drawings you should have marked the outside edges of the footings.  This should of course also show any building bump-outs on the north side of the building.  It is the northern edge bump-outs of the building that determine where the East West reference string will go.  These bump-outs need to fit within the excavated level area.  Also allow 1 foot for the drainage gravel and another foot to give room for solid soil before getting to the batter board assembly.  Allow further distance (say 2 inches) for the batterboard assembly.  The "East West" reference string will be used to represent the northern outer edge of the Form-a-drain for the main house rectangle.

After adding up all the bump-out distances that you need to allow for, measure from the northern most edge of the excavated flat area along the north south reference string and that is the point at which the East West reference string will cross at 90 degrees.

Foundation With Batter Boards 

The amount of space added on for the batter board assembly is a variable.  If using steel stakes and a bit of creativity you can get this number down to close to about 2 inches.  Getting it to 2" means that the inner edge of the batterboard will be 2 feet out from the outer edge of the Form-a-drain.  Even though steel stakes cost $5 each and you need between 4 and 6 per batterboard, that is still my recommended way of implementing batterboards, particularly when fitting a building into an excavated hole.



Two tools are useful.  A 100 foot steel tape measure and a Laser Measurer.

Tape measure 100 feet    Laser Measurer 

Copy north-south reference to be eastern footing edge

The north-south reference string was just used to establish the direction that the building will face.  Now you want to establish the east or west wall of the outer house rectangle.  If the eastern wall is the most cut into the excavation then choose that wall to become the reference as it is the most difficult to accommodate.  You need the outer edge of the Form-a-drain for the east wall to be just over 2 feet in from the excavated bank (to allow for the batterboard etc).

After deciding where the eastern outer edge of the Form-a-drain needs to be, make a copy of the north-south reference string at that position.  Do this by measuring the same distance from the north-south reference string at two points.  Setting the eastern edge (if that is the most excavated) will ensure that you have the building nicely fitting within the excavation.

Lower north south reference string 

The string seen in the above photo is a lower down copy of the north-south reference string.  The eastern edge of the batterboard is set by measuring from that north-south reference string.

Batterboard design

The horizontals of the batterboard are made from 2x8 lumber.  I cut my own lumber using my SawMill.  The stakes are steel stakes because the ground is so solid.  Steel stakes also take less space.

Metal stake 3 foot 

It is important that the batterboard assemblies are very strong.  A lot depends on the accuracy with which you mark-out the foundations, and that means you don't want the batter boards to move.

The exact positioning of the batter boards does not matter as it will be the strings that you attach to the batter boards that set the accurate position for the foundations.  It also does not matter if the stakes go into the ground crookedly, just as long as they do not move around.

It even does not matter too much if you want to put one or two of the batter boards further out than normal to for example give room for an access driveway.  If you are using corner batterboards (like I typically do) then you can only move them out by about an extra foot or else the legs will not be long enough for the strings in both directions.  Typically it is good to stick to having the batterboards between 2-3 feet from where the Form-a-drain edge will be.

You bang in the stakes using a big club hammer.  With rocks in the ground, the stakes will probably not go in completely vertical, but that's ok.  Being solid in the ground so they don't move is what counts.

Batterboard North East of main house 

Sometimes the batterboard assembly has to be very much set into the side of the excavation.  It may be necessary to chip away at the edge of the excavation to get the batterboards in the required place.

Batterboard corner set in 


You need to set the rest of the batterboard assemblies so that their positions all correspond with the house foundation layout drawing.  It is best to do the most difficult ones first, ie the ones that are tight up against the edge of the excavation.  In my case I mainly used a convention of the inner edge of the batterboards being 2 feet from the outer edge of the Form-a-drain.

Initially it is about getting the batterboard assemblies in the right places as per the plan, rather than setting the height of the batterboards.  At this stage, the batterboards are not screwed to the stakes. 

Later when forming the footing you will also add wood at the bottom of the stakes so that you have something to brace to.

Wood At Base Batterboard


Set the height of the batterboards

Once the stakes are securely in the ground, you will then screw horizontal 2x8 boards to the stakes such that the boards are all the same height all the way round the building.  They do need to be exactly (better than +/-1/8") the same height because you will want to use the strings as a vertical reference point when implementing the foundations and slab.

The height you chose for the top of the batter boards is not that important (as long as they are all the same).  In my case 2'2" above what I decided was going to be the top of the basement slab level was what I chose.  That also happens to make the batterboard strings at reference grade height which is nice.  When you have decided on this batterboard height, make sure to add it to your excavation cross-section drawing because you will be using the strings attached to the top of the batter boards as a height reference.

Excavation Heights East West (Cropped) 

Excavation Heights East West 

Here's the equivalent drawing I used for the evaluation building...

Excavation cross-section

The easiest way to get the batter boards all exactly the same height is with a self leveling laser level.  A good one that self levels and makes a horizontal line is about $150.

Self leveling laserDetails are here .

Place the laser on a level platform in the middle of the house area.

Laser Platform On Deck 

It is good to have a laser target on the far side of the build site to ensure the laser height has not moved.

Laser Target 

Initially it is good to set the horizontal beam just above the batterboard height that you have decided to standardize on so one batterboard will not block the laser to a batterboard behind.  You can do this with a temporary 1" wood spacer under the laser unit.  Rotate it to shine in the direction of each batterboard in turn and get the top of the batterboards to all be at that distance (eg 1") below the beam.

Initially it is best to set the batterboard heights using rocks underneath.

Batterboard set right height with rocks 

Once you are happy with the height then you can screw the stakes to the horizontal lumber.

The batterboard horizontals need to be accurately level as well as accurately the right height.  A check worth doing is to move your laser level to one batter board and shine it at another batter board because that will make sure your laser really is working accurately.

Batterboards right height south side 

Site - Batterboards set

Setting the outer edge footing strings

Once the batterboards are all installed in about the right locations and their height has been accurately set then it is time to add the strings that will accurately represent the outer edge of the outer Form-a-drain that forms the footing.  The east-west reference string should be about right for the north wall of the main house rectangle, and the copy of the north-south string should be about right for the eastern wall, but there is still an opportunity to make them even more accurate.

Strings are implemented by putting a wood screw in the top edge of the batterboards. Tie small loops in the end of the strings and stretch them between the screws.  They need to be as tight as you can possibly get them.  They want to be just short of the breaking point of the string.  You will likely need to tie multiple loops in the end of the string so you can stretch it to the tightest one.  The tighter you make the strings the less they will move about and the greater the accuracy you will achieve.  Be careful because when strings break you can get a nasty string burn on your arm, leg, or face!

You will likely need to move the screw positions a bit to get everything dimensionally correct and square.

Using string between batterboards may seem a bit low tech, but it really is the most accurate way.  It relies on the point of crossing of the strings rather than the absolute position of the batterboards.  Avoid the temptation of using glue where the strings cross because accuracy relies on the string's ability to move with things like string age or ambient temperature.

The taught strings extend beyond the footings (to reach the batter boards) and need to cross each other at exactly a right angle (this is the outer corner of the footing).

Use a 100ft tape measure to set the other wall strings so they correspond with the foundation layout house drawings.  By measuring from the reference strings you should be able to preserve the right angle that was previously set using the 90 degree laser tool .  Note that sideways force from the lip of a tape measure will move the strings slightly even when they are very tight, so try to avoid and compensate if necessary.

I find that making temporary marks on strings is best done using pieces of electrical insulation tape.  You may well need someone to hold the other end of the tape measure because you don't want to accidentally move the strings by hooking a tape measure to them.

Using loops to attach the strings to the wood screws is better than using a tight knot because it allows you to temporarily remove the strings if they get in the way, eg get in the way of the digger.  What is actually the most important is that the screws do not get moved.

Refining the 90 degree accuracy

We need the 90 degree corners to be very accurate or else the building will be skewed (and the roof won't fit properly). 

If you have an accurate 90 degree corner, then the distance from string crossover to string crossover should be the same across both the diagonals of the rectangle, ie the hypotenuses should be equal.  We're looking for better than a 1/8" difference here.

Measuring between string crossover points is hard because even very tight strings have some movement if any side force is applied, eg by the hook of a tape measure.  I made assemblies (pictured below) out of plastic card that could be fitted over the string crossovers.  On the underside, pairs of 1" squares of plastic card form string width grooves to make sure the strings were accurately in the right place.  You just need one assembly that forms a vertical flat target for the laser and one assembly with a vertical edge against which the middle of the laser measurer will be positioned.  You use the pair of assemblies to measure one hypotenuse and then you move the pair to the other hypotenuse.

Laser measurer target 

Laser measurer on corner mount

From your AutoCAD drawings you should know exactly what the hypotenuse length should be.  If the measured length does not match the drawing then it probably means the leg lengths of the rectangle are wrong.

Do fine adjustment of the positions of the string screws in the batterboard top edges to gradually increment until you have the hypotenuses the same length and matching the figure from the drawings. 

Realistically some adjustment will be needed to ensure you have perfect 90 degree angles and you also have exactly the required width and length for the outer footing rectangle.  Keep iterating until these requirements have been accurately met.  Draw a trapezoid on a piece of paper and note down the lengths of all the sides and both diagonals for each iteration you do, because it is easy to get confused about which direction to move the strings and by how much.  Every time you move the screw that the string is hooked on to, try to use it as an opportunity to get the string just a little tighter.


Accuracy is very important

Any error at this stage will cumulatively multiply as you build your house and may even cause the roof not to fit properly, so it really is worth taking the time to get it right.


More complicated shapes

Typically a house foundation is more than just a simple rectangle.  In my case I have portico protrusions on the north and south sides of the house and a couple of internal concrete walls.  These all need proper footings.  The footings for these also need to be marked out using additional batter boards and strings.  Use the same techniques as described, but of course do it relative to the main house rectangle.

Here is my foundation layout plan...

Foundation With Batter Boards 

Marking out the deck support pillars

In the case of discrete foundation blocks such as are used for the pillars that support concrete decking, it is not very efficient to use batterboards and strings.  A better strategy is to implement the wooden squares that are going to be needed later anyway to form the concrete for the pillar foundation.  You can then position the squares on the ground to mark where the various holes need to be dug.  (You don't actually need or want diagonal wood to hold the squares square as PL-Premium glue is sufficient to hold them square.)

Deck pilars marked out

Once you have the squares in the right place then you can draw round them with paint to show the digger where to dig.

Deck Supports Marked With Paint 

The following shows the site with the discrete foundations marked using their wooden forming frames.

Site - Flat site with batterboards 


Digging Footing trenches and under slab

It is way too much work to try to dig foundation trenches manually - you need a machine.

A mechanical digger is a big tractor like machine with a big digger arm on the back or front.  Sometimes they have big rubber wheels and sometimes they have tracks.  They go by various names such as backhoe, trackhoe, excavator, or JCB.  You can use a friendly generic name such as "digger", but I tend to say excavator.  They are way too expensive to buy your own for building your house, so you will need to rent one, and it will probably come with an experienced operator.

Every minute counts

They are not cheap to rent, so you need to avoid it sitting idle for more than a few minutes.  That means you need to be super prepared ahead of it arriving on site.  Here is a picture of the one I used on the evaluation building.

Mechanical digger 

For the main house I needed a full sized excavator because I was dealing with very hard rocky ground and a much bigger area.

Site Digger For Septic 


Do very targeted excavation drawings

You will have done detailed drawings of your house floor plan and your foundation design, but these drawings have lots of unnecessary information that does not pertain to the marking-out and the digging you need done.  Ahead of the excavator arriving, you need to produce a couple of drawings that just describes the specific digging that is needed.  You will need to transfer these drawings to the real ground using wooden stakes and spray paint so the excavator knows where to dig and how deep.  You will need a nice simple "Excavation Plan" and a simple "Excavation Cross Section" so it is easy to explain to the excavator operator what's needed.

Do the excavation drawings accurately in your CAD package.  Add a layer that shows where the excavation cuts should be and then turn off all the other layers.  Take a JPEG of the drawing.

Make sure that your excavation drawing reflects the fact that the excavation goes further out than the Form-a-drain strings (to allow for the drainage pipe and rock).  The drawing below correctly allows for this.

Foundation With Batter Boards 

 On the JPEG color in the different heights to make the different depth clearer...

Excavation Plan (colored) 

Here's a cross section showing the different excavation heights...

Excavation Heights East West (Cropped) 

Excavation Heights East West 

A few useful numbers

For external walls
        From outside of wall concrete to the outside of the outer Form-a-drain is 1'4".
        From outside of wall concrete to the outside of the inner Form-a-drain is 3'4".
        From outside of wall concrete to center of vertical rebar is 5-3/4" (both 8" and 12" cavity walls).

For internal walls
        From outside of wall concrete to the outside of the outer Form-a-drain is 2'0".
        From outside of wall concrete to the outside of the inner Form-a-drain is 2'8".
        From outside of wall concrete to center of vertical rebar is 4".

Where there is a drainage pipe round the outside then add 6" of extra excavation width.
Where there is not a pipe (eg on the inside) then just add 1" of extra excavation width.

Note that the Form-a-drain strings on the batterboards are different for internal walls compared with external walls.  That means there will sometimes be two outer strings and two inner strings on one batterboard.  The offset difference is 8".

Different Strings For External And Internal Walls 


Excavation batterboard strings

During the excavation for the footings etc you are not interested in the position of the Form-a-drain.  You will remove the Form-a-drain strings and just have strings that tell the excavator where to dig.  You will not remove the screws that the Form-a-drain strings attach to so it will be easy to put them back later.  You will add the excavation strings using the Form-a-drain screws as the reference for measuring.

Your CAD drawn excavation plan will tell you where the excavation strings should go, but typically they are 6" further out than the outer Form-a-drain string position, and 4'9" inwards from the outer Form-a-drain string position.

When measuring the excavation depths it is the height of the batterboard strings that is the reference that you measure down from.  For more accuracy use a self leveling laser set at the batterboard height and measure down from that.


Strings will be detached or broken constantly

The reality is that both the outer and inner strings will be in the way when the excavator starts digging.  What's important is that the wood screws in the top of the batter boards stay in place. You can unhook the strings from the screws when necessary and when strings get broken you can implement new strings.  To help the excavator operator you can use spray paint on the ground and/or temporary stakes, that you place by being guided by the strings before you unhook them.  If the excavator operator is color blind then it is best to use white spray paint.  Once in a while you can re-hook the strings to make sure the digging is proceeding well and re-implement temporary stakes as necessary.


Figure out where the dug soil will go

Soil Wooden Retainer Inside 

Soil Wooden Retainer Outside 

Use paint on ground

The painted lines are determined by the positions of the strings.  They are visible to the excavator operator after the strings have been removed are the actual digging is happening.  The paint gets erased as the excavator drives over the lines are excavates down so you need to keep re-attaching the strings and repainting the lines.

Site Ready For Footing Excavation 

Don't dig until properly marked out

The actual foundation digging (ie the footing trenches and the under-slab level) must not be done until the batter boards and batter board strings are accurately set in place or else the digging with the excavator will be inaccurate and you will have a lot of manual digging to do to finish off (and correct).


Excavate Footings and under-slab areas

The order in which the following excavation steps are done is up to the excavator operator.  Some operators like to dig the footings first and then do the slab area and some like to get everything down to under-slab depth first before doing the footings.  Some do both at the same time.  What they all aim to do is not get snookered from getting access.

Dig the footings

Digging footing 

Footing corner 

Don't forget about any internal concrete walls in your house design as they also need to have footing trenches.

In my design, the depth of the footing trenches is 5' 8-1/4" below grade.  I put the batter board strings at reference grade, ie 2'2" above top of slab).

Excavation Heights East West (Cropped) 

Excavation Heights East West 

The width of the footing trenches needs to be 4'9" in from the outer footing marker strings (allowing for 1" extra on the inside) and an additional 6 inches outside of the outer footing marker strings for the weeping tile drainage pipe.  That 6" corresponds to a 4" perforated pipe surrounded by 1" of crushed rock on each side.  That means the footing trench will be a total of 4'9" + 6" =  5'3" wide at the bottom.

With all the digging, it is worth finishing off the last inch by hand as this will achieve the best accuracy, so get the excavator operator to subtract an inch from the depth on the drawings when they are digging.

In practice, the cut away earth verticals round the outside will be a steep angle rather than actually vertical, but that's ok.  Even so, try to keep them as vertical as possible as that undisturbed soil is useful for preventing lateral movement of the building in an earthquake.

Dig under-slab area

The excavator will be used to dig the non-footing under-house area down to the under-slab depth.  Operating an excavator requires lots of skill and an important part of this skill is avoiding snookering yourself so you can't now get access to something you need to dig.  Make sure the excavator operator knows all the details of what you want for the footing depth and location and the under-slab depth and they will figure out the best digging sequence to achieve the right final result.

Footings half done 

As per the ground markers and Excavation drawings

Prior to the dig and as needed during the dig, you will have transferred the batter board string positions to the ground using (white) spray paint and some temporary marker sticks.

You will continuously check depths throughout the digging process by measuring down from the batter board string height.  Remember that it is important not to go deeper than the depths shown on the drawing because you want to build the foundations on undisturbed soil.

If you do accidentally go too deep in a particular small place, eg to remove a large rock, then it will be necessary to fill the area with bag mixed concrete.  Never just add lose soil as it will not work properly and you house may sink (unevenly).  It is safest to have the excavator stop digging about an inch less than on the drawing and finish off the last inch by hand.  You need to hope you don't hit the tip of a big rock in that last inch (which is why you don't want to leave more than 1" for manual digging).

Excavation Heights East West (Cropped) 

Excavation Heights East West 


Marker sticks 

Dig pipe trench from the building

Used for sewer, drainage, water, electricity, and data

Before the excavator leaves your site you will need a sewer and drainage pipe trench dug from the building to the septic system and for the drainage water to a drain area well away from the building.  You may well also choose to put other utilities such as electricity and data etc in the same pipe trench.  It's certainly not a bad idea to have some spare electrical conduit in there.  All these details should be on your plot drawing.

Must be proper slope

The pipe ditch with the sewer pipe must be sloped downwards at a uniform angle of 1/4" per foot (+/-1/8").  This slope also works well for a drainage pipe.  If you cannot maintain this proper slope then you will need to route the sewer pipe to a collection tank and then pump it from there to the septic system (see Sewage System).

The starting point for the top of the pipe trench slope is the depth at which the drainage pipes go under the footing Form-a-drain at the furthest side of the building.  You need to maintain the drainage slope under the building, so that determines the depth of the drainage ditch on the side nearest the drain-away.  It is the drainage pipe that determines the depth of the trench from the house because the sewer pipe actually exits through the wall rather than through the slab.  Mark the depths on your foundation excavation plan.

Foundation With Batter Boards 

Excavation Plan (colored) 

Photos of main house footing excavation

Day 1

Footing Excavation 01 Starting

Footing Excavation 02 Starting Underslab Height

Footing Excavation 03 North East

Footing Excavation 04 Middle East Deck Support

Footing Excavation 05 South East Deck Supports

Footing Excavation 06 Filling Soil Cage

Footing Excavation 07 Digging To Underslab Level

Footing Excavation 08 Soil Cage Full

Footing Excavation 09 End Day 1 Under Digger Arm

Footing Excavation 10 End Day 1 Soil

Day 2

Footing Excavation Eastern Deck Support

Footing Excavation Dave With Laser Depth

Footing Excavation Eastern Footing

Footing Excavation South East Corner

Footing Excavation Dirt Mountain

Footing Excavation View To East

Footing Excavation Coming Down Dirt Mountain

Footing Excavation From North East Above

Footing Excavation Looking South East

Footing Excavation End Day 2

Day 3

Footing Excavation 301 

Footing Excavation 302

Felicity On Big Rock 

Footing Excavation 303 

Footing Excavation 304 

Footing Excavation 306 

Footing Excavation 307 

Footing Excavation 308 

Footing Excavation 309 

Footing Excavation 310 

Footing Excavation 311

Footing Excavation 312

Footing Excavation 313

Footing Excavation 314

Dig the drainage ditches

The thin ditches for the drainage pipes are the lowest part of the excavation.  You want them to be as narrow as possible (eg 6" wide) so as not to disturb too much soil (as you want undisturbed soil to resist lateral movement of the house).  In my case I needed to dig these by hand because it was not possible to find an excavator that had a sufficiently narrow bucket and get had enough power to cope with the large rocks in the soil.

When handling rocky soil , the best solution is to use a jackhammer to break up the soil so you can shovel it.

JackHammer  JackHammer

Drainage trench from the house...

Digging House Drain Ditch With Jackhammer

Under-slab drainage ditch... 

Drainage Ditch Across Slab In Progress 

Dave Jackhammering

A heavy duty field hoe is a useful digging tool to remove the soil from the ditch.

Field Hoe 8in  Field Hoe

The drainage ditches have a slope of between 1/8" and 1/4" per foot.

The thing that determines the depth of the drainage pipes is the need to drain away any water that has collected in the bottom inner Form-a-drain.  That means the 4" drainage pipe needs to pass immediately under the Form-a-drain on the furthest side.

At the far end of the pipe a slightly different arrangement can be used to allow the 4" pipe to be as high as possible...

Form-A-Drain and 4in Pipe Far End 

Far End Form-A-Drain Drainage Takeoff Parts

FarEnd Form-A-Drain Drainage Takeoff Assembled 

That means the ditch needs to be 5" (6.5" if using the standard arrangement) below the footing excavation depth at the farthest away point.  The standard way of doing the connection is shown below...

Drainage pipe under Form-a-drain 

The trenches for the drainage pipes as they exit under the footing are the deepest part of the whole excavation.  It is necessary to keep them sloped at an angle of between 1/8" and 1/4" per foot.

The length of the drainage pipe from the furthest point to the exit point is a worst case of 72 feet.  With the 1/4" per foot slope that means the ditch at the exit point needs to be about 18 inches deeper than at the furthest point.  It is worth allowing an additional 1.5" just to be safe with small variations in the actual slope.  At the exit point the drainage ditch will be 18" + 5" + 1.5" = 24.5" (2' 0-1/2") below the footing depth.  That is 5' 8.25" + 2' 0.5" = 7' 8.75" below the batterboard string reference height.

The details of the drainage pipe are covered in the section on UnderSlab Plumbing .  It is actually best to implement the drain even before you have finished the detailed digging of the foundations because that will stop the dig site getting flooded by rain water.


Finishing off the dig accurately by hand

Need good accuracy

It is important to get your excavation as close as you possibly can to your drawings to avoid cumulative errors later.  The best way to do this is have the digger stop 1 inch short of all the figures on the drawings and finish things off by hand digging.  It is ok to accidentally go down an extra inch as a bit of extra concrete will fill in the extra, but try to avoid it.  If it is lower in a couple of places (eg where a rock was removed) then bag mix concrete directly into the hole can be used to get it up to the required uniform height.  If you happen to have soil that sets like concrete then you can potentially use this to fill in rock holes.

Using a self leveling laser go round the whole excavation and put markers to show the actual heights left by the excavator.

Excavation Measured Marker 

If it turns out that the excavator has gone too deep over the whole excavation site (as determined by the median of the values) then the best thing to do is to decide to set your house a little lower in the ground.


Transferring points to the ground

The strings between the batterboards accurately give the outer edge position of the Form-a-drain.  What we need is to transfer the position of the Form-a-drain to the earth at the bottom of the footing trench.  It's good to use 8 oz Plumb Bobs because they are heavy enough to work but not too heavy that they will cause the batter board strings to sag excessively.

Plumb Bob 

Plumb Bob Sets Footing Trench Edge 

Refining Digging

PlumbBob Setting Form-A-Drain Position 

To get a reference for the footing and under-slab heights it is best to use a self leveling laser.  Use a length of white 2" PVC pipe with 1" indelible marker marks and the particular heights of interest marked.  It is a bit difficult to see the laser line on sunny days so you will need to do most of your height measuring on overcast days or in the early evening.

White Pvc Pipe Measure Stick

You should establish an immovable reference platform for placing the self leveling laser.  If necessary you can setup secondary reference platforms that are closer to where you are actually digging (to get a brighter laser line) but when final accuracy counts then you should use the primary reference platform.

Standard laser reference platform 

Excavate width beyond string locations

The batter board strings mark the inner and outer locations of the actual footings.  Round the outside, the excavation needs to be an extra 6" to allow for the 4" perforated drainage pipe and crushed rock.  Round the inside, the excavation needs to be an extra 1" to allow enough space for the stakes that hold the Form-a-drain.  These extra width areas need to be excavated to the same depth as the footings.

Actually, where the peripheral pipes go to the T pieces that take the water to the sloping drain that goes across the site, it is good to go a couple of inches deeper under the pipe to give the pipe a slight downward slope.

Use Form-a-drain jig

To check that there are no high spots it is best to temporarily fit lengths of Form-a-drain with wooden spacers to hold them the right distance apart.  Get the outer edges in the right place using a plumb bob down from the batterboard strings.  Measure from the self leveling laser to the top of the Form-a-drain to check it is sitting at the right height.  The height should be the footing excavation depth minus 10" for the height of the Form-a-drain.  In my case that is 5' 8.25" - 10" = 4' 10.25".

Wood jigs for bottom Form-a-drain 

Form-a-drain jig 

Form-A-Drain Temp Jigs Pipes 

Finish between Form-a-drain jig

Once you have the Form-a-drain jig sitting in the right location (right height and right lateral position) then you can use a 10" wide wooden plank with lips on the end to check that the trench surface between the Form-a-drain is the right height (and correct if it's not).

10" wide wooden plank with lips

The most important thing is actually not that the ground between the Form-a-drain is exactly the right depth, but rather that it is a smooth surface that will not puncture through the EPS that will be under the footings.  Having said that, you should get the height within +/-1".

Compact the soil

Hire a compactor (if required)

Depending on the soil characteristics, the bottom of the excavated footing trench may need to be compacted, particularly in areas where large rocks were removed.  (Do not try to compact the under slab area because it will likely cause the edges to cave into the footing trench.)  The best way to do the compaction if it is needed is to hire a compactor machine for a day.  You need one that is 5HP or more.

The subgrade needs be compacted to 95% relative compaction.  Move the compactor over every square foot of soil until the soil is firm and smooth looking.  It is possible to have the soil tested to measure the compaction.

For my soil type, compaction was not necessary as it was well compacted by a glacier during the last couple of ice ages and I was careful during excavation not to disturb the soil at the bottom of the footing trench and under the slab area.


Holes for deck columns

These are rectangular holes.  These are separate from the house foundation so it does not matter when they are actually done (as long as you can still get access for the excavator).

Deck Pillar Foundation Excavation 

Portico Side Post Area


Finishing off the dig

The digging is pretty much at an end (big sigh of relief as it's hard work).  It is however worth doing a bit of checking and finishing off.

Check list

Measure all your excavation depths in all places.  Make sure the trenches are the right widths and in the right places relative to the batter board strings.  Have you allowed 6 inches all the way round on the outside of the Form-a-drain marking plumb bobs for the outer weeping tile pipe?  Have you allowed an inch clearance on the inside of the inner footing strings?  Make sure there is no place where the footing trench is not deep enough or not quite wide enough.  Even one sticking up rock will stop you placing the forming for the footing (the Form-a-drain).  It is ok if the ditch is slightly deeper than the reference figure below grade because the concrete will fill up the difference, but it cannot be less than the reference figure under any place that the Form-a-drain will go.

Make sure your drainage pipe ditches are cut across the footing trenches.  Note that the drain pipe ditches are lower than the bottom of the footing trench.

Make sure there is room for the vertical drain pipe clean-outs to come up outside the footing (on the opposite side of the building from the exit point).

Make sure the drainage pipe ditch continues on from the exit point on the building and maintains the proper 1/4" per foot slope.

Sweep the ditches

As a final step, sweep the ditches with a broom to get out the last of the loose soil.

Swept footing ditch 

A few extras

If appropriate, dig the trench or trenches (or at least the beginning of the trench or trenches) to bring electricity, water, data, and anything else you need to the wall behind where your services will be received in the house.  It's good to do these excavations now so the digging does not disturb the intricate forming of the footings.  It may even be that you need to route some of these services under the building, possibly in the same trench used for the drainage pipes.

For these various services it is best to route them through the north wall of the building near where you plan to have your services area inside the building.  It's best to come through the wall rather than through the slab as the ground water table pressure will be a bit less and it will be more accessible to fix any problems.  Even though it is through the wall, the water pipe does still need to be below grade and below the frost line.  It's good to put the electrical feed at a similar height to keep it hidden out of the way.