Home Site Map - Steps - Foundations -
Form Footing - Peripheral Drainage
Round the outside is perforated drain pipe for additional drainage.
Outside the lower outer Form-a-drain
Round the outside of the footing trench there needs to be a 4" perforated drainage pipe, or as it's sometimes called, a "weeping tile". The outside drainage pipe needs to be installed at this point in the build process so that crushed rock can be placed behind the lower outer Form-a-drain to balance the forces associated with filling the under-slab area with crushed rock.
Drainage 4" pipes will not have filter cloth sock
Many people say that 4" perforated drain pipe should be covered with a sock of filter cloth to keep soil from getting in and blocking it. I prefer to leave off the sock and instead have a good arrangement of cleanouts, down which a long cleanout brush can be inserted and/or a high pressure hose pipe. I worry that over the long life of the house that any filter cloth would itself become blocked and there would be no way to clean it.
Line the outside with geo-fabric
The geo-fabric goes from the bottom of the lower outer Form-a-drain, under the perforated drain pipe, and then up the outside of the excavation. The first strip runs in parallel with the Form-a-drain so is only wide enough to go part way up the excavation. The actual width of geofabric does not matter as long as the width is greater than 4 feet. The Form-a-drain goes up the perforated side of the Form-a-drain to its top. More geo-fabric will be added later before it comes to adding more crushed rock.
Bed of drain rock
Prior to installing the 4" perforated pipe, lay down a thin layer of 1.25 inch crushed rock without fines for it to lie on. Near the drainage outlets the bed should be about half an inch, but it should very gradually get thicker as it goes back in order to give the pipe a slight slope. Something like 1/16" per foot is sufficient. Certainly you should check that there are no places where the gradient is the wrong way.
1.25 inch crushed rock without fines
If you buy crushed rock that has not been washed after the crushing then you will need to wash it yourself (which is hard work).
Install the drainage pipe
The drainage pipe round the outside of the footings connects into the connectors previously installed in the drainage pipe that runs across the building area and away. The perimeter pipes join to the T pieces of the drainage pipe at both the exit point and at the far side of the building.
The job of the perforated 4" PVC pipe is to catch the bulk of the water before it even gets to the Form-A-Drain. Some water will get through to the Form-A-Drain because in some places the 4" pipe is slightly higher than the bottom of the Form-A-Drain in order to provide a slight downward slope to the pipe.
There are cleanout Wye fittings that connect to vertical 4" pipe. Pairs of these are at the east and west sides as well as on the north west of the north portico and south east of the south portico.
East side
North side
West side
There are also cleanout pairs in the north west and south east corners.
Regular (tight turn) bend right angle pieces are used at the building corners. Using these regular tight turns keeps the pipe close to the Form-a-drain .
Note that you can glue together the piping ahead of the geofabric and bed of crushed rock as it is easy to lift up the pipe after it has been glued.
The pipe needs to go round the outside of the portico deck support pillars because the portico deck pillars are right up against the Form-a-drain.
Put geofabric on inside Form-a-drain
This is so that crushed rock can be put on the inside at the same time as it is added on the outside so that the forces can be balanced.
Use 4' wide geofabric.
Add crushed rock to height of pipe (inside and outside)
Add crushed rock on top of the peripheral drainage pipe. Do not at this stage add too much crushed rock or it will cause the Form-A-Drain to bend inwards. Fold the geofabric over the top of the crushed rock to keep soil from dropping into it.
When adding crushed rock to the outside you also need to add crushed rock to the same height behind the inner lower Form-a-drain in order to try to balance the forces.
