Drain tile and stepped footings
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| Posted in General Questions on
Our house will have a walkout basement and in the design diagrams the architect has placed the exterior drain tile at the same level as the footings even where the step footings are located. At a couple of locations this places the exterior drain tile close to 5 feet beneath the slab. Does this make any sense?
Is there a reason to place the exterior drain tile any deeper than the interior drain tile depth at the step footing locations? I have a slope to my building site but I would have to run the drain tile out a few hundred feet from the house to reach day light for a 5 foot deep drain tile.
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Replies
Tom,
Does your basement have several different slabs at multiple levels? Or just one slab at one level?
Just one slab at one level.
If there is just one slab level, then it's acceptable to place the footing drains between 5 and 10 inches below the top of the slab. There is no need to go much lower than that.
Tom, I suspect the architect has drawn them that way to take into account the normal sequencing of construction. The perforated pipes are installed, covered in drain rock and back filled. In order to keep the perimeter drains at at height half way up your foundation wall, the backfilling must be done in stages. Another concern with having the perimeter drains higher than your footing is that sub-surface water can pool against your footings and make its way under the slab. In your case it may well be worth doing this to avoid the problems of sloping to daylight, and it's probably a good idea to bringing it up with your architect.
Malcolm, I have attached a foundation wall detail diagram that shows my area of concern. If the exterior drain tile is tied to a sump basket then the sump basket bottom will need to be something like 7 feet beneath the slab. The attached diagram is actually for the walkout wall which spans most of my southern wall. The step footings are along the east and west walls and look similar to the attached diagram except the T-mass wall goes up to 9 feet. There are steps at both 5' and 3' below the slab.
I can't imagine that my house is much different than other houses with walkout basements so I'm surprised I can't find examples describing the drain tile system.
Tom, I'm afraid I don't see the attachment. Don't know if the problem is mine or it didn't attach.
Sorry, user error on my part. I browsed to the file but failed to press the Attach button before posting.
Tom, I'm not familiar with the whole drain system your architect has proposed, but here if I was connecting the interior and exterior perimeter drains I would simply tee them together and run a solid line to daylight. Why is the sump basket necessary? Perhaps this doesn't make that much difference as you are already down five feet anyway though.
Looking at the section I agree with Martin. As long as you include a layer of clear crushed under the slab to act a capillary break against water which may make its way into the foundation, I can't see that running the exterior drain at a higher elevation would cause much of a problem.
Edit: It would be interesting to know what your building code says on the issue. That may be informing your architect's choices.
Malcolm, good idea about checking the building code. I will be building in Wisconsin and the code notes:
(d) Drain tile or pipe installation. Drain tile or pipe used for foundation drainage shall comply with the following requirements:
1. a. Except as allowed under subd. 1. b., the top of the tile or pipe shall be at or below the top of the footing.
b. Where the top of the footing is more than 4 inches below the bottom of the floor slab, tile or pipe is required on the interior of the foundation only and it shall be placed directly under the floor.
Note: This situation will commonly occur with a walkâout basement.
So it appears per the code that no exterior drain tile is necessary at the step footings or along the walk-out wall footings.
So my question now is: Is there value in doing what Martin suggests along the walls with the step footings and the walk-out wall?