Sloping a Pad for Drainage
Hi all,
I’m designing my foundation and want to have a good water management plan, I figure this starts with the house pad. I’ll be cutting/filling the pad from the local soil here, which is gravelly/silty stuff. It compacts well, but doesn’t drain as well as I’d like. So I planned to add 4” of 3/4” washed stone on top of this for drainage. The footings will bear directly on this drainage layer.
Im wondering if should design a mild slope into the soil layer below to give any accumulated water a path to drain?
As for the layer of washed stone layer, does this need to cover the entire house pad or just under the footings?
Thanks! (Marine 4c, 12” frost depth)
Adam
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Replies
I think it would be a mistake for your design to vary from whatever is the local custom is. Whatever the local custom is it has passed the test of time and is know to work with the local soils. If you decide to reinvent the wheel and start telling people how to do something they have been doing for decades seems likely to end badly for you.
Locally the custom and perhaps the code requires the footing to rest on undisturbed soil anything else seems more likely to shift under a load. To my ear drainage under the footing seems likely that any flowing water would remove some soil and shift the building resting on that footing.
Walta
Thanks Walta, that’s good advice. I’ll ask around. I can probably rely on interior/exterior footing drains as well, since I can drain to daylight.
Adam
AdamPNW,
I'm not sure I see any value in building with drainable material under the footings for a couple of reasons.
First would be a code issue, as here when you move from placing the load-bearing elements on undisturbed soil, it triggers the need to engineering.
The second objection is that the drain rock has now created a path for water to move under the building that typically doesn't exist. When stem-walls, or thickened parts of slabs are placed on undisturbed soil (or compacted largely impermeable fill), the exterior perimeter drains carry any water away so that it doesn't get under the building. If there is a source of water under the building footprint, then an interior drain can be used to move it beyond the footings. The layer of drainable fill directly under the slab is there as a capillary break to stop moisture migrating upwards. To place the whole foundation on drain rock, inviting water to deliberately use a horizontal path under the building doesn't seem like a good strategy to me.
I see, you’re saying that the drainage layer I’ve suggested is now a two-way street for water. Instead we want to hinder it from getting under the footings (exterior drains), but allow it to escape if it gets there (interior drains). Appreciate the explanation, that’s one more problem solved!
Adam