Waterproofing Below Slab Concrete
Hey All,
Building a house on a steep lot in CZ 6. The uphill-side of the home’s basement concrete wall is the basement is the actual basement wall and will be fully waterproofed on the exterior with dimple mat and waterproofing. The concrete walls then start to step down, eventually going partially below the slab height, then fully below grade and acting as retaining walls for the slab above. On the downhill sloping side, almost the entire concrete wall is below the slab and holds the soil basement fill in place. The entire concrete slab will have a vapor barrier of course.
It makes sense to waterproof the concrete walls that are above the basement slab height but below finish topsoil grade. All good there.
My questions:
– Is there any reason you would want to waterproof any of concrete that is below the slab height? I figure it would be futile since the earth beneath the concrete slab has moisture in it and the entire bottom is open. But maybe there’s something I’ve not considered.
– The interior backfill below the slab will be a combination of drainable gravel and dirt fill. Thinking it would be wise to place some perforated pipe in the backfill beneath the slab and poke it out the downhill facing side through a solid PVC pipe. The thought here would be that any water that gets below the slab would be able to drain easily, but again, maybe this doesn’t matter whatsoever because the entire space below this backfill is open to water and moisture entering and exiting the space. Just thinking that if this wall is more of a retaining wall, you wouldn’t want to have any water on the back of it challenging it.
Thanks for any thoughts
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Replies
The most important part of the basement water management plan is your exterior footing drain it needs to be several inches below the basement floor and drain to daylight on your property. With that drain working it is simply impossible to get standing water in contact with the slab.
Below the slab should be a plastic vapor barrier that should block most of the vapor movement.
Assuming the slab is resting on 4-6 inches of clean stone that will allow water movement there is no reason to spend money buying pipes. If you are worried that the exterior drain will fail for very little money you can have an interior sump or two installed and only populate it with a pump should you get water in the basement.
You may want to add a passive radon vent under the slab that would need some pipes.
Walta
Thanks Walta,
You actually bring up an interesting point... I was always planning on running the french drain alongside the stepped footing, but I guess it wouldn't make sense if that footing is far below the slab. You're suggesting just run the french drain at the slab level, like in the attached drawing, correct? This side has a more dramatic step where the other side is more gradual and frequent.
There will be a passive radon system installed and a complete vapor barrier, yes.
I guess my question was also, do we care about the soil beneath the slab's water content?
SeanRyan,
I will have to counter Walta about water movement on the inside of your foundation. There definitely are reasons for controlling water on the interior of a foundation's low end as you have. In my locality the bedrock is composed of sandstone layers which often rise near the surface grade unpredictably. Spring melt water arriving from up slope can take many paths depending on the overburden soil types as well as travel in the sandstone layers. I know of local stepped foundations that developed interior pooling on the low end of the foundation thanks to spring melt water. Imagine the foundation as a dam set into a hill side. The higher elevation wall impedes flow which will drop under the footing to then flow under slab until it is again impeded by the interior side of lower elevation stem wall as you have noted. There is a potential for build up that at minimum will keep that wall wet. Water will not flow around the foundation like a boat wake. So, yes, the lower stem walls may be sitting in high moisture conditions and how you isolate your sill plates is very important. Yes, there is a good reason to provide an exit path. Yes having soggy soil under the house and slab might not be a good option.
Now the usual TL/DR part.
The relevance to your situation MIGHT be moot if your soil profile site-wide is truly capable of allowing up slope water movement to pass around and under the foundation footings freely. Maybe you are on a coarse mix of great depth, but if you have clays or close bedrock layers I would strongly suggest adding in an interior collection pipe and under footing exit to daylight on the low side stem walls. I would also strongly suggest having a soil profile done professionally to confirm that you do not have potential expansive clays on the site. It is an expense that could save major issues from appearing in the future once your foundation presence changes the movement of water through your site soil. Expansive clays can deposit in bands that confound reliance on adjacent site experiences as a guide. The radon expert I hired told me not to bother attempting to assess my site prior to building as there are no really sure fire ways to test. I simply did as he advised, which was to embed the radon pipe system under the slab and be ready if needed. You already seem to be on top of that.
I have outsulation on the foundation (and house) which may differ from your plans. I do enjoy a bone dry basement and have no fear of placing my lumber and sheet goods against the walls directly nor boxes directly on the floor. Some important details went into ensuring these conditions.
My own site has about five feet of drop across the short axis of my foundation. My lowest footing elevation is about 28" below the slab top on the down slope side. I chose to use Xypex additive for all my concrete to simultaneously control for capillary movement upward as well as act as waterproofing for all vertical walls. My footings and floor slabs are stepped to accommodate the terrain. The under slab insulation is 3" of reclaimed XPS with vapor barrier on top.
My footings all rest on the local rock layer and the exterior perimeter drain rests next to the side of the footings NOT the shelf created by the footing top and wall. A full burrito of the fabric wrapped pipe was created by first setting filter fabric, then pipe, then a bed of washed pea gravel over pipe and pulling the filter fabric over the pea gravel and pinning. The backfill soil is thus impeded from clogging the pea gravel bed around the further filtered drain pipe. The pea gravel is fully wrapped in fabric to prevent infiltration on all sides. Silting out the drain stone WILL happen if just fabric is set over the top. Silting out the pipe will happen faster if not the fabric socked type.
Inside the foundation perimeter I opted to place washed stone entirely even where the depth was close to 28" because I was on the bedrock which indeed does weep water every spring. The cost was actually only (10 yrs ago) a few hundred different from the cost of bringing new non clay fill and the man hours of compacting. This had the added benefit of making radon piping installation a breeze. A short pipe against the interior side of the down slope footing gathers the spring run toward the lowest corner which has a pass thru pipe to the final drain run to daylight. A witness pipe I set in the floor at that corner confirms my belief that the bedrock would weep each spring. I know the water stays well below my insulation thanks to the washed stone.
I am not an engineer, but I would raise a question now about the up slope wall and its footing/rebar. How your floor joists are set may have an effect on the resistance to lateral load of your tallest walls. I had to deal with a garage under house situation where the two wing walls holding back the adjacent soil had begun to roll over despite having been thickened at the base. My joists are set to counter the tall walls in my plan, but I did choose to create wide offset footings with larger rebar to control for potential movement in the wing walls of my exterior basement access. These are not pinned to the foundation because of the exterior insulation details.
Thanks for the very detailed response, onslow -- all appreciated. It sounds like poking a couple small pipes drained to daylight through that bottom wall would be a good insurance policy, which is what I was thinking. I was just thinking about how retaining walls are often configured with such drainage, and even though the walls are connected here, might as well help prevent water from challenging it. All these walls have big footing toes on them and lots of bar.
Sean,
I think Onslow has given you good, detailed advice.
My less exhaustive response would be, you need two things:
- Exterior perimeter drains at the footing level wherever the slab is below grade. Whether you continue them the whole way along the sides and low end, or simply terminate them and provide solid piping to daylight from there on doesn't matter much.
- An interior outlet for water that may make its way under the slab. The easiest solution is a small rock pit at the low end with a perforated pipe that goes through the stem-wall and connects to your exterior drainage.
Sean To my eye in your drawing, it looks like your drain pipe is too high and your slab is to low. You need that differential to create the pressure to ensure the water finds the drain. Like this drawing.
Onslow, can we agree that foundations are very different based on the local conditions?
What work here may well be a failure over there. If you have 4-6 inches of free flowing gravel under a slap the water level under that slab will be uniform pipes or no pipes. If you somehow get water 12 inches above a working drain, I don’t understand why the water would not find that drain somehow.
Walta
Hi Walta -- The slab height and drain pipe will be as you've included in your image attachment. I was just roughly sketching it yesterday and not to scale. Thank you for the clarification.
The main goal of making the image I sent yesterday was to confirm that it would be okay to run the exterior drain tile below the slab to daylight, rather than have it jog along the footing that ends up being very far beneath the slab on the low side of the house.
The problem is it is easy to draw the drain as showed but it is a ton of work to build it. Mostly because the footing for the house, are not made with forms but use dirt as the form and no one want to dig out the dirt to get the drain in the right place.
The drain almost always ends up where the blue circle is in the drawing and not deep enough to really keep the floor dry.
Walta
Walta,
Trench footings are a regional practice which may or may not be common where Sean is building. One aspect of that section I don't like is having the slab bearing directly on the inside of the footing with no fill in between. That both makes it more prone to cracking due to differential settlement, and leaves no path for incidental water that makes it through the joint between the footings and stem-walls above.
I total agree more gravel would be better.
I shamelessly stole that image from a .gov website.
Walta
Walta,
I think it's a pretty common detail as people want as much head height as they can get with 8 ft forms.