Split slab Drainage in Bleow Grade Condition
KTH
| Posted in Expert Exchange Q&A on
Hello!
We have a condition on a residence in a 5B climate with a new garage to be built over a new basement. The space below the garage is usable (non storage). Our structural engineer is going with a split slab approach, and we are thinking that there needs to be a drainage cavity with waterproofing between the topping slab and the structural slab to prevent water from infiltrating below.
If we do this, my question is, how can it drain, since the slab is below grade? The attached shows a wall section through the space and an enlarged detail at the edge of the slab and how we are thinking drainage could work. Is this feasible?
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Can you share more drawings, including an overview? This is a complicated design and I need more context to understand what is going on.
Absolutely! Attached is the structural section through the garage that show the split slab location / condition over usable basement space, the floor plans for both the basement and garage level, and an architectural section through the space.
Overview wise, our structural engineer is utilizing a split slab construction for the structural floor of the garage that creates the "roof " of the basement that is supporting a 2nd story above. We are concerned about possible water infiltration from the garage floor to the basement below from snow / water off of the cars on the garage floor. To help eliminate this, we will be providing a concrete sealant on the garage floor, as well as providing a monolithic waterproofing membrane with a horizontal drainage layer between the topping slab and the concrete structural slab to allow for any water that might get through the topping slab and sealer a way out, and not into the ceiling below. The idea of this would work similarly to like an above ground parking garage split slab, however in this case, the slab is below grade, and the drainage plane would extend below grade. We are looking for a solution as to how to ensure that any water that enters the horizontal drainage cavity is able to be directed away from the foundation wall / building.
If the runoff is to drain by gravity it has to go to a place that is at grade but at a lower level -- ie, downhill of the house. A pipe would take it there. If the layout of the lot doesn't make that possible, what you need to do is have it drain into a sump and then pump it to the surface.
I've only done a split slab over living space once, on a panelized Passive House, which had its own set of challenges.
My first thought is that the driveway is too high where it meets the garage. Frost heaves will lift the driveway over time, directing water toward the garage. I spec a 2" step down from garage to grade, and slope the driveway at least 6" in 10' away from the house. The edge of the garage slab can be beveled to ease the transition. Car tires have no trouble getting over a 1" lip.
I would consider using a trench drain outside the garage. Here's one brand of many: https://www.duratrench.com/precast-trench/.
I haven't read all of the notes on your drawings. Is your engineer requiring a friction connection between the slab layers? If not, a waterproof membrane there would be simple and effective, similar to how tiled showers are built.
You can use high-PSI concrete for the topping slab. At 5,000 psi, concrete is pretty close to waterproof. It is more prone to cracking the higher the psi though so you would want to take as many steps as you can to limit cracking.
Otherwise I think a waterproof product on the surface is your best bet. It will probably require regular maintenance and recoating.
Personally, after doing one of these, I will do everything I can to avoid them in the future--it's just too much risk and complication for me to deal with. But others do them regularly so I'm sure there are reliable ways to handle it.