Choosing the Right Kind of Slab
Hello all!
NE Wisconsin zone 5 my client wants a Pretty Good house built on Washington Island with little top soil on hard Dolomite limestone. We can have lots of snow melt and heavy rains so I’m a bit worried about a real “floating” slab. Stem wall may only be 18″ deep (insulated of course). I’d like 4″ of EPS under either fines (no slab) or slab. Is it just a matter of raising things up? How do we keep water from lifting or infiltrating? Is it just a matter of raising the whole foundation up on gravel without fines? Can inner and exterior drains prevent it. I like 6″ of 3/4 for drainage purposes but again, is that enough?? Also, not much slope in the south facing yard so drains would have to be very long to reach daylight.
Also, they are worried about burying plumbing in concrete so that is where the slabless slab comes into play. Certainly easier to lift a wood and insulated floor if there are future problems.
Any help is mush appreciated!
Thanks, PK
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Paul, the sub-slab prep should include drainage that prevents standing water. A few small pools won't hurt anything. Areas directly below the slab should never freeze, so it's the perimeter to be most concerned with. Although frost wings aren't required in all areas I always include at least a 24" wing, sloped 5-10° to 4" perforated pvc that drains to open air. If it's hard or impossible to drain to open air I would either raise the whole slab zone (slab + 10' out). Concern about repairing pipes is one reason my clients asked for a concrete-free slab when I did one several years ago, but there are millions of concrete slab-on-grade homes built and with good-quality plastic I don't think it should be a big concern.
Thanks Michael!
Sounds like you had another concern not written as you wrote "If it's hard or impossible to drain to open air I would either ... . Just a typo, or was there another thought you needed to mention?
Paul, I think I was going to say that I might go with a crawlspace system that would allow for a sump pump, or build on piers--preferable helical metal piers, which are fast and have relatively low embodied carbon. But I don't see why you couldn't raise the house site elevation enough to get above the surrounding grade, unless it's a very small site. There may be aesthetic reasons your architect doesn't want to raise the house's height above existing grade.
Here's what I got from the architect.
"Drainage as draining to grade is not an option on this site."
Why would this be? Can't we just raise the stem foundation walls up and add a deeper gravel base? What am I missing?
Any responses are appreciated,
PK
Is there a code for drain depth? If a site is absolutely flat, can the drain tubing be just under the heightened surface until about 10' out from the building?
Can anyone dive into my last two concerns? Thanks!
Paul, I don't know why your architect says you can't drain to grade but assume they have a reason--I would keep asking them until you get a satisfactory answer. It could be an elevation issue or a zoning issue, or it could be for aesthetic reasons--there's no way to know from here. There is not a code requirement for drain depth but the drains should start adjacent to or below the foundation footings and run with a slight slope until they reach open air. If the drain can't reach open air it would do no good to just run it flat out 10', or any distance.
It's all about elevation. This is very flat land on Washington Island on Lake Michigan with limestone bedrock only 2' under a sandy soil. He's very worried about during our heavy snowmelt in spring when there is substantial water near the surface. So I say "just raise it up with lots of gravel base" why not? Then the drain can be on the surface away from the house. Large quarry on the island so gravel is easy to come by. I'd love to go with frost protected slab to avoid stem walls. Mono Slab EZ? Do you see any problems with this?? Thanks, PK
Paul,
That sounds like a good and workable idea. Another option would be to just have the stem-walls extend higher above the existing grade - although that might mean the relationship between the first floor and surrounding lot wasn't what you wanted.
Paul, I agree with Malcolm--I often do raised slabs.
BTW - the house will be 2 story, about 1,800 sq ft.
Thanks all!!
Our architect asked for a civil engineer to review the drainage plan. Could you request a civil engineer to sign off on the plans AND take responsibility for the outcome? We ask a lot of builders; this seems beyond reasonable.