Do I need a footing drain and/or dampproofing or waterproofing for my slab on grade?
My understanding is that neither IRC nor my local jurisdiction (Kirkland, WA) require a drain for slabs at or above grade. Even if that’s correct, some questions:
1) SHOULD I put in a footing drain anyway (i.e. risk reduction worth the extra cost)? Would that be along all 3 sides of the new slab?
2) Should I add damproofing or waterproofing or ok without any?
(2a – if I should what good bang-for-buck product would you recommend?)
3) Would lowering the footings / raising stem walls so that my slab is further above grade be a better investment?
Some add’l facts that might be pertinent:
– I am adding 8′ of length to an existing 16’W slab on grade
– The added 8′ runs parallel to natural grade of 2:12 with very good native soil drainage. All existing and new downspouts connect to drains that daylight at end of lot.
– The existing slab is uphill from the new slab and 10′ further up the slope is an existing retaining wall built in 1978. I don’t know how that foundation wall was waterproofed but we have never observed any seepage to the slab we are extending. It’s often drizzling here but Seattle area’s 39″ annual precip is kinda middle of the pack for US as an annual average, and it’s spread out over more months… not sure how relevant that is vs. the fact that we simply aren’t battling groundwater elsewhere)
– On one side the 8’L stem wall is under an 27″ eave. The other side is under a covered deck offering 12′ of protection. The long side (perpendicular to natural grade) will be under a 36″ eave 18′ above.
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