20x30 Foundation

Started by hpinson, January 05, 2015, 03:07:13 PM

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Hi John. I am modifying your 20x30 plan in Sketchup to 20x20.

I have a question about the slab on grade foundation plan.

In the "Foundation Section", the cross section shows the 2x6 P.T sill overhanging from the concrete slab over the Dow foam Blueboard by 2 inches. Why the overhang, as opposed to seating all six inches of the sill completely over concrete?


John Raabe

The detail I show allows for cement millboard or metal flashing to physically protect the insulation on the slab edge. Note that there is 3.5" of direct bearing on the slab - the same as a 2x4 wall.

You could also build the slab full width with the sill to the edge and find another way to insulate the slab edge. This detail, for example, could eliminate the foam exterior wall insulation and have a tapered 2x with drip stop to get water out and over the edge insulation (which is commonly 2" thick).

Here is another link for some alternative slab insulation details. http://web.ornl.gov/sci/buildingsfoundations/handbook/section4-1.shtml
None of us are as smart as all of us.


Thanks John. That makes sense.

Another question - is it preferable for sand to go on top of the 6 MIL. Black plastic, or the plastic on top of the sand? I've seen it done both ways - for instance, in my house, which has a brick floor, the plastic is on top of the sand.  Bricks are on top of that.  Not sure which is preferable though I would think on top of the sand which would prevent moisture in the sand heading upwards.

I sprung for Sketchup Pro.  The Layout tool provides a lot of missing features that the free Sketchup modeler does not have, and while not perfect, you can do architectural drawings for presentation with it.  I would have liked to get the full Chief Architect as that is much easier to use and has better built in libraries, but it was way too expensive for occasional use.

John Raabe

I have seen slabs done both ways too. For a poured slab I think sand on top of poly allows for physical protection of the poly and allows the concrete to cure evenly from both top and bottom. But check with the local batch plant for their suggestion.

Sketchup Pro seems like the best tool to me. CA is powerful but $$$ and doesn't have as big a free support community. However either one can do the job.
None of us are as smart as all of us.