I'm working on a 20'x12' monoslope tiny house like this:enter image description here

enter image description here

The foundation is 3x 20' 2x8 sitting on 4 helical pile/concrete pier on each side (8 total).

As far as I understand, the load bearing walls of this type of building are the front and rear walls. The side wall are non-load bearing. If I use big enough floor joist (10" seems sufficient for 40-50psf live load based on span tables I checked), I should be able to span 12' without any support in the center?

The roof area is 22x16=352 sqft. I'm designing for 27psf snow load and 15 deadload for total load 42psf.

352*42 = 14784lbs or 7392lbs each side

If I understand this correctly, my concrete pier/helical piles need to support these weights?

I have clay soil so I assume 2000lbs for 12" concrete pier at 4' deep. would 4 concrete piers per side enough or I need more for safety margin?

  • 2
    $\begingroup$ Get this checked by a structural engineer who knows the standards in your area. $\endgroup$
    – Solar Mike
    Apr 4, 2022 at 4:39
  • $\begingroup$ Wind load? Seismic load? $\endgroup$
    – Forward Ed
    Sep 1, 2022 at 8:12

1 Answer 1


You already forgot the floor load; the wind, and seismic loads.

Also, the wide window needs a header and two posts that transfers a concentrated load to the foundation, which needs to be calculated. We need to provide lateral support via shearwalls or acceptable alternatives.

Sites not verified by a soils report are usually assigned strength of 1000psf or less, not 2000psf.

You need to have connections and fasteners including those for the crawl space to footings, thought out and detailed.

Many municipalities have typical plans for these small structures on a handout.

Otherwise, if this is going to be used as a dwelling you need to hire an engineer.

  • $\begingroup$ I already stated that floor live load is 40-50psf $\endgroup$ Apr 5, 2022 at 0:18
  • $\begingroup$ you didn't add it to footing load. $\endgroup$
    – kamran
    Apr 5, 2022 at 5:39

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