Workshop Notes

Various notes used in deriving the plans for the workshop we built in our backyard. I'm trying to gradually adapt this page to be both a historical record of how we arrived at the building we eventually did, and what we eventually arrived at.

This isn't there yet.

The building started out as a "we should get a TuffShed™", to getting a quote for that and realizing that for the same price we could do better, to expanding our reach and thinking that we could build a living roof to give us better climate control for the space, and more room on our limited lot.

A bunch of Workshop Pictures from the construction if you don't care about the details

Format of plans and drawings

Preferred scale: 1/4 inch per foot for structural and architectural; 1 inch = 20 feet for site plans


Per Irene Borba at the petaluma planning department, 2009-07-09:

The setbacks for a detached accessory structures is 5' from the rear and 4' from the side. If the existing structure was not permitted and you tear it down and built another structure, it would have to conform to the required setbacks. The height limit is 15' and that is measured as the vertical distance between the average finish grade and the midpoint between the eaves and ridge of a gambrel, hip or gable roof, or the highest point of a flat or shed roof, or the ridge of the roofline of a mansard roof.


Per Edward Hamer at the Petaluma Building Department:

The manufacturer should have some specifications on to what is required. Typically we see a poured slab with a thickened edge with a couple of #4 rebar's around the perimeter with a #4 rebar grid 18" OC each way. The California Building Code is on-line at

Also if the structure is going to be within 5' of any property lines the wall facing the line will need to be one hour fire rated construction. If the wall is within 3' the can not be any normal openings in the wall such as windows or doors.

Foundation calculations

CBC ch18 table 1804.2 says that for soil type 5 (clay), which I believe we have, allowable foundation pressure is 1,500PSF, lateral bearing is 100psf/f. Luckily we have a flat surface.

Table 1805.5(5) says that the wall must be 7.5" wide if it's <5' high, and doesn't need to be reinforced(!). It does say that vertical reinforcement must be WallThickness - (1.25" + BarDiameter /2) +- 3/8".

Concrete must be 2,500PSF after 28 days.

140lbs/sf roof times half the 14 foot span is only 980lbns.

Footings are poured as upside-down T, we can go with the 7.5 or 8" wide specified above, and put the width in the bottom of the T, and it looks like even with the heavy roof that can be as narrow as 12".


After much back-and-forth, we decided on a 6" slab with 18" thickened edges, going 18+" below grade.

Minimum of 3 1/2" with 6 mil plastic vapor barrier lapped at least 6" (not required, but we're doing it anyway). We didn't do insulation, though we probably should have, mostly because the city sprang the Title 24 energy compliance rules on us late in the game and I didn't run those numbers until then.

2-3" of sand, tamped to within 1/2". 8-12' spacing for contraction joints is reasonable.

Rebar on 1' centers.


It's massive overkill, dirt is 60 lbs/cubic foot, water is 62 or so, and we have 6" or so of dirt, but we've engineered for 120 lbs/square foot on the roof.

We tried various calculators, in the end it was cheaper and easier to give the roof loading and span numbers to a truss construction place and let them do all the details, even though that ended up with trusses that are less aesthetically pleasing than the ones I wanted to do.

This joist span calculator says that Doug Fir-Larch 2x12 No.1 grade can carry 100 psf live load and 20psf dead load over a 12 foot span, but the right technology here is to use an i-joist.

Proposed Roof structure

Adapted from "Colbon Building Products Enkadrain Case history 0406 division 7 drainage & waterproofing" PDF.

  • Sheathing
  • Felt to allow for a slip layer
  • Waterproofing layer: EPDM (should cost circa $150 for a 14x22 sheet, includes splice instructions) or WR Grace Bituthene 4000 (From w/store in Rohnert Park, 36"X66' with conditioner is $223.84)
    • 4" of Polystyrene
    • 6 mil plastic
    • Enkadrain 3615R (See also GeoPad UltraGuard from
    • 5.5" of topsoil

    Engineering the Wall

    We ended up using a relatively standard staggered 2x4 stud on 12" spacing 6" thick wall assembly, with 5/8" plywood, nailing schedule 4" exterior 8" interior. I forget what I came up with for shear strength, but our engineer used that as overkill.

    At User "RockEngineer" says on 2004-04-16 that:

    The ADS manual for Engineered Wood Construction Structural Lumber Supplement Table 5.2a and 5.2b gives axial compression allowables for 2x4 and 2x6 lumber of various grades and various lengths. The table assumes that the studs are braced along the 2" nominal width and the limiting buckling is the 4" and 6" nominal width. Table 5.2a is for duration 1.0 and 5.2b is for duration 1.15 (snow). For No. 2 Spruce-Pine-Fir the allowable load in pounds are

    8 ft30008710
    10 ft20106700

    As far as the framing controversy on 2x4 vs 2x6 studs the 2000 IRC Table R602.3.1 shows the prescriptive allowable length of wood stud walls based on the number of floors supported. In essence it says that a 10 ft tall 2x4 stud wall 16" o.c. can support a floor and a roof for their "model" structure. 2x6 walls 24" o.c. can be prescriptive up to 14 ft without engineering.

    But "jheidt2543" says:

    Your examples from the ADS Manual seem to relate only to axial loads.

    Wall Assembly notes

    What about blocking? Blocking is unnecessary in an 8' wall.

    Barrier Technology Corp has a two hour wall assembly that has shear strengths of plywood sheating. Coated side against framing Blazeguard is 200psf as a rooth sheathing w/16" centers, 94PSF on 24" centers.

    We ended up just using sheets of 5/8" drywall on both sides of the staggered stud wall assembly. Except for the shear panels, this is a stock UL 2 hour wall assembly.

    Seismic Class

    From using the app at and 38.224829,-122.627707

    Period (sec)Sa (g)
    0.21.500 (Ss, Site Class B)
    1.00.600 (S1, Site Class B)

    S1 is < .75g, so the SDC isn't automatically "E". Table 1613.5.2, I'm not sure if the soil profile is "C" or "D".

    Category: Workshop