The shed designer framed every floor the way a house is framed: 2x6 joists 16" apart. A builder who walked through it with us said most members check at about 15% of their capacity. The floor joists are a good example. On the 8x12 starter they sit on skids 48" apart and use 15% of what they can carry under 40 psf live plus 10 psf dead.
That much spare capacity is money spent for nothing. The cheapest lever turned out to be the joist spacing, not the size, because there is a second rule involved: the subfloor's span rating.
The rule that actually limits spacing. Both subfloors the designer prices are 23/32" tongue-and-groove panels. APA's single-floor ("Sturd-I-Floor") ratings tie panel thickness to the widest joist spacing it may span:
- 19/32" panels are rated 20 oc (joists 19.2" apart);
- 23/32" panels are rated 24 oc.
(APA data sheet.) So 24" joists are within the panel's own rating. The listings don't show the grade stamp, so check that yours reads 24 oc, or 48/24 for a sheathing panel, before you rely on it. The designer now checks this as its own row on the Structure tab. It passes at 24" and fails beyond.
What it saves, from the designer's prices (materials only, before tax). The figure in brackets is the joists' use of capacity:
| Preset | Joists at 24" | 2x4 joists at 16" | Both |
|---|---|---|---|
| 8x8 | $28.46 (23%) | $32.49 (32%) | $55.11 (48%) |
| 8x10 | $30.46 (23%) | $39.85 (32%) | $61.18 (48%) |
| 8x12 starter | $27.98 (23%) | $47.85 (32%) | $67.44 (48%) |
| 10x12 | $71.02 (16%) | $48.98 (23%) | $108.96 (34%) |
| 12x16 | $90.90 (23%) | $71.18 (33%) | $145.08 (49%) |
Two cautions before you take the 2x4 column:
- The 2x4 price is not from a listing. We have no readable listing for treated 2x4, so its price is scaled from the treated 2x6's per-foot rate. It says "(scaled)" on the shopping list. The 24" spacing saving needs no such guess: it is fewer 2x6 boards at the listed price.
- A riding mower is a point load. The check spreads 40 psf over the floor. A lawn tractor puts a few hundred pounds on each tyre's footprint, and that is not checked. If one will live in the shed, keep the stiffer floor.
The "bring the price down" search now tries both moves. It keeps one only when the joist check and the subfloor rating still pass. On the 8x12 starter it takes 2x4 joists and then 24" spacing, among its other steps.
The designer is not engineered or code-checked. Its structure tab is a screening check, and you are responsible for verifying it.