Woodshed Engineering

April 10, 2016 12:58 PM

I want to build a woodshed, AND I don't want to go to that much effort, only to have the floor collapse. I am not an engineer.

So my plan is to build a little enclosure to keep all my firewood off the ground and out of the rain. In my mind, I want this, only with a footprint of 16 ft wide by 4 ft deep.

I'm concerned about the floor loading. I plan to have a stack of firewood in there about 4.5 feet high-ish. According to this wood weights table, I would need a floor loading capacity for that of up to 250 psf. (!!) That sounds like a lot, maybe I'm doing this wrong.

And then I can only find span tables that go up to 60 psf live load.

I'm worried that I'll do it like the video and the floor will collapse. But I can't figure out what I should do. Deeper joists, smaller spacing, but how much deeper/smaller? Assume I don't want to just pour a concrete floor and call it good, or is that what I'm going to have to do?

posted by ctmf to Home & Garden (8 answers total)

A 250lb person standing on the shed floor will exert considerably more pressure than that (the area of their feet being much less than a square foot). Fat people can usually stand in sheds with no risk of collapse.

If anything, that shed base looks rather overengineered to me. I've built much flimsier wood shelters and never had a problem with the weight of the wood.
posted by pipeski at 1:06 PM on April 10, 2016

Best answer: I think you are over estimating the weight of the wood. Presumably you have something like Douglas Fir at 38lb/cu ft when green. Assuming about a 30% reduction in density for air space and bark, which is much less dense, that gives 120 lbs/sq ft at 4.5 high.

You are going to build your shed with 4ft (actually 3' 9") long joists. At 100lbs live and 20lbs dead load (the max in the calculator), the AWC calculator indicates you could build this with #2 SPF 2x4s in wet service conditions and still be fine at 4'1". Your actual span is 3'5" once you account for 3.5" of support on each end. That's a really short joist span. I'd feel very comfortable with 2x6s for the joists for wet maple or another hardwood firewood. If you are really concerned, put another pressure treated support at mid-span or use 2x8s.

On a practical side, I've seen many semi-rotten old woodsheds built on 2x4s and 2x6s and they are just fine. You are probably overthinking this. Chances are you could throw it together with a pile of scrap 2x4 and it would be absolutely fine.
posted by ssg at 1:38 PM on April 10, 2016 [10 favorites]

Yeah, that's an over-engineered shed, which isn't a bad thing. You'd be totally fine modifying those plans to your dimensions with the floor joists running short-ways as opposed to having 16 foot joists running the length of the structure.
posted by cmoj at 1:38 PM on April 10, 2016

If you are really concerned, put another pressure treated support at mid-span or use 2x8s.

That was my thought as well. I agree that 2x6 joists running the short way will be more than strong enough. But if you were still worried or thought that you might later stack wood higher than you are thinking, just add a third support in the center and you will have a huge safety margin.
posted by Dip Flash at 2:59 PM on April 10, 2016

Best answer: Throw another 4x6 or whatever those foundation rails are made of underneath the middle of the shed, and your problem (if there ever was a problem) is solved. I'm not sure how you got your calculation for weight, though. To get up to 250 psf, you'd be looking at filling your shed with a 5' 4" high solid cube of seasoned white oak. I think you're probably underestimating how much of the shed will instead be full of air, bark, dirt, etc. And even then I think you might still be OK, because your spans are small.

For reference, the heaviest framing I regularly deal with is deck framing. Decks are supposed to be rated to 70psf these days, on the assumption that you might one day pack that thing wall to wall with dancing drunks, and then God help you if your second-story deck collapses and everybody comes crashing down. We typically use 2x10 joist framing for decks, spanning up to about ten feet between beams.

Understand that that's for a structure where failure may (and has in the past, hence the new stricter codes) mean dozens of people dying messy and traumatic deaths, and the structure has to remain viable for several decades without major maintenance. You're talking about a four-foot span (don't run your joists lengthwise, that would be silly) with possibly a center beam making it actually a bit less than a two-foot span, and the worst thing that happens if it fails is that some wood falls on the ground and you've wasted your time. And if in ten years' time your shed is starting to get a little wonky, that's not really a big deal.

I don't know if there's anything in the code that covers a comparable situation, but I would be totally comfortable doing that in 2x6 joists. I'd use pressure treated ones rather than the KD stuff that the video depicts though, because that KD lumber is going to rot away fast, sitting on the underside of a shed like that. Definitely go PT. Do your floor sheeting in pressure-treated plywood as well, that OSB they depict is going to turn to mush in a few years, sitting there in the dark between a pile of wood and the dirt.

Other things that you can do if you want to beef up your shed's floor: use pressure-treated decking as a floor instead of plywood, upgrade to construction screws (e.g.) instead of nails as your fasteners (or just use bigger and/or spiral nails), and put in a row of blocking to keep your joists from racking. You could get some joist hangers and/or hurricane ties up in there too if you wanted, the former to help keep your floor frame together and the latter to keep your floor securely attached to the foundation rails. All of those things increase your price, of course. The only thing I'd actually bother to do would be to use screws, and that's mainly because as well as making the floor stronger it would also make my life easier putting it together.
posted by Anticipation Of A New Lover's Arrival, The at 4:34 PM on April 10, 2016 [1 favorite]

Best answer: ctmf: "And then I can only find span tables that go up to 60 psf live load."

You have zero live load so don't worry about that column.

Are you doing something besides storing wood? 'Cause I wouldn't even have wood on the floor. Put a rim of 2x6 pressure treated on edge around outside of the shed; add a couple going side to side the short way to hold the long edges in place; then fill the cavity with washed gravel. It'll let your wood dry; won't retain moisture from snow/rain; won't wick moisture from the ground and will be cheap because you don't have to support the weight of 2 cords of wood. If you want to get really fancy pour a strip foundation out of concrete below your frost line.

Basically you just need enough foundation to keep your walls/roof squarish.
posted by Mitheral at 5:03 PM on April 10, 2016 [5 favorites]

Just wanted to come back and say that I am currently estimating a deck job with a 330 gallon hot tub on it right now, and the only modification we're making is adding in an extra support beam. We're still looking at 2x10 framing with joist spans of sixish feet. We'll have to check that design at some point to make sure that it's sufficient, but we're not expecting any surprises.
posted by Anticipation Of A New Lover's Arrival, The at 10:34 AM on April 15, 2016

Response by poster: Oh yeah, that's a super-solid deck with 2x6's 16" o.c. and 15/32 plywood. The gravel was a good idea, but I decided a couple of 4x8 sheets was a lot easier to carry than a ton of gravel. As it turns out, pounding 16d common nails by hand is a serious PITA. Slow going.

Thanks for the reassurance, you all were right.
posted by ctmf at 7:46 PM on April 18, 2016 [1 favorite]