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I'm planning to build a 10x16' cabin. I need help sizing beams and spacing piers for the foundation. As of now I am planning to use two 4x6 16' beams with three ez tube precast footings per beam. This would leave 8' between piers. I am planning to use 2x6 10 foot floor joists. enter image description here It seems maybe slightly under built.

I'm trying to strike a balance between cost and simplicity because I haven't done this before. Adding two more footings and digging two more holes isn't ideal from a time or cost perspective. Do I upsize the beam? Or do I need four footings per side regardless.

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  • Don't use 2x6 floor joists. Your floor will bounce and 10 feet is too far of a span. Secondarily, you want a joist that is deep enough to install some insulation, at least R-10 (you mention cabin and not a shed). I would use 2x10s or 2x12s for the 10 foot span - 2x8 are the smallest that I would use. I prefer 16 inch on center and not 24 inch on center. That helps make the floor stiffer. You can cantilever a foot or two, but that cantilevers most of the weight (walls, roof) which will cause problems for a flat, no squeak floor if you try to use 2x6s or 2x8s. Apr 2, 2023 at 4:13
  • You don't actually need the 4x6s if you select the correct floor joist size. If you do want to use the beams, which is a good idea, then add a middle row to reduce the span to 5 feet. Then you can use 2x6 or 2x8 (preferred) floor joists 24 inch on center. Apr 2, 2023 at 4:17
  • Don't 2x6s provide plenty of room for insulation? Is your concern the span for the 2x6s or room for insulation? The span table's I've been looking at seem to specify 2x6s can span 10' if they are 16" OC Apr 2, 2023 at 4:55
  • A 2x6 Selected Structural will span 9' 11" (24 oc). No 2, which is typical in big box stores, is 9'1". Using a 2x6 is pushing the limits even at 16 oc (No 2 is 10'9"). Then you have the deflection issue which can cause all kinds of little problems: floor squeaks and bounce, drywall cracking, etc. The cost to go to 2x8 or 2x10 is not that much usually. At my last quote 2x10 and 2x12 were the same price. And yes, I prefer enough room to install R-19 (2x8) insulation so that the floor average is at least R-10. For 2x6 you will need to hand select each one carefully and that is expensive in time. Apr 2, 2023 at 5:22
  • Note: don't forget to add blocking between the joists. This is to prevent the joist from twisting due to load. I typically install blocking every 3-4 feet between each joist. Apr 2, 2023 at 5:27

2 Answers 2

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Rule of thumb without an engineered design is about an inch of depth of wood per foot of open span.

6(actual 5 1/2) inch wood should span 6 feet safely for a house.

Two 2x6s nailed together as a beam is usually slightly stronger(and cheaper) than a 4x6.

For a cabin/shed 8 foot spacing will probably be okay. Pressure treated wood better than plain wood on cement close to the ground.

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  • Thank you crip669. Maybe I should upsize the beam to be safe? 3 2x8s nailed together or something Apr 2, 2023 at 0:20
  • Two extra feet is usually okay buy the expected weight holding goes down some. Should only hold five sumo wrestlers instead of ten. It will depend on what you plan using the shed/cabin for, a couple of people sitting around or a ten person hot tub.
    – crip659
    Apr 2, 2023 at 11:12
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The piers and beams don't need to be at the edges and ends. The beams and joists can overhang the piers and beams (respectively.)

So, squeeze the piers together a bit more, and cantilever the ends of the beams and joists. Or just the beams, if you prefer. Having 6 feet between piers and 2 feet overhanging at each end works just fine to support 16 feet.

Having 4 feet between piers and 4 feet overhanging generally does not, without considerable design effort and overbuilding, so keep it reasonable if Vern A. Cular is your architect (as in, "vernacular architecture" - look it up if need be.)

"Proper sizing" would depend on a bunch of information that varies with location (such as snow, wind, and/or seismic loadings) as well as intended use for design floor loadings. Vern does pretty well with this size of shed/shack/cabin, though, generally.

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  • Thanks for the response and the vernacular architecture rabbit hole. I will slide the piers in two feet on each end. From what I've read 2x6 can span 10 feet so not sure if I ought to move the beams inward in relation to the joists or I'm good as is. What would you recommend for beam sizing for the two beams resting on the piers? Apr 2, 2023 at 1:12
  • @Foopysfarm - I would not cantilever 24 inches with 2x6 or 2x8 joists. You are limited to 1/4 the span of the joists. For 2x6 16oc the max cantilever is 13 inches. For 2x8 24 inches. Cut those numbers in half because you would be cantilevering two ends. That is the maximum and I would not go that far for a small cabin. If you use 2x10s then 18 inches on each end is fine. Just remember you have little weight inside the walls while putting the building weight (walls and roof) on the cantilevered ends. Your floors will bow up (will hump toward the middle of the joist) while also twisting. Apr 3, 2023 at 1:00
  • @Foopysfarm My advice is to not cantilever at all. Put the walls directly over the piers. Apr 3, 2023 at 1:10
  • I'm with ^foopysfarm^ on this one. Cantilevering and moving piers closer together wouldn't save on cost or work hours and would only increase the possibility of settling and warping as the lumber dries and shrinks. The more direct path of load transfer will result in a structurally stronger construction. Sep 25, 2023 at 8:28

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