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Fungineering
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# Posted: 31 Mar 2025 11:20am - Edited by: Fungineering
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Hey gang,
Building a 12x14 cabin with an inverse lean-to roof which is 12' at the back and 7' at the front using an Ondura product for roofing material.
I'm building on Pylex 50" foundation helix screw since it goes below the frost line an is easier than dragging sonotube and concrete through the bush.
The front and back ceiling-bearing spans have four helix screws at 3.5' spacing and the centre will have three across with two standard concrete deck blocks on a bed of compacted 3/4" rough gravel to help fill the space.
I'm going with a drop in joist which will be supported at 6 foot intervals between front, middle and back spans so a single run of 12' lumber 16" OC making a total of (rounded up) 11 joists across with joist hangers.
This is a VERY rough layout of the support and spans.
All pressure treated although I am not sure what the wood itself is.
This is a rough layout of how I plan to build and place things inside the cabin. Live weight might be 3 - 4 people max. I plan to have a small loft for a twin bed but only over the only room which will have walls rather than span the entirety of the back.
My question is would this be doable with standard 2x6 lumber for the spans, joist and load? I know the smaller you go, the less span as well as load bearing they are but with the foundation support I have would this offset any deficiencies?
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Grizzlyman
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# Posted: 31 Mar 2025 12:11pm - Edited by: Grizzlyman
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Just my .02- others may disagree . I hate bouncy floors. If the span table says 2x6 is fine then I’m sure it is… But I’d do 2x8s. Not that much more $ and guaranteed to be solid. My sauna is on an 8’ span and 2x8s are very solid.
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gcrank1
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# Posted: 31 Mar 2025 03:34pm
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Yeah that
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Fungineering
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# Posted: 31 Mar 2025 03:49pm - Edited by: Fungineering
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Quoting: Grizzlyman Just my .02- others may disagree . I hate bouncy floors. If the span table says 2x6 is fine then I’m sure it is… But I’d do 2x8s. Not that much more $ and guaranteed to be solid. My sauna is on an 8’ span and 2x8s are very solid.
Would you consider this for the three span joists only or for all joists in general? Costs wouldn't be much more of an issue, maybe like 7$ extra per piece from my solitary lumber store but I'd have to see what they have available. I'm pretty deep in the backwater so I work with what's available. 
I did find out that the species is spruce though.
Should I consider sistering the span joists under this type of load or is that really overdoing it? I don't plan to have a dance party or a bear sleep in the mini-loft.
Also, super amateur hour question, but when I read the footage on a span chart that means the distance it can run without any support under it, right?
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MtnDon
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# Posted: 31 Mar 2025 06:26pm - Edited by: MtnDon
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Quoting: Fungineering ...foundation helix screw since it goes below the frost line...
Quoting: Fungineering the centre will have three across with two standard concrete deck blocks on a bed of compacted 3/4" rough gravel
Do you think that mixing helix screws, designed to resist frost movement, with deck blocks that can be easily moved by freezing, is a prime idea?
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It is not clear to me what you intend to place on top of the helix screws to support the 12 foot long floor joists. We would call these beams. As drawn, they look like a pair of 2xsomething fastened to something on top of the helix. (A 2x6 nailed/screwed sandwiched over a 4x4? That would not be the best method, but the drawing is not clear. Could you clarify that?
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You mention " drop in joist" and using " joist hangers." so the floor joist span is not 12 feet but more like approx 6 feet. Slightly under 6 feet between each joist hanger pair.
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You mention that the floor joists will be spruce. Is that a graded S-P-F spruce? #2 is the normal stocked grade; just wondering if it is graded or from a local sawmill source.
The AWC Calculator can be a handy tool for sizing joists. Depending on the species a 2x6 can span up to about 8-1/2 feet, so 2x6 could be fine with an effective 6 foot span.
Quoting: Fungineering when I read the footage on a span chart that means the distance it can run without any support under it,
The span is the horizontal distance between supports, measured from the inside edge of one beam to the inside edge of the next one. The actual bearing length of the joist should also be noted. The AWC calculator provides that information as well as the maximum span.
I mention horizontal distance because when you calculate for rafters we use the horizontal span, not the rafter length.
With your 12 foot span built as a shed roof (that's what we call a roof such as you describe) the rafter span is 12 feet (less the thickness of the walls. If you built a gable roof with a central ridge beam, the rafter span is the wall-to-ridge, horizontal distance, approx 6 feet.
Speaking of roofs, what is your snow load? That can have quite an impact on sizing the rafters.
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If you look at the AWC calc, for floors we usually use L/360 for the deflection limit and 40 PSF for Live Load (LL) in a normal habitable room. Rafters are different. There is a tutorial page link on the AWC page that helps explain all the info the calc needs and outputs. A minimum snow load of 20 PSF is used for rafter specs. (That also helps cover rain and wind loads).
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Grizzlyman
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# Posted: 31 Mar 2025 06:35pm
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Fungineering
I believe it is unsupported span distance. And when you say “span joists” you’re referring to the beams that will be directly supported by the foundation supports- correct?You’ll want to double them at least- and possibly use 2-2x10s. They will carry much more load than individual joists. You should be able to find a beam calculator online as well.
I also just think 2x6 are probably not good for floor joists unless a really short span- that’s not scientific- just my opinion. I used 2x6 joists when I built my shed and they’re fine for a 7’ span in a shed… but you probably want something more solid- especially when it’s just a few more bucks.
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DRP
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# Posted: 31 Mar 2025 07:58pm - Edited by: DRP
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We usually use face to face clear span when talking about span. Technically span includes half the required bearing length at each end as well (Usually that would add a couple of inches to the clear span.)
"Bounce" is a tricky thing to pin down. In either way I think of it, deflection or vibration it is related to elasticity. Use stiffer materials and methods. Deeper is stiffer, dougfir is stiffer than spruce. Those are materials options. Stiffening the girders, that is the correct term for a beam supporting other beams (joists are beams in that sense) So joists supported by girders supported by flimsy posts. By which I mean, the screws are going to give the most vibration. Girders contribute to the deflection and vibration of the joists. Compared to a conventional perimeter foundation wall where there is no deflection or vibration from a rim girder or non rigid foundation elements. In that case the joist is the only thing ringing, in this assembly there is a system of several elements ringing.
Or another way to think about it is, careful chasing bounce free with the methods described. You can help within reason but some of it is baked into the methods we chose. Don't forget the roof load in sizing the girders... and the deck block comment hit me the same as MD, probably not a good mix.
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Brettny
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# Posted: 1 Apr 2025 05:36am
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It's only 12x14. I would use blocks and only two girders/runners. It's easy to relevel if you ever need to. I have a 10' wide shed with 2x4 floor joists. It's terribly bouncy. At 12' wide I would use 2x8 floor joists.
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DRP
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# Posted: 1 Apr 2025 08:19am - Edited by: DRP
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The code girder and header tables are at R602.7 here;
https://codes.iccsafe.org/content/VARC2021P1/chapter-6-wall-construction
Joist span tables, these will be the same as Mountain Don's awc spancalc link above. His link is to a calculator by the people who write the codebook tables. Table R502.3.1(2) would be correct for code minimum deflections;
https://codes.iccsafe.org/content/VARC2021P1/chapter-5-floors
Note that a 2x8 in SPF on 16" centers is about maxed at 12' span. I've built that plenty of times without complaint but that is close to minimum.
Scroll down a paragraph or 2 below the joist span tables for the cantilever tables we were talking about last week.
In one class about annoying floor vibrations (it can be a big callback problem), the professor explained the combined effect of girder and joist combination systems vibrating at lower, more annoying frequencies than the codebook tables address. I'd have to find my notes but his recommendation was to build to significantly lower deflections than codebook minimums. I've had designers often working closer to L/720 deflections than the code minimum of L/360.
To explain those fractions a bit, 1/360th of a 360" span would be 1" of deflection, or sag, at design load... uhh I was doing stairs yesterday, my calculator is on the bench at work, in a 12' span that would be in the neighborhood of 3/8" deflection at design load. So an "improved" deflection (the usual specifiers term) the deflection is half of the minimum. The base vibration frequency goes up proportionally which gets it above our irritating frequency range and annoyance goes down. Where I've had this stuff come into play is (tile for one!) but usually on longer spans with lighter loads. The rattling china when someone walks across the floor. Deep in the weeds for a camping cabin but there's a bit of background.
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gcrank1
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# Posted: 1 Apr 2025 08:40am
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Ive Never heard anyone complain about having too firm a floor 
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DRP
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# Posted: 1 Apr 2025 07:48pm
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I rotate between plantar fasciitis and neuromas from bad shoes and concrete floors, a little give is probably a good thing.
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rpe
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# Posted: 1 Apr 2025 07:52pm
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I would reconsider the Pylex screws, and go with something more substantial. The difference between the Pylex product and a properly engineered screw pile is huge.
https://postechpiles.com/screw-piles/
See the critical reviews here:
https://www.homedepot.ca/product/pylex-adjustable-50-inch-foundation-screw/1000746285
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gcrank1
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# Posted: 1 Apr 2025 08:10pm
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Even though this is a 12x14?
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