1.5 story floor joist changes

Started by Skfarmgirl, February 26, 2024, 12:12:51 PM

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I'm planning to start building the 1.5 story 20x30 home this summer. Full loft. Im milling my own lumber but can only mill 16.5'. I also don't want to handle 20' lengths as I'm working alone and female so they are just so heavy and awkward. I plan to add another beam down the centre of the floor, both levels. So the span is now 10'. Can I double up (full size 2x6) Douglas fir cut 2x6x10? 16"oc. I don't want a bouncy floor but there is also another reason to do this. Between both levels I am saving 12" of height(6" vs 12" floor joists) I would like to drop the whole roof down by the 12" to decrease the overall pitch of my roof. I could still maintain my room heights this way. But not sure how it would change the integrity of the roof? Located in western BC gulf island. So not lots of snow but when we get some it's usually wet and heavy.plan on putting up a tin roof as well as covered deck around all four sides. Hope this makes sense?


Quote from: Skfarmgirl on February 26, 2024, 12:12:51 PMCan I double up (full size 2x6) Douglas fir cut 2x6x10? 16"oc.

It is not clear to me what "double up" means.  If it means two 2x6 doubled up (side-by-side) in the space where a single joist would normally be that would be stiff enough. Read below.  However, doubling in that manner would mess up insulation batt sizes if you were to insulate.

Non-standard joist sizes also means that all the usual and readily available hardware like joist hangers will not be suitable. That might not matter if the joiusts are all placed on top of the beams. However, when you get to the rafters' true size 2x lumber would mean that the very good to use H1 hurricane brackets won't work. There are often complications when doing something that doesn't fit neatly into the usual way of doing things.

A very handy tool is the AWC span calculator. It is available on the webpage.... https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/   It is also available as an app for Android and iOS.

There is a tutorial link on its use on the page I linked to. Bsically one selects the timber species from a drop down list, as well as sixe, grade, joist spacing, deflection limit, and a few other variables. Click calculate and it produces a table of results.

A possible issue is the use of your own sawn timber IF there will be building inspections. The lack of a grade stamp won't pass inspections under a strict interpretation of the code.

I ran the calc using grade #2. That is the common grade you will find in lumber yards and big-box stores.  Most likely your sawn lumber would be grade #2, but that can vary up and down. It takes a lot of training and experience to obtain a grader's license. The results I ran came up with a maximum span of 9'9" using a deflection of L/360, which usually is great for a stiff floor. L/360 allows for the installation of ceramic tile flooring. By using a true size measurement rather than the surfaced commercially available sizes you would gain some strength if it was a real grade #2.   But you would still be on the maximum length for that size.

Link to the tutorial (is well worth a read)
Just because something has been done and has not failed, doesn't mean it is good design.