Joist size calculation for attic loft conversion

I'm familiar with the concept of sistering ceiling joists to make them appropriate to hold a live load as a floor, but I've found the various joist calculators and tables either wrong or confusing. I'm in British Columbia so I'm hoping to get an answer specific to our residential code.

I currently have 2x4's every 16" on centre for a span of 12ft. Here's the PDF for BC's structural code. If I'm reading it correctly, I believe I'd need 2x8" joists at "select structural" grade for northern species. I might be able to use spruce 2 grade considering that this is a sistering application and the existing 2x4s will be glued and nailed to the new joists - I'll confirm with a structural engineer at some point. Can anyone confirm that I've read the tables correctly?

I don’t know anything about metric, but I’ll try.

The “Tables” you refer to have footnotes that are not available. I’ll assume the footnotes qualify the loading, etc. Because the Tables indicate “Maximum Spans for Floor Joists” I’ll assume they’ve added a dead load factor and live load factor.

I don’t know why you’d want to use “Northern Species” or “Spruce”, when you live in the middle of Douglas Fir Larch country. However, I’ll try to answer your questions.

So, Douglas Fir Larch grade Select Structural 2x8’s at 16” on center spanning 12’ can support 40 lbs. per square foot (psf) live load plus 10 psf easily.

Likewise, your Table says Northern Species (whatever that is) 83x 400 can support a floor load spanning only 2.41 m. with a grade of Select Structural.

Likewise, your Table says Spruce can only span 2.85 m. with a grade of Select Structural.

So why not use Douglas Fir Larch. It’s probably what’s in your local lumber yard.

• I'll have to do some price comparisons. Spruce looked to be the cheapest option. If I can save headspace by going for fir then I'll definitely do that but I don't think the difference in strength can save me a whole 2". Oct 17, 2020 at 22:10