Two 2x4s instead of one 2x8?

[Stambro]

I am about to replace the floor joists. I have a bunch of two-by-fours C24 and I wonder if I could join two together to double the joist height?! Is that possible or would it be worse than having a two-by-eight?

 

[hempularen]

I have a feeling it will become a bit worse. If you can bring them together so that they become a homogeneous beam, it should be equivalent. But then we're talking about something like plyfa on both sides, and 30 - 50 nails per meter.

 

[Stambro]

Okay! Thanks for the answer. Then it's off to the brädgård.

 

[epw]

What is the span and what is it supposed to be for a room?

 

[Stambro]

About 4 meters, a load-bearing outer wall. Intended to become a bedroom.

 

[epw]

45x195 c24 supports 3.94 m at cc 60. So if it's longer up to about 4.32, then 45x220 c24 at cc60 applies. To reduce the dimension, you can decrease the cc between the studs.

 

[KnockOnWood]

But then we're talking about something like ply on both sides, and 30 - 50 nails per meter.

?
Plenty of wood glue in between, and then screws, e.g., 6x160, 5 per meter should be strong and good, right?

One could also wonder what the dimensions of the studs in the old joist were.

 

[justusandersson]

Theoretically, two 2x4 inch beams glued and screwed together vertically become stiffer than a single 2x8 inch beam. However, it's not worth the effort. Additionally, the dimension is not sufficient for that span, as epw points out. To meet the deflection requirement, even a 45x220 with glued and screwed particle board isn't sufficient, although it's on the borderline.