Doubling Up Every Other Floor Joise

Place the lumber block between the two joists.

Doubling up every other floor joise.

Set a hydraulic jack and post under the beam and jack up the joists about 1 8 in. To achieve this doubling effect in strength and stiffness you will indeed need to gusset the upper and lower joists together. Use joist hangers for all connections. Nail the block in place with 16d.

If we use strong enough gussets and fasteners we know that the top and bottom joists will deflect by exactly the same amount when loaded. If you double those joists you can span 12 feet 3 inches. Tack a beam under the sagging joists. Create space by doubling joists if you need to remove part of a joist double the joists to each side then add doubled joists between them to carry the load of the severed joist.

The main reason for doubling up floor joists is to double the strength of a single floor joist. A day until they re level. Not even a 12 micro will span that far. Nailing two 2x4s together will work to span about three joists unless the sag is under a weight bearing wall.

I would think the doubled up s would span further because you can attach one to the other more thoroughly. For example a joist made from doubled 2 x 6s can span a distance about 25 percent more than a single 2 x 6 but a 2 x 12 can span about 80 percent more than a 2 x 6 even though it has the same amount of wood as a doubled 2 x 6. So instead of having a post run all the way from the roof to the ground it can stop at the double floor joist. So i made some double joists for where my deckboards will butt up to each other but about a week after puting them in i noticed that some are already trying to seperate by cupping away from each other.

Jacking them up too fast may cause cracks in the walls and floors overhead. I thought i had used way more than enough screws to prevent this i even put them together cup side in if they were slightly cupped and now they. I don t think doubling up a 16 2x8 will all of a sudden span 30.