Building codes specify the minimum joist depth and spacing typically 12 or 16 inches on center for spans up to 20 feet but those requirements are intended to prevent plaster ceilings from cracking not to.
Add joist to existing floor.
Find a replacement joist that is the same size as your floor joist.
Use a tape measure to measure the length width and height of your existing floor joist.
Purchase the type of lumber that matches the width of the existing joist.
For example if 2 inch by 10 inch material is in place the sister joists must be at least 10 inches wide.
Make sure you also buy the same type of joist.
Even if your joists already have a row of bridging at the center of the span adding a row on each side of the existing bridging will stiffen the floor.
Select or cut sisters that also match as closely as possible the span of the existing.
Hoping there s no more than a couple.
Of the existing joist.
Measure the width and length of the existing joist.
As a footstep falls on one joist some of the force is transferred to neighboring joists.
Thanks to all for your helpful replies and suggestions.
If your floor joist is an i joist for example buy an i joist.
Depending upon the conditions it is possible to strengthen or repair existing framing members such as floor joists or roof rafters by adding reinforcing material.
Sandwiching the member on either side with plywood is sometimes worthwhile but the plywood must be installed correctly for greatest strength.
If the attic joists are not adequate one way to strengthen the floor for live loads is to sister the old joists.
Bridging or x bracing allows joists to share weight.
Sistering is the process of adding a new joist next to each existing joist.
I do have at least one existing joist that is pretty well rotted.
Looks like i will remove the old decking before adding additional joists.
The best way to limit annoying flex in a floor is to make sure that joists are sized correctly before a house or addition is built.
Use a sledgehammer or a pry bar to force the bottom of the new joist along the mudsill and center support beam of the floor system or the opposite mudsill on short spans until it s flat against the existing joist.
When the sagging joists are level apply a generous bead of construction adhesive to the existing joist.
But it s usually best to run the sister joist over the entire span.
Then attach the sister joist using three 16d common nails driven every 16 in.
Nail the new joist to the existing joist with two rows of 10d to 16d nails spaced 6 in.
Most new construction uses 2 by 8 lumber for joists although 2 by 6.
In the case of 2x6 joists you can pair them up with additional 2x6 joists by nailing them together side by side.
