Thanks to all for your helpful replies and suggestions.
Add joist to existing floor.
I do have at least one existing joist that is pretty well rotted.
Find a new floor joist with these exact same dimensions.
Sistering is the process of adding a new joist next to each existing joist.
But it s usually best to run the sister joist over the entire span.
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.
Bridging or x bracing allows joists to share weight.
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.
Most new construction uses 2 by 8 lumber for joists although 2 by 6.
Find a replacement joist that is the same size as your floor joist.
Measure the width and length 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.
Purchase the type of lumber that matches the width of the existing joist.
In the case of 2x6 joists you can pair them up with additional 2x6 joists by nailing them together side by side.
Hoping there s no more than a couple.
If your floor joist is an i joist for example buy an i 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.
Looks like i will remove the old decking before adding additional joists.
Select or cut sisters that also match as closely as possible the span of the existing.
Cutting the new joists 1 16 longer makes lots of sense as i will be doing lots of the work by myself.
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.
As a footstep falls on one joist some of the force is transferred to neighboring joists.
Of the existing joist.
Sandwiching the member on either side with plywood is sometimes worthwhile but the plywood must be installed correctly for greatest strength.
When the sagging joists are level apply a generous bead of construction adhesive to the existing joist.
Nail the new joist to the existing joist with two rows of 10d to 16d nails spaced 6 in.
If the attic joists are not adequate one way to strengthen the floor for live loads is to sister the old joists.
Then attach the sister joist using three 16d common nails driven every 16 in.
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.
