How to Reinforce an Under-Built Floor

Quick-Fix Sistering

• 1

  Cut two pieces of plywood with your circular saw for each joist to be strengthened. Make each piece of plywood 1/4 inch more narrow than the joist to which it will be attached.

• 2

  Apply construction adhesive to both sides of each joist with your caulk gun. Apply adhesive to every exposed surface on both sides of the joist while avoiding the face of the bottom board that is only visible from below.

• 3

  Press a piece of plywood into the adhesive on both sides of each joist. Cover the entire length of both sides of each joist.

• 4

  Nail the plywood onto the joists with your framing hammer. Placing a row of three 6d nails every 8 inches along the entire length of each joist.

Heavy-Duty Sistering

• 5

  Choose lumber that matches the exact dimensions of the lumber in your existing joists. Cut a board to match every piece of wood in each joist to be strengthened.

• 6

  Apply construction adhesive to both sides of every joist with your caulk gun. Press a matching piece of lumber into the adhesive at every point. Cover every board in each joist with a new piece of lumber on each size.

• 7

  Nail each new piece of lumber to the existing joist on both sides. Place a row of three 16d nails every 8 inches along the entire length of each piece of lumber covering the joists.

Adding Extra Bracing

• 8

  Choose lumber for your braces. Pick only high-quality lumber that is straight, cut free and matches the dimensions of the lumber used in your existing joists.

• 9

  Measure the distance between the existing trusses. Cut the new lumber to this length with your circular saw.

• 10

  Nail the bracing lumber perpendicular to the joists. Drive three 16d nails through each joist to hold the ends of the braces in place.

• 11

  Place a series of braces every 24 inches along the length of the joists. Stagger the braces from one set of joists to the next so that you can drive your nails straight through the existing joist and into the end of the brace.