OSB, or oriented strand board, is a common building material that is similar to particle board but much denser. OSB is made by mixing together wood chips and adhesives, then molding the mix into sheets or into a block shape. OSB is used for construction and framing. You might have OSB sheathing in your roof, walls, or subfloor. OSB is denser than plywood but not as strong. Over time, the OSB used in your building’s construction will warp, crack, or rot. When this occurs, you must replace the damaged sections of sheathing as soon as possible.
Step 1 – Isolate Damage
Determine the extent and scope of the structural damage to your building. If the OSB is rotting, you must identify the source of the moisture and remove it before proceeding. Otherwise, the problem will just happen all over again in a short time. Measure out the area of roof shingles, siding, or floor covering that you must cut away to finish the repairs. Obtain replacement materials before beginning. If you are repairing the roof sheathing, it is recommended to use roof jacks and safety planks for structural support during the repair.
Step 2 – Prepare OSB
Oriented strand board is treated with a waterproof sealant as it is manufactured. However, cutting through the board breaks the seal. Cut all your new OSB sheets to size before beginning the repairs. Then, coat the cut ends with waterproof sealant. Give them about 4 hours to dry before installing.
Step 3 – Remove Damaged OSB
Using the crowbar and utility knife, remove the shingles, siding, or carpet that is covering the damaged OSB. Expose the full section of cracked sheathing. Pull the nails and screws holding the panels to the stud or joist. You may have to mark the locations of the joists on the panels, and cut the panels free with a circular saw. Once all of the damaged OSB is removed, inspect the framing elements for signs of damage as well.
Take the replacement sheet of OSB and place it in the gap left by the damaged original. It should fit snugly, with the sides flush against its neighbor. Lay a thick bead of adhesive on the studs or joists before making the final attachment. Screw in the new sheet of OSB with 1.5-inch woodworking screws.
Step 5 – Cover New OSB
If you are working on the roof, cover the new OSB panel with tar paper and tuck the ends down into the seam. Lay new rows of shingles over the tar paper until the replacement sheet is covered. The top rows should well cover the fixed area, but it may not be possible with older roofing where you'll have to work it into higher rows of shingle, or possibly have to redo the whole section. For walls, you will have to tape, texture, and paint a new piece of drywall. For floors, you will have to replace a carpet, vinyl, or tile. If these repairs are beyond the scope of your skill, contact a professional contractor for an estimate before fixing the OSB.