After addressing a previous leak in my garden shed, which was previously used as a home gym, I discovered some water damage when I removed the compromised OSB board from the shed’s ceiling.

I’m hesitant to replace the water-damaged joists shown in the attached photos, as this would require undoing the work done to replace the outer damaged OSB boards and re-felt the roof. I’ve closely monitored the roof for nearly a year, and there haven’t been any new leaks.

What kind of repairs can I carry out on the damaged joists before I attach a fresh new sheet of OSB back on the ceiling? I’m aware that quick fixes aren’t always successful but I want to at least try.

In the photos I’ve attached, please pay attention to the joist that is slightly detached. The last image is an example of what kind of garden shed it is. It was custom built about 17 years ago and the roof is one of those slanted type where the highest point is at the back and the lowest point and gutter is at the front of the shed.

Also, the photos are from one small corner (at the front part of the inside of the shed) where the damage occured.

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2 Answers 2


You may find rot in the wall section too, but if the top plate is in good condition, you can sister the existing roof rafters without removing them or any roof. You will need to remove the blocking that is between the rafters so the new rafters can be set on the top plate, tight beside the rotted ones. Once in place, the new ones can be fastened to the originals so everything is tied together.

You will need to remove another section of ceiling finish to get enough of a tie into the original rafter. If the rot is 2 ft or so in, run the new material twice that and securely fasten with 2 1/2" or 3" screws, 2 every 12" apart or so. screws will keep everything from shaking so badly while setting them. Keep the screws about 2" from the ends. If you can get the new rafter to meet the top, all the better, but not critical. If you do, you can use the same number of screws, just over the whole area.


Those rafters (and joists? it's hard to tell) look quite bad. Take a screwdriver and poke them all over. If the screwdriver goes in more than say, 1/2", you have to replace the wood. For joists, you can always sister them (nail another joist to the existing one), but if the rim joist is also rotten (as it looks like), then your chicken is cooked. Yes, you can add supports and replace the pieces of wood one by one, etc. But by the time you're done, you will wish you had demolished the shed and installed a new one.

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