Replacing rotted OSB that goes under a dormer....


Old 05-10-15, 08:15 AM
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Replacing rotted OSB that goes under a dormer....


We're getting ready to have our roof replaced and have a question about the proper way to replace a rotted piece of OSB that goes under a dormer.

If you look at the attached pics, you'll see an area in black. This is the area I temporarily "patched" until we can get the roof replaced. That piece of OSB is going under the dormer. The red X on one of the pics shows the area around either side of the rafter where the OSB rotted out.

Since that rotted area of OSB can only be cut flush with the rafter. should the roofer attach another 2x4 that runs along the existing rafter to support the new piece of OSB? See pic with the red line running along the rafter. I would think that would be a requirement. Otherwise there would be no support for the OSB.

20150510_093008_zpsz494ml8v.jpeg Photo by hikerguy1 | Photobucket

Also, I think there's a pretty good chance that the OSB that's sitting under the dormer is probably rotted out as well. Should we have any concerns about this? I know it would be impossible to replace without tearing up the dormer.

On a side note, is it customary for a roofer to charge for a whole sheet of OSB for each section they repair, even it that area is less than half a sheet? For example, if we end up having three sections that require less than 1/2 sheet for each area, would they typically bill for two sheets or three sheets?


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Old 05-10-15, 09:17 AM
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Patching on the inside of the roof ??? Get the roof fixed right away. Your patch is likely to make things worse. Instead of the water leaking through you've sealed it on the bottom side so the water has nowhere to go but stay in the wood and seep further, damaging more wood.

As for the repair I wouldn't worry too much about the details until the roof is off and you can truly see what damage you have. From the pictures the OSB doesn't look rotted so if you get the leak fixed ASAP the repair might not need to be too extensive.
Old 05-10-15, 09:22 AM
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Once he opens up the roof, he will know exactly what needs to be done. If the repair falls outside the area of the truss and therefore does not have a nailing surface one will need to be added. The small area that is rotten under the framing of the dormer, probably isn't big enough to cause structural issues. So, it most likely can slide. Key is proper flashing on the repair.

Each roofer will charge differently, I don't think that there is a standard as to how to charge. You can ask during the negotiations, doesn't hurt to see if you can get a break. Call him out, you got nothing to loose.
Old 05-10-15, 10:32 AM
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Yes, I know it's not the best solution, but the roof will be repaired by mid-July at the latest (depending on who we go with). The reason I did this is the hole is right over the porch (just outside the front door). If I don't do this, I have to go up in the attic every time it rains and soak up the water. Otherwise, the "plastic ceiling" (for lack of a better word) over the patio will collapse from the weight of the water.

My thought is that with it patched the way I did it, the water is traveling the way it's probably been travelling the past several months (if not longer) until the wood finally rotted through. Will two more months make that much difference? They wanted around $900 to repair it. Not sure if it's worth that expense. The water shouldn't "pool" in that area, should it? I was assuming any water would just run down the roof (granted, it will be flowing between the shingles and OSB, but it will eventually hit the gutter, won't it?).

The pictures don't show it, but the OSB is very rotted in two areas (one area that I patched, another area that hasn't rotted all the way through yet). I can push my hand against the area I patched and it just "flexes" very easily. The hole itself wasn't that big. It was just enough wood rotted out to allow water through. I just patched around that whole area in case any other part was going to rot through as well.

One thing one of the roofers who bid on the job found was that someone nailed right through a shingle near the dormer. He thinks that was done when installing the siding on the dormer (I guess some time of wood brace) and that's what caused our issue.

Last edited by hikerguy; 05-10-15 at 12:00 PM.

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