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This week's rainstorms brought a leak in my roof above a set of windows that are at the low end of a single-story addition with a shallow roof.

Our search for the source of the leak (which turned out to be a small hole in the EPDM membrane roof) revealed rot in the framing of the house, probably mostly from a previous leak from before we acquired the house. That leak was shoddily fixed — the owner sistered the rotting roof beams (see photo) but did not deal with the top plate, which is in pretty bad shape.

What can we do about the top plate?

Rot closeup

Rot context. The wall below is all windows.

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  • This is why you nobody should build a modern structure out of wood unless they really know what they're doing.
    – iLikeDirt
    Oct 19, 2014 at 18:30

3 Answers 3

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It looks to me like the proper fix for this is going to end up being a lot more work that what you probably want to hear about. If I was the contractor recommending the repair scenario I would be saying to remove whole upper structure and replace it anew.

Such a flat roof is always a recipe for a mess like this. It it was at all possible to rebuild the roof to add some rafters above to give some more slope it would be the right thing to do.

Short of that, if you end up deciding that the rest of the roof structure is basically sound except in the area where these leaks have been then repair probably entails supporting the roof on temporary supports and then removing the window and the wall covering back to the corners. It may then be possible to fit in complete new plates and rafter / joists to completely replace the wet and rotted materials. Still a pretty sizable job. :^(

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In addition to what Michael said, I see an another problem: this roof structure is a moisture trap if you live somewhere that's not a desert and where it ever gets cold. Moisture-laden interior air that's trying to get out will migrate up through the ceiling drywall and the air-permeable fiberglass insulation until it hits a cold surface. That cold surface is likely to be the boards of the roof deck which are exposed to the cold exterior temperatures and insulated from the warmth generated within. So water is going to condense on the underside of the roof deck, run downhill, and pool... exactly where the top plates are rotten. It's likely that even if you replace them and fix the roof leak, they will eventually rot again due to this additional problem (again, assuming you live in a place where it gets cold and is not a desert).

There are two ways to fix this problem, while you're already having to make repairs to this part of your house:

  1. Remove the fiberglass batts and apply an air-impermeable insulation like spray foam to the underside of the roof deck to prevent interior air from being able to touch the cold roof deck
  2. Attach rigid insulation boards over the top of the roof deck to keep it warmer so that moist air that hits it doesn't condense on it. You'll need enough to keep the roof deck above the dew point in winter. Needless to say, this will entail replacing the roof as well, as the roofing material has to go above the insulation boards.
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  • Would a vapor barrier on the underside of the ceiling joists help here? There are no eaves on the addition, so no ventilation or cold space above the fiberglass batts. Would a vapor barrier on the warm side help or hurt any condensation problems?
    – Wesley
    Oct 20, 2014 at 15:53
  • Adding a dedicated vapor barrier is almost always risky. If the room is air conditioned, the vapor barrier itself will get cold and serve as the condensation plane for any water that gets in! I prefer to use an air barrier right up against whatever it is you don't want moist air it to hit.
    – iLikeDirt
    Oct 20, 2014 at 15:58
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Tear it all off and rebuild a roof that's not made out of 2x4's.

To fix-at-it, tear off all the ceiling drywall. Temporarily support the beams and remove the sisters (the tacked-on LVL's). Insert new 2x4 sisters that go the entire span, tucked above the header that would of been behind you while taking that picture, short enough to slip up into the cavity, just short of sitting on the existing header above the window.

Keep them as big as possible while still being able to psychically insert them. A slight angle cut on the bottom of the 2x4 where it will sit on the header behind you helps squeeze it in there. A slice off the side might help get the other end up into the cavity as you will be trying to angle them in there.

Then attach a new header to the face of the old window's header, ideally the entire length of the room, across the top of the windows; a cripple for your new joists. (yes it will show and stick out into the room a little, just pretend its a decorative trim, perhaps bevel it or route the edge)

A framing nail gun may split the existing joists, you might want to use deck screws for attaching the new sisters to the existing joists. Stagger the spacing on the fasteners; one high, one low, ect., every 8-12 inches.

Fix the roofing before you do anything else.

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  • The somewhat rotted header concerns me less than the poor job done to strengthen the obviously rotten joists.
    – Mazura
    Oct 19, 2014 at 18:03
  • Thanks to you and the previous commenters for taking a look and providing some advice. In an ideal world we would indeed replace the roof, but I think that is beyond our abilities right now. At the moment the plan is to support the beams with a temporary wall (as you and others describe above), take out the windows, and replace all the rotten beams.
    – Kate Golden
    Oct 20, 2014 at 15:43

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