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This vinyl bow window was installed about 15-20 years ago:

window overview

Overall it looks and works OK, but the underside is wood and has severely rotted around the outer edges:

entire underside

Here's a detail of one corner. There is a cut hole in both outer corners, with what appears to be a rusty bolt head visible inside. The wood appears to be plywood, and around the outside edge it has rotted to the point that you can easily break it away with your fingers. My guess is that this is due to water wicking back along the underside of the window when it rains, and it should have been constructed or flashed differently to avoid that.

corner detail

How should this be repaired? In particular:

  • Is it possible to disassemble and replace the rotted piece from the outside in a bottom-up approach, or would you remove the inner window sill and work downward?
  • Is this even repairable or does this effectively guarantee there are deeper problems with the window?
  • How would I prevent this rot from occurring again in the future?
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    it is hard to see if there are any screws or nails holding this in place. I would examine the construction carefully because the bottom is usually structural in nature if it is a seat. There are probably deeper problems looking at the extent of the damage, With that said the trim may be ceder and it holds up much better to water damage. but you do need to stop the water from getting in there. It may be possible to remove any rot and scab in new supports all stuff a DIY person could do. – Ed Beal Mar 23 '16 at 19:02
  • This is a vinyl window. The only part that is visibly wood is the bottom (on both the exterior and interior). There may be wood structure inside the vinyl but the trim is all vinyl. Aside from the two big bolts in the corners, there are no screws/nails visible in the bottom board. The lower piece of vinyl trim actually wraps around the edge of the bottom board, so at some point it must have been laid in from above or the vinyl was wrapped around it later. – Shimon Rura Mar 23 '16 at 19:12
  • Also, the window is not a seat - it's about 3ft off the floor. Not sure if that affects how it would be structured, but I believe this bow window replaced a casement window that was previously in this opening. Regarding examining its construction -- how can I do that when it's all covered in vinyl? Remove interior trim? – Shimon Rura Mar 23 '16 at 19:17
  • I would be concerned that the rot could affect the support. It looks like a 3 piece unit to me if it is 1 piece the support is not as important but I would want to clean the rotted material before it proceeds any further. It looks to be a double pane so I wouldn't think that it would be sweating and dripping down the inside. Were there any plants in this window that could have leaked the water? just looking for a cause as the plywood looks to be recessed several inches and water doesn't run up hill. – Ed Beal Mar 23 '16 at 21:05
  • The plywood is not recessed at all... it just looks recessed because the rot around the edges is dark and creates a shadow effect. The white edge is just a piece of extruded-L-shaped molding that wraps the side/bottom of the rotting plywood board. There have been plants and some interior drips perhaps, but nowhere near the sort of consistent rate of them that would create this type of rot (and the interior window sill is in OK shape). – Shimon Rura Mar 24 '16 at 15:08
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Due to its age, I'd leave it & paint it a few times after a stain blocking primer. To correct it, just caulk & nail or finish screw a shoe molding onto the white outside edge to stop & eliminate the capillary action. A pre-drilled PVC molding would be perfect for this.

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    I like the molding suggestion. On painting, parts of this are so rotted that I don't think it would hold paint well. How can I know if there is structural damage or if the bottom board is just a cover? – Shimon Rura Mar 23 '16 at 19:29
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    As long as the windowsill inside isn't going anywhere there's no reason to screw around with anything. You can do a wood putty fill-in to ensure a good primer & paint bond or even just smear on paintable latex caulk with a putty knife. This isn't structural to the window unit & is just a sill backer. Typically these are boxed out below & insulated, that's why the plywood wasn't finished or covered in vinyl. Just a quick & cheap improper installation from a "Pro". – Iggy Mar 23 '16 at 19:35
  • I totally agree with what you are saying. I only wanted to say that silicone should work better than caulk for this and that a tube of silicone is more affordable than a skirt. I second your comment that the plywood should be covered in vinyl. – Ben Welborn Mar 24 '16 at 14:59
  • Thanks. Yeah, silicone is wonderful, but it doesn't take paint well. Unless it's one of the semi-silicone hybrids. I wish they would just slightly insulate them & sheath them. But, once the manufacturers put out installation instructions (whether or not provided or available), their cost cutting becomes fully justified & fully legal. Welcome to screw you land. – Iggy Mar 24 '16 at 15:03
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I would cut about 3/8" off (around) the edge of the plywood. Fill in the gaps with silicone, but be sure the silicone is not flush with the wood. If you do decide to replace it, then the easiest way to avoid the damage is to cut the replacement board a little smaller and again use silicone around the edges so that water can't run down and under the window and contact the wood.

enter image description here

  • Be wary of edge cutting. Many times these bottoms are edge nailed from the outside before getting the vinyl outer skin. – Iggy Mar 24 '16 at 15:06
  • True, I don't see any nails... if they are present then they are covered-under the skirt. Obviously, if you can take the skirt off (without damaging it), it would help immensly for whatever you do to fix the edge. But the skirt is probably integrated with the rest of the window (hence the question). If you cut the edge plywood, then it might fall out (unless there is glue). If it falls out, then fishing out the rotten wood should not be too difficult. – Ben Welborn Mar 24 '16 at 15:54
  • Yeah, that's why I personally wouldn't be inclined to remove any of it & would just pretty it up. Maybe go with a wider shoe molding beneath to hold the edge in or re-face the bottom entirely, but then you add a seam into the water's path that is able to wick very far inward or even collect. – Iggy Mar 24 '16 at 15:59

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