What would I do to fix rotted window sills?
Question:I'm fixing it myself and need an idea on how to do it.
Answers:
This really is not a good DIY project.
First, remove the old sill. You may be able to do it with the lower sash raised (assuming a double hung window), better to remove at least the lower sash. Cut the sill in the middle with a reciprocating saw (Sawzall) or handsaw. Pry up in the middle to work both sides loose. You may need to do some trim work on the sides to remove nails or staples from the bottom.
Using the piece you removed as a pattern, shape the new sill. This is where it gets tricky. You will need to bevel the inside and outside edge, as the sill will be placed in the opening at an angle to make the water run to the outside. You may even need to work on the bottom, depending upon the thickness. The actual procedure will vary, but the sill has to be placed with the top at the same angle as the old.
Measure width and cut to length. I would prime and paint at this point, then place in the opening. You may need to use shims underneath to get the angle correct and bring it up against the bottom of the jambs. Nail in place with finish nails. At least 3 nails from the side jams at an angle into the new sill, a couple down through the sill.
Caulk the joints where the jamb meets the sill with a quality latex/silicone caulk. Touch up your paint and replace the sash if you took it out.
This is a bit over-simplified. Probably best left to a pro, preferably with some restoration experience.
Replace with new wood, preferably when you put in new windows.
Remove all of the rotted wood, and replace with new wood.
If you just have a small spot of rot, there is a product that you can use to fill in the bad area and repaint. I don't remember the name of the product but your local home center can tell you. First you dig out the rotten wood then apply a hardener and then fill in the hole with a putty/resin material that comes in the kit. After it cures, you can sand it smooth and paint it. I used it on a couple of spots years ago and it is still in good shape. It is only for spot repairs though, if a major part of the sill is rotten, you will have to replace it.
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