How to Repair a Rotted Window Sill

Object: This is an article about how to repair a rotted wooden windowsill or brick molding yourself without having to hire a professional repairman, while saving lots of money by doing it yourself

First of all, this assumes that you know that you have a rotted windowsill and/or other components of the window construction.  Normally just a visual examination is all you need to make this determination.  However, if you are not sure, probe the wood with a small penknife or an ice pick.  If the wood resists puncture, it probably is ok.  If the knife blade or ice pick easily penetrates the wood, you have problems.  A telltale sign otherwise is the bubbling of the paint and shrinking of the wooden surface.

More modern windows have sills that are made in two parts which are “finger jointed” together.  If you are lucky (and for the purposes of this article, we will assume you are lucky), then the rot has only entered the outer piece of the windowsill.  This will be the piece that juts out into the weather and is not underneath the window upright pieces. 

Begin by digging out the rot with some type of tool such as a knife of wood chisel.  If the rot goes far enough inward, the seam between the two pieces of wood will become evident.  When you reach this point, begin to try to separate the two by inserting a chisel or putty knife into the seam and trying to wedge them apart.  Be careful not to damage the inner piece of wood while doing this.  Finally, you may be able to insert hammer claws or a crowbar or a wrecking bar into the crack and force off the outer piece of wood.  It might have been glued.  The one I just completed was not glued, but was nailed.  I say it was not glued.  If it had been glued, the glue had deterioriated along with the wood.

Be careful of the brick molding while pulling this piece out.  Most likely it will have a nail into it from the bottom of the rotted piece and there is the possibility that you may splinter the brick mold when the nail pulls out.  I had this to happen.  The brick mold was in good shape otherwise, so I put Elmer’s Wood Glue into the crack where it splintered and then clamped it while it dried.

I was not so lucky on the other side of the window.  There the brick molding was rotten for about 4 inches up from the sill.  I had a choice of trying to cut out the rot and replace it with a new piece of wood, but that seemed counter productive, so I removed the entire piece and replaced it in its entirety.

To replace it, you must first take a sharp instrument such as a putty knife or construction knife or chisel or pocket knife and cut through all the caulking which seals the brick molding to the rest of the window and the siding on the house which might be brick, but in my case was “hardy board”.    Once the caulking is cut through, then a pry bar and a hammer will remove the molding.  Again, be careful where it connects to the top or cross piece of molding, because there is probably a nail or nails holding these two together, also.   Removing this without damaging the top piece can be tricky.  You might have to try to cut the nail with a metal cutting jigsaw blade or chisel.  You might be able to just “wiggle” it out, the way I did.

At Home Depot, I was given a choice of replacing both of these pieces with either vinyl or real wood.  I chose to replace the brick molding with vinyl and the sill with wood.  I had carried a short piece of each of them with me to the hardware store to be sure that the new matched the old.  The brick molding was not a problem matching with either vinyl or wood, so I chose the vinyl, meaning that I will never have to do it again.  However, with the sill, I could not find an exact match at either Lowe’s or Home Depot.  The grooves that make a “finger joint” were a match, but the thickness of the wood was not the same in either case, so I chose wood, which was cheaper. 

When putting the two together, I did a “dry fit” first to be sure that the wood would go together and that I had made the correct measurements and cuts.  Upon doing the dry fit, I determined that a small portion of each end had to be shaved off because the plywood siding which was nailed to the stud wall stuck out a bit further than the actual window casing, so the sill had to be trimmed a bit to get a snug fit.  This is hard to explain without being able to view it, so it is important to “dry fit” to make that determination yourself.

Once the fit was perfected, I soaked the wood with a wood preservative and/or wood sealer such as that used to coat decks with.  Then a bead of construction glue (Liquid Nails) was placed into the finger joint seam and then pressed it into place, using a hammer to tap it into place.  I then nailed it with 10 penny finishing nails.  Use galvanized nails to prevent rust later.  Of course, the wood sealer needs to be dry before applying the adhesive.  I let it dry in the sun for a couple of hours before applying the glue.  For some of the sealers, you will need to wait 45 days or more before applying latex paint.

After the sill is in place, the brick molding can be replaced.  Make the top cut a 45-degree angle, assuming that the window is a normal rectangle.  Measure the space where it is to be placed and then cut the exact length and nail it into place with some 3” finishing nails on the thick side and some 2 or 2 ½” nails on the thinner side.  I placed the nails about 18” apart.

The last thing I had to do was to put a protective piece of molding underneath the sill.  This is necessary because there will be a gap left under the sill between the sill and the framework of the house if it is covered with siding.  I used a piece of molding that is designed for placing on top of windows and doors in a frame building.  I think it is called “drip cap”.  I your sill sits on brick, this will not be necessary, as the sill will most likely sit directly on the bricks.  If it does not, then you have purchased the wrong piece of sill.

Once nailed, use latex exterior, paintable, caulk to seal the cracks on all the pieces so that there is no possibility of rainwater getting in behind the surface. 

As stated earlier, if you used a sealer on the unpainted wood, you will need to wait to paint it.  The can should give you the proper waiting period before painting.  The wait is not as long if painting with oil paint, but can be a couple of months with Latex paint.

With luck, you have just saved your self enough money to go out and have a nice weekend.