How To Repair Wooden Windows

repair wooden windows
No Plastic

I decided to renovate and install insulated glazing sealed units to my wooden casement windows rather than fit plastic. There was a little rot here and there which was not a problem except that one of the tops hung sashes was beyond repair and would have to be replaced. Having a carpenter fit a new one though was out of the question; it would have cost more than a plastic replacement window. The only answer was to make a D.I.Y wooden casement myself.

D.I.Y Top Hung Wooden Casement

I needed new molding from which to fabricate a sash, but the price of new window molding from a sawmill was prohibitive at a third of the price of a completely new window for just ten feet of it. If I made a mistake in the construction it would be expensive.

I decided to try and make my own molding using three pieces of batten from the D.I Y store screwed and glued together. Problem was that windows are jointed with mortise and tenon joints, which are way beyond my expertise.

The only way I could think to construct the casement was by way of miters, but mitered joints are not load-bearing; they’re just decorative. To overcome this limitation and provide the necessary rigidity I used anodized corner plates at each corner of the frame, having first glued them to hold the joint firm enough while I chiseled out for the plates. You need to chisel out to a depth of maybe 1/4″. I set the plate in a bed of car body filler and screwed it down then filled in over it. It was only necessary to fit the corner plates to the outside of the frame, though it would do no harm to fit one to the inside too providing the two sets of screws do not meet in the middle.

Another option to hold the miter firm enough to chisel out for the corner plate would be to ask a local picture framer to glue and underpin your miter. I achieved further rigidity from the shoulder piece that was screwed to the inside of the frame. This element was added to the frame after it was assembled. It was butt jointed and screwed across the mitered joint.

Make Sure The New Sash Fits

The new sash obviously has to be the same size as the old one so far as the external dimension goes.

Because your D.I.Y molding was matched to the rebates in the frame before it was glued and screwed, it will fit by default providing you cut your frame to the right length.

When taking your measurements, measure the outside edge; that is the edge that will form the periphery of the frame This will be the long edge, and then cut your miter inwards towards the inner edge where the glass will sit.

To test the accuracy of your measurements before committing to cutting the entire frame; cut and miter one long length and one short length and try them for size. If it’s too short you can cut another piece. It may be beneficial to temporarily remove the old sash to give you room to maneuver. The molding should fit easily with a little play at either end, maybe a 1/8″ to 3/16″ overall. This play will diminish slightly when all the mitered sections are assembled. When assembling your new frame, make sure it is square; the main frame will be.


Remove the old sash carefully. You will have to replace it with trial fittings while construction continues on your new one. Do not order the glass until the exact size of the new sash is determined.

The third element of my molding was the shoulder that fitted on the inside of the new sash and against which the double-glazed seal unit would seat. The positioning of the shoulder is dependent upon the fit of the new sash in the closed position. The old sash had to be removed for the trial fitting of the new one. That done the new sash should fit easily into the frame with a little play all around. At this point, the position of the shoulder can be marked.

Have in mind that for me this was part of a larger project to renovate and modify my single-glazed wooden windows to take double-glazed sealed units. The shoulder I was fitting matched that modification. In your own case you may prefer to fit a shoulder and so form a rebate within the width of the new frame rather than screw one to the inside edge, a frame within a frame, using more decorative beading that will more closely match the moldings on the rest of your window.

This could be achieved using beading, mitered at the corners, glued, and pinned into place. Within the width of your new sash, there would still be plenty of room to install double-glazed sealed units if you wish.

Fitting The New Sash

sash windows

Fit the new sash before you put it in the glass. I used the old hinges positioned on the new sash such that I could screw straight into the old screw holes in the main frame. Depending on the type of hinge you may have to chisel out some wood on the new sash to let the hinges in. The hinges should not alter the fit of the sash in the frame. If they do you may have some small adjustments to make. How far in or out the hinges are is important.

Too far in and the top inner edge of the sash can foul the frame and prevent proper closure. With the hinges fitting into the old screw holes in the frame; all the adjustments in or out will be on the new sash. It pays to fit the hinge with just one screw so that in the event of adjustment you have new wood to screw into with the other two.

You will also have to fit the window furniture on the inside. The window will have to open and close and latch properly before the glass goes in. You will have putty in the glass, but getting the putty to sit neatly into the rebate can be tiresome in the extreme. The video below will help.

With the glass fitted and puttied in it’s all over bar the paint job.

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