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Window Sash Repair: Part One

Although most of my work with Historic Design Consulting consists of consulting with owners of historic buildings, I occasionally so some restoration/conservation work myself.  As people have become interested in retaining historic windows I find myself working on window sash fairly often.  This Spring I have been restoring some sash from an 1881 railroad depot from southern Minnesota.  Window sash can deteriorate quickly if they are not maintained properly because the joinery between the stiles, rails and muntins provide many places for water to soak the wood and cause decay.  Although the sash from the depot are generally is good conditions, there are a few issues that I need to address,

Good end of the bottom rail showing the solid tenon.

Here is the bottom rail from one of the depot’s windows. This end of the rail is in good condition.  The tenon is not split and remains solid.








Bottom rail showing the broken tenon.

The other end of the same rail is another story.  It had been saturated with water and had decayed.  Most of the tenon had turned to dust and was not holding the joint together.







The tenon has been removed and a slot cut which will serve as a bridle joint for a new tenon.

What to do?  Since a solid joint is critical for holding the sash together I decided to replace the tenon with solid wood.  This requires making a Dutchman, or a piece of new wood fitted in place of the rotten bit.  I sawed away the the remains of the old tenon and cut what woodworkers call a bridle joint. One advantage of my Civil War era tool set is this kind of work is fairly easy. No table saws or jigs to set up. Just lay things out with a marking gauge and saw away with my dovetail saw.





The new tenon is glued into the slot or bridle joint.

The original tenon was 3/8″ wide so I ripped a piece of old growth pine and fitted it into the bridle joint.  A bit of glue will hold it in place as I trim it to length and use a gouge to match the ogee molding profile on the inside sticking.  A bit of sanding and it will be ready to be fit into the sash’s stiles.

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