Designing a Hopper Window

Hopper Window Open

I’ve never built a window. I never had a real need to. That is until someone contacted me about building a window sash for a stained glass window she designed. As much as I tried to convince her that a fixed-pane window would be easier, less expensive, and quicker, she insisted on a ventilating hopper window. My first thoughts on a design are shown here.

Hopper Window Layout

My immediate design concerns revolved mostly around keeping it weathertight. How do you design a window from scratch to account for keeping out wind and moisture?

Obviously, carpenters and millwork shops figured this out 100 years ago or more. Plus, common sense dictates allowing for drainage so water doesn’t accumulate and cause decay of the wood.

The design of this window started with the sash. The stained glass will be trapped  and vacuum-sealed between two panes of glass. This assembly will be installed into a rabbet on the exterior side of the sash and set with modern glazing compound by The Stained Glass Store.

The customer wants me to make the sash from kiln-dried walnut from trees that were harvested on their property. The sash with be treated with a clear penetrating oil finish like Penofin before delivery to the stained glass shop.

The sill and jamb will be made from a less-expensive wood and will be primed and painted for weather resistance. The sill will be sloped 12 to 15° to help shed water. The head and sill will be joined to the jambs with dadoes with waterproof glue and screws for added strength.

I plan on using butt hinges at the bottom of the sash. To hold the sash open for ventilation, there are a variety of window stays I can use. I’ll want to have them on hand before I start building just so I can make sure everything will work as planned after assembly.

For weatherstripping, McMaster-Carr has a wide variety to choose from.

This seal will be used on the bottom of the sash to keep out wind and rain.
This seal will be set into a rabbet in the stop that surrounds the jamb and header on the exterior side of the sash.

Hardware stores and home centers have a variety of weatherstrip materials to choose from, too.

There’s no guarantee that any of my plans will actually work until I get into the assembly process. But for me, half the fun is finding the answers to some of these problems and watching it all come together in the end.

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