A solid brick (10″ walls) rambler is about to get all new windows and doors, plus 2″ exterior rigid foam, 1×2 furring strips and cedar siding. My plan is to frame out the new window opening with plywood boxes, and had assumed 3/4″ would be necessary for support, rigidity and durability, but I read somewhere about someone building out with 1/2″ plywood. My aim is to have a quality frame that will last a couple of lifetimes, but I don’t want to pay for or carry more thickness than I really need. Any recommendations as to whether 3/4″ is overkill, or how a mere 1/2″ might perform?
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Whatever thickness that will hold the window/door openings in position and can be adequately fastened into the brick structure will work. You can hold the box of whatever thickness to the brick, can you hold the window to the box with that thickness?
Another consideration would be how you intend to finish the openings inside and out. Do you need the meat of a thicker frame?
Don't forget to seal this plywood, it will be able to pick up moisture from the brick, which might cause it to move all over the place. Detail the install of this box to keep out water as well. Think one whole opening, not just box and window.
Best of luck.
1/2 inch ply not a good idea
I have done this many times. We used to retrofit old brick buildings into offices at a steel mill I used to work for.
I have never used anything less than 3/4 inch thickness. On many occasion, we used to plane down 2x12s to a 1 inch thickness and used that material to build our boxes. I preferred this over the 3/4 plywood cause it was even stronger. If going that route, then pre-drill to avoid splits. With plywood, you dont have to worry about splits, but you do worry about edge grain showing ( especially on the interior side) when trimming out.
We used to build our box, attach our window to the box, and then go and install this "unit" into our rough opening.
Your box needs to be thick enough to stay ridgid and not sag, and must provide good holding power for your window screws and provide enough width for attaching trim; if needed.
1/2 inch ply may be tempting, but I would not use it...especially if any shimming will be incurred between studs and box. I/2 inch is more prone to bow if shimmed and that may cause your window to bind when opening/closing.
Afterwards, we normally just wrapped the outside of the box with coil stock, and trimmed the interior side with a stock moulding. Other times, we just painted it. After all, it was an old mill.
Good luck on whatever way you go.
Davo
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