Frame for sliding screen - how to make flush with the wall?

[Federico Mena Quintero]

Our house has iron windows that are more or less embedded in the concrete. Here is one of them.

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I need to make a sliding screen to keep the mosquitoes out. The sliding part with the screen (basically a window with a screen instead of glass) is not a problem, but I have some doubts about the fixed frame with grooves so that the screen can slide.

The fixed frame is pretty narrow depth-wise, about 5 cm, so that it can sit on the concrete ledge pictured above. I intend to make the frame with a simple box-like construction, and then nail some strips to define the grooves/rails for the sliding screen. Then, I'll put some molding around the frame and the wall to cover the gap.

My problem is that the concrete "ledge" is not completely parallel to the metal window; the depth varies by about 3 mm at most depending on which corner you measure.

Should I simply assume that the final molding around the frame will hide that variation? Or should I plane the frame so that it sits flush with the wall when installed - so that the molding rests almost dead on the wall?

Elsewhere in the house, where the walls are not completely flat, there are door frames with molding that is separated by quite a few mm from the wall due to the irregularities - and I hate it and it looks terrible. What makes me unsure about planing the window frame, so that it is flush with the wall, is that it may look non-square.

Thanks for any advice; this is my first window frame ever, so I'm plagued with questions as you may imagine :)

[Pat Barry]

This is an interesting problem. I suspect the best answer will be to make the frame slightly oversize and then 'scribe' it to fit the finished space. However, this is certainly complicated by the fact that your frame has to fit inside of 4 fixed concrete walls, therefore my thought is to use a template (actually 4 templates). I'm thinking that you would make 4 strips, one for each wall surface that would be 'scribed' to size, then use these to transfer the scribe positions to your screen frame and then 'scribe' the frame to fit in the opening.

And there's always a bit of caulk to fill in the small gaps.

[Federico Mena Quintero]

Thanks, Pat! Scribing the frame all around sounds like the best way to go.

I think I'll use dovetails for the frame so it holds together without glue, then fit it in the wall, scribe it, disassemble it, and plane the strips to the scribed lines. Then I'll glue it and fit it again.

Hadn't thought of caulk for the other gaps around the house - good idea.

[Zach Dillinger]

You could use dovetails, but a properly fit mortise and tenon will also hold together without glue.

[Paul Incognito]

Frederico,
Here's my take on it. You say you have 5cm to play with, I'd make my screen frame 4 cm and push it tight to the window frame. That will give you a reveal that will make a 3mm difference less noticeable, and you won't have a gap at the screen/window joint. Then if you want, you can apply a small scribe molding to bring the wood flush with the wall.
This is one of those questions that if you get 10 answers, they're all likely to be different, and none of them will be wrong.
Paul

[Federico Mena Quintero]

Paul - thanks, leaving a reveal is an interesting idea. Time to experiment.