Flush Casement Windows


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Chunky Monkey

Established Member
28 Nov 2013
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I'm currently making my first windows, Flush casement windows, and have done all my drawings/dimensions on the principle that they'll all be opening, so the sashes have a 3mm gap all around.

However, I'm now thinking the don't all need to be opening, some could be fixed, but I'm not sure how I should handle to this, do I:
a) Make all the sashes identical in size as if they're all opening, and then using spacers and screw and seal the fixed sashes to the frame
b) make the fixed sashes 3mm bigger in section so they're a good fit in the frame, maybe with an edge rebate to maintain the gaps with the opening sashes.

Any advice would be gratefully received.

I do option b. Make the sashes all the same size tight in the frame then plane off your gaps on the opening ones. I've never bothered rebating a shadow gap so it matches the opening ones.

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The window setup for flush on butts I do is basically:

Make all sashes, fixed and openers the same margin. Usually 3mm all round.

On fixed sashes I do a 6mm rebate.

Then when fitting up, I put in a 9mm packer on top and sides, chamfered so the front edge can be siliconed before the sash goes in.

Sash and frame get a drip groove.

Bottom of sash gets a big drip or rebate.

Why do I go to that much effort?
Its because a 3mm or 4mm gap will allow water to get in and run down, then out of the bottom chamfer.

If a sash is cut in tight, capillary action will allow water to creep in, but it cant escape.

Modern joinery design needs consideration for protection of paintwork and gaps are better to allow water to shed quickly.
Dont forget a 2 to 3mm radius to external edges and ideally put in a V at the sash joints
These make a huge improvement on the length of time to the first maintenance cylcle.