Hi Alex,

The brickwork opening is usually the window size + a small tolerance to allow for fitting.

If the external finish is brickwork, then its important to have these sizes in multiples of half bricks horizontally and full bricks vertically.

There are other facors. Kitchens and bathrooms usualy have a cill height of 1050mm from finish floor level. 900mm to unit or appliance height plus 150mm splashback. The head height is usually governed by the finish door height if downstairs. ie 2100 which is 28 bricks high. [75mm multiples not 65mm allowing for mortar bed]

So mostly windows are designed to brick sizes if possible, and this fits in nicely with other criteria [and is less costly]. However this is by no means critical, and if the house is rendered then you can vary things pretty much however you like. Just expect your brickie to have a good moan

What's the story with reveals anyway? My cottage (in Scotland) has them, seems to be a full brick width at the sides. The front windows have been replaced with really bad aluminium frames but these are set (loosely) into the opening, rather than behind the reveal, as the frames are so narrow.

When I buy new PVCu or wooden windows, how should I specify the size, is 40mm a good rule of thumb? How much frame should be visible beyond the reveal? Should I chop the reveal out and make full use of the opening?

Thanks David (again).

I used to work in architure designing and doing construction drawings and I can tell you these 'standard' windows sizes used by Jeld-wen, Magnet and others are a pain in the bum. I think the sizes are based on old imperial brick sizes rather the current metric brick sizes (which have been is use for at least twenty odd years).
I also seem to remember that the measurements in the brochures include a tolerance, so the measurement is the actual structural opening size rather than the physical size of the window frame itself.
Most of the big housing developers use these windows and it causes problems when dimensioning. Do you set the full length of a wall to brick dimensions or the piers in between the windows to brick dims, either way something gets screwed up. I was always told by the developers that they 'fudge' the dims on site, to be fair they usually did a good job.

That's interesting neilu. Thanks for that addition.
I called Jeld-wen and the person I spoke did not sound terribly knowledgeable but did point out that lot of their range - ie their bog standard sovereign range are just continued production of old legacy John Carr windows so that certainly gives the impression that they are probably based on old imperial sizes. There newer DreamVU windows have widths of 600, 900, 1200 etc unlike the sovereign 630, 915, 1200 etc. So for the sake of clarity, does any body know what widths are most suitable for a new build with windows placed in the opening with say, 10mm all around.
I believe 1200 by 1200 is a common size and perhaps 1800 by 1200 or 1800 by 1350. Given that openings must be matched with bricks do these make sense. As Mike George has pointed out, shouldn't all openings be multiples of 1/2 bricks ie 107.5mm and with say, each 1/2 brick on LHS of frame and corresponding 1/2 brick on RHS, full brick on LHS and full brick of RHS of frame alternating up each side of frame. I am sure I have seen walls with non-matching bricks each side of window jambs.

I emailled Jeld-wen today and the answer came back that they're all basically made to measure, so there's no excess cost to specify your own size. As tony said.

There is also a problem with bricks -- they vary in size ans even a 1mm smaller brick will make a house 50mm longer or shorter

we once had a second load of bricks come from the same maker 3mm shorter causing immense problems on the upper floor = banding course!!

From different makers sizes can vary as much as 6 or 7mm for "metric" bricks (bricklayers never work to whole brick sizes just lay the first course and if it doesn't work the other end put a cut in under a window or in the middle of the next course).

I've just been asking a similar question in relation to entrance doors, where the "made to measure" solution seems less of a possibility - how do I deal with, for example, fitting a 1981 x 838mm door, external frame size 2067 x 926mm, into a metric brick opening?

Height wise there's 33mm left over for a 2100mm opening, and width wise 96.5mm left over for a 1022.5mm opening. Is this just packed out? seems like a lot of packing... Counterwise, it wouldn't work in a smaller opening (910mm) as the "check" would only be 13mm depth, BR asks for 25mm.

Any thoughts?