Double glazed sash wndows and catches

[GMM]

Plenty of time to scheme today as I've been trying to strip old paint (see earlier post!)

A litle while ago, I asked for advice on replacing sash windows. Plenty of discussion, followed by getting completely absorbed in Cassell's joinery book that Mr Rumm pointed me to. Still no action though, as I've had the same sort of year as Tim Watts described the other day.

Now: One window will definitely need replacing, so I've been building up to making up a whole box frame and sashes ready to fit when the weather's kind enough for a day. Then the following hit me:

Sash windows close with a catch (of various designs) working across the meeting rails (ie the bottom of the top window and the top of the bottom one). This catch is normally about an inch wide on each side (give or take, depending on design).

The sashes are 2" planed, which approximates to 44mm in new money, which IME is always less than 44mm. If I put a narrow DG unit into that (4/6/4 = 14mm), a seal (say 2mm total) a 12mm rebate and 12mm beading to hold it all in, that will take up 40mm, leaving only 4mm for the catch to sit on, which is not enough for any catch or lock I've seen. I could make the whole thing deeper to account for this but that would look a bit strange, and out of keeping with the rest of the house.

There are plenty of firms advertising DG conversions for sash windows, so presumably they have solved this but none of them mentions it in their online material that I can see. Does anyone have any idea how this is done?

[mattscantlebury]

Hi GMM,

The fitch catches are fixed to the top of the bottom sash. These are the bulky bits and they latch into a much thinner fitting attached to the top sash.

The glazing units are nowhere near where you put the catches so should be no problem.

[harryagain]

Traditional timber sash windows are draughty energy losers. You would be bette replacing with uPVC fakes (with a similar appearance) on buildings that warrant it (ie whose appearance would be spoilt by other types).

There are some good fakes about these days.

[fred]

In article , GMM writes

My original SG sashes are a true 50mm depth so there is more room to play with.

I had some DG replacements made up a few years back and they too are

50mm finished section. In that they have (I think) 20mm (4-16-4) DG units.

There is 15mm or so meat left in the profile after the rebate is taken but there is an extra 8-10mm or so at the stile where the upper and lower sashes meet as that is a different profile. Actually, I think they used the same profile all round then added custom mouldings to widen the stile so they would meet in the middle. In the old SG sashes I can see that lower sash was built out more and the upper sash less so, so that there was a tapering and sealing joint. In the new one both upper and lower are built out by the same amount and the seal is taken care of by a rigid plastic weatherstrip.

The 15mm of rebate material plus the 10mm extra at the stile gives enough to fit the narrow half of a screw down catch to the upper sash.

The 50mm thickness doesn't seem out of proportion but that are big windows.

FYI, the parting beads are 14-15mm on the olds and 15-16mm on new.

Thanks for pointing this issue out as I will be retrofitting DG into some of my existing sashes and had forgotten to allow room for the sash fixings in my rebate calcs.

Found a sketch:

[GMM]

That's very useful and seems to be the solution then: A thicker section. The window I'm replacing is (approx) 6' x 4', so can probably take a bit more depth without looking too chunky. Actually, your sketch indicates an even thicker section than than 2" finished (57mm? - maybe that was just to indicate the profiles). Even then, the sketch shows that the catch only just fits.

I guess I was planning on 44mm as it approximates to the original design, it's readily available and I didn't want to add any (more) weight, as the sashes will be pretty heavy as it is, with DG fitted.

On the other hand, it does solve a problem or two to make the section thicker and as I am building the boxes I can make it how ever it works best. Those are pretty meaty parting beads on yours too, which also goes some way towards solving the issue. I'd be a little reluctant to go that thick, but only because I was hoping to use the standard beads (8mm?) on the grounds they would be easy to replace if that was necessary.

Like your originals, my current lower sashes are built out to the thickness of the parting bead. I guess there's no reason why the top sash can't carry most of the extra thickness if the angle of the taper is reduced to a minimum: The taper would have been the only seal originally, but I would add a proper sealing strip there anyway, so it could probably be closer to square.

It's always the details that are the gotchas in these things!

[GMM]

There are certainly some very good plastic ones about. Unfortunately, one of the joys of being in a conservation area is that they are banned, so I don't have that option. They don't even like double glazing very much as it detracts from the original appearance, allegedly, but can be persuaded to accept slim DG units.

[fred]

Mine are a similar width but a tad taller and the extra thickness on the new ones doesn't look out of place. Yes, they are thicker than I first thought (and measured!), sorry for misleading you. The originals are definitely 50mm though. To fit in with existing internal panels I fitted the frames direct to the stone rather than with the usual half inch or so of fill (parging?) and foam filled before finishing with frame sealant.

To confirm, the units are 20mm but that makes the build 4-12-4, with low-E on the inner leaf.

I'd say by far the greatest weight was in the glass, that became obvious when moving the lot around before assembly. Having looked at the price of lead weights, I made my own from 2" round steel bar salvaged from a scrappy.

Thick beads seem to be the standard in Scotland, I've only seen the thinner ones in England, notably on Georgian terrace windows. I quite like the thick ones as it enhances burglar resistance and makes it easier to fit a brush strip seal in a rebate on them. As you're rolling your own though I'd be inclined to machine rebates on the sashes to take rigid plastic sliding seals, there are router cutters that cut the rebate and slot (for the seal retainer) in one pass (seen but no refs I'm afraid).

Here's the detail of the vertical seals on mine, in those rebates:

A flitch catch seems to take up less room on the upper sash (drawing of narrower profile sash with thin DG units):

Can't say I fancy using units that thin though, diminishing returns and all that.

Quite, and I hope it goes well for you, it would be beyond my skill to attempt a build from scratch.

[Andy Burns]

Any use?

they also sell pre-grooved parting beads

[fred]

The reinforced foam seals there are probably more suited to conventional hinged windows, I don't think stand up well to the friction of sliding sash windows.

Found these in their "vertical sash window seals" section which look more like the ones on mine:

slidex_seal

or

Hard plastic so they can take the friction but possibly a less efficient seal on anything but a perfectly smooth mating surface.

I've used that company for other stuff in the past and found them fine, good prices too.

[GMM]

I'm sure you're right about the weight: The timber is minimal compared with the glass. I hadn't thought about the scrappy for weights, but I find my friendly local one very useful for a lot of things. Much useful stuff to be had for the price of a pint.

I hadn't really thought about thicker parting beads being more secure (which would make sense). All the ones around these parts are the 8mm jobbies but certainly thicker ones would take a groove for a seal more easily. Again, they would need a deeper frame/box, but that's no problem for me as I'll be putting some PIR insulated plasterboard up at the same time as doing this window, so a bit more depth might actually give a better result internally.

I had been mulling the merits of routing the sash or the bead for seals. I have some staff beads that I've routed and fitted with "sofseal" from Reddiseals, who are not only close-ish to me, but a source of good advice:

Of course, the main reason for doing that is because you can deal with a lot of the draughts just by changing plain staff beads for some with seals (routed, painted and sealed off the job), so an easy task to fit.

The reason for going for thin DG is simply the conservation planners. They have a thing about windows in general and will only allow DG if it's 4-6-4. I guess 4-6-4 is better than 4 (!) I was also trying to fit it into a 44mm stile, of course.

I can't guarantee that building the whole thing is within my skills either, but there's only one way to find out. I intend to make the whole thing up as a bit of a winter project before fitting, so any adjustments will be possible before I do anything irreversible. Compared with the cost of getting one made though, I can afford a few boo boos along the way and still wind up quids in. If it works out to be too tricky then I'll have learned my lesson!

Many thanks for all the detailed info. It's very useful to get the low-down from someone who has been there.

[Martin Bonner]

Have you looked at

? They have quite a lot of interesting bits and pieces. They seem to aim more at trade than domestic customers but you do need to do some proper joinery to use their stuff.

My only connection is as a satisfied customer.

[fred]

In article , GMM writes

You're very welcome and please do let us see the results, perhaps consider a heroic DIY wiki entry with stage photos if you have the time and inclination.

[GMM]

Thanks Martin. Yes, I have seen their range and they seem to have a lot of useful things. I've a feeling I might well become a customer as this progresses.

[GMM]

I'll certainly try to remember to take some shots. So often, I forget about photos until things become too advanced for it to be worthwhile, but this one could be a more considered project...

[fred]

In article , GMM writes

Oi!, no excuses.

Take the pics, then you can decide whether to publish later :-)