How to make a sash window

telos · Post by **telos** » Mon Oct 06, 2008 11:59 am

Yeah, I know it sounds ridiculous but the spacerbar seals are very sensitive (poor things). If a unit is exposed to a fluctuating pressure gradient (ie varying pressures which are not equal on both the internal and external faces of the unit) then the seal is repeatedly stressed, which can cause failures.

Completely filling the gap with mastic potentially causes two problems as not only can the unit not move freely depending on the pressure across the unit but water has an amazing ability of getting everywhere. Even good, intact mastic seals can let some water vapour through (as can painted wooden frames as well). If this cannot evaporate and vent into external air then it can accumulate on the DG seals which can also cause failures.

Amazing innit?

Post by **jfc** » Mon Oct 06, 2008 3:45 pm

Hmmmm , lucky i'm tight with the mastic then .

Main frame jointed .....

And a dry fit before glue up ....

Post by **jfc** » Mon Oct 06, 2008 4:57 pm

Now i have the frame sizes i can start making the sashes . The sashes need to overlap at the center so i need to make each sash 32.5mm longer than the middle of the opening ( I am using 65mm stock for the sash rails .)
I have also made the meeting rail on the top sash 10mm thicker than the rest . The traditional sashes have an angle on each meeting rail to take up the gap left by the parting bead but this makes fitting a lock on the sash very tight so by putting it all on the top sash it gives more room . Its suprising no one has picked up on this and started to supply thinner sash locks ::)

mrgrimsdale · Post by **mrgrimsdale** » Tue Oct 07, 2008 6:21 am

Telos wrote:Yeah, I know it sounds ridiculous but the spacerbar seals are very sensitive (poor things). If a unit is exposed to a fluctuating pressure gradient (ie varying pressures which are not equal on both the internal and external faces of the unit) then the seal is repeatedly stressed, which can cause failures.

Completely filling the gap with mastic potentially causes two problems as not only can the unit not move freely depending on the pressure across the unit but water has an amazing ability of getting everywhere. Even good, intact mastic seals can let some water vapour through (as can painted wooden frames as well). If this cannot evaporate and vent into external air then it can accumulate on the DG seals which can also cause failures.

Amazing innit?

I don't have anything to do with DG but the air gap idea is interesting.

It's also a basic principle of trad external joinery. Properly done, every door frame or window is fitted with an air gap between it and the masonry, including the cill. It's closed by the minimum possible amount of mastic at the outside, and usually by lime plaster on the inside - not allowing it to fill the gap.
The bottom of a window frame will stand on a few bits of packing, slate, tile etc or have a designed masonry cill with a fall etc.
Also the back of the joinery where it faces the masonry is never ever painted, so it can 'breathe' i.e. get wet occasionally but also be able to dry out.
That's why it lasts so long esp sashes which have a well ventilated box construction.

Where modern stuff is painted on the back, and packed tight with impervious mortar/mastic all round the gap - you get rot. In fact you can get water-logged cills, where once the paint outside or other detail has failed, the cill can literally fill up with water and rot away in no time.

So it's good to know that modern window installers are slowly learning tricks which trad joiners have known all about for generations ;D

cheers
Jacob

andrewking · Post by **andrewking** » Tue Oct 07, 2008 7:27 am

I don't have anything to do with DG but the air gap idea is interesting.

It's also a basic principle of trad external joinery. Properly done, every door frame or window is fitted with an air gap between it and the masonry, including the cill. It's closed by the minimum possible amount of mastic at the outside, and usually by lime plaster on the inside - not allowing it to fill the gap.
The bottom of a window frame will stand on a few bits of packing, slate, tile etc or have a designed masonry cill with a fall etc.
Also the back of the joinery where it faces the masonry is never ever painted, so it can 'breathe' i.e. get wet occasionally but also be able to dry out.
That's why it lasts so long esp sashes which have a well ventilated box construction.

Where modern stuff is painted on the back, and packed tight with impervious mortar/mastic all round the gap - you get rot in no time. In fact you can get water logged cills, where once the paint outside or other detail has failed, the cill can literally fill up with water and rot away in no time.

cheers
Jacob

Closer to the fact would be that its to allow for the fact that not all bricklayers lay plumb and level.
Using a rod to take site measurements, the joiner will work to the smallest size, check diagonals etc, still allowing clearances to pack and fit the frames squarely within an opening that isn't always as good as it should be, unless the brickie has built to dummy frames.
It's common practice.
(I've also removed or replaced plenty of old joinery where its been painted on the backs during alterations, so not 100% gospel on both counts)

mrgrimsdale · Post by **mrgrimsdale** » Tue Oct 07, 2008 7:43 am

andy king wrote:snip
(I've also removed or replaced plenty of old joinery where its been painted on the backs during alterations,

Really? Why would they paint the backs? Waste of paint amongst other things.
I think you are probably imagining it. Or it wasn't that old.

cheers
Jacob

Post by **jfc** » Tue Oct 07, 2008 8:09 am

I think the waste of paint thing is more like it . When i worked for a preservation company the golden rule was to prime everything . It was quite interesting to go through a rotten building ( sometimes as big as churches ) and see where the rot had left the primed timber alone and totally destroyed unprimed timbers . I think with todays paints its worth priming everything .

mrgrimsdale · Post by **mrgrimsdale** » Tue Oct 07, 2008 8:30 am

jfc wrote:I think the waste of paint thing is more like it . When i worked for a preservation company the golden rule was to prime everything . It was quite interesting to go through a rotten building ( sometimes as big as churches ) and see where the rot had left the primed timber alone and totally destroyed unprimed timbers . I think with todays paints its worth priming everything .

What for if it's out of sight? In the wrong place may keep water out but it can also keep it in. Thats my experience after removing 1000s of old windows and a few modern ones.

cheers
Jacob

Post by **jfc** » Tue Oct 07, 2008 9:16 am

What for if it's out of sight?

To prevent rot and insect attack . Like you i have seen a few windows and seen where priming has prevented rot attack .

mrgrimsdale · Post by **mrgrimsdale** » Tue Oct 07, 2008 9:41 am

Well all I can say is, since I first noticed in about 1986 that old joinery (door & window frames pre 1900ish) is never painted on the backs, that I've never seen a single exception. Not one. I've seen a lot of stuff, Britain and Ireland, and I'm a bit obsessive about details ::).

On the other hand the very worst water logged frames have been fairly modern and well painted behind and under, and crammed in with mastic, mortar etc.
Or old frames of course when paint (outside faces) has been neglected for a very long time like 50 years or more.

If I was a betting man and someone out there was about to remove a pre 1900ish frame, I'd put a big bet on there being no paint on the backs.
Bound to be the odd exception due to odd circumstances, but I've never seen one.

I can only say what I've seen!

cheers
Jacob

Post by **jfc** » Tue Oct 07, 2008 9:52 am

I agree most of them are not painted on the backs but the main reason i take them out is because they are rotten .

agbagb · Post by **agbagb** » Tue Oct 07, 2008 12:12 pm

Hi JFC, I managed to follow you over to here. I've had a read through this thread, your windows are coming on nicely

.
Thanks for comments and suggestions about design details regarding my window build.
If any here is interested I'm bloging my progress at http://slidingsash.blogspot.com/ These are double glazed sliding sash windows with weights and trying to keep the section as slim and traditionally proportioned as possible.
I found some good information regarding modern design principles on the TRADA web site http://www.trada.co.uk (search in the library for High Performance Wood Windows), how to keep water ingress controlled, some info on painting as well. I think I'll be priming everything and micro-porous painting visible parts.
I also found useful information about double glazing in timber windows at http://www.thewindowman.co.uk Lots of info about misting and other failures, using glazing tape etc.

Definitely DON'T fill the space in the rebate! 5mm air space all round and vented seems to be the minimum.

PS. I've put too many post on blog yesterday and its been picked up as potential spam, hopefuly it should be OK again in a couple of days

Post by **jfc** » Tue Oct 07, 2008 12:19 pm

;D Glad you found us . Not sure if ive been banned again over there . It was a quick in and out to tell you to wedge the tenons .

mrgrimsdale · Post by **mrgrimsdale** » Tue Oct 07, 2008 12:25 pm

My trad window drawing here. Very different from a modern one:

http://www.owdman.co.uk/wood3/sash1.jpg

cheers
Jacob

andrewking · Post by **andrewking** » Tue Oct 07, 2008 6:10 pm

Well perhaps its regional, perhaps age as well or a combination of both. Load of stuff around the 30's era that was primed on the backs that i've taken out at our local hospital when it was operational.

Post by **jfc** » Tue Oct 07, 2008 6:34 pm

Got the frame glued up today and started marking out for some fluting on the face .
This clamping wall / wall saw is so handy for glue ups .

daves · Post by **daves** » Tue Oct 07, 2008 9:00 pm

jfc wrote: ;D Glad you found us . Not sure if ive been banned again over there . It was a quick in and out to tell you to wedge the tenons .

Strange that you got in with your old username. It still doesn't show you as banned, and a nice bit of advertising for this forum to boot!

Post by **jfc** » Tue Oct 07, 2008 9:18 pm

I'm not sure i was banned , more like one mod got the ar*e and blocked my IP . Now its changed i can log in again . But now that i have said that they will probably block it again . Strange as they let other banned members back in that have done far worse than question a moderators actions ::)

daves · Post by **daves** » Tue Oct 07, 2008 9:32 pm

jfc wrote:I'm not sure i was banned , more like one mod got the ar*e and blocked my IP . Now its changed i can log in again . But now that i have said that they will probably block it again . Strange as they let other banned members back in that have done far worse than question a moderators actions ::)

They have? ???

Post by **jfc** » Tue Oct 07, 2008 9:43 pm

Yup ::)
Anyway i'm back on these huge 5mm gaps people are leaving for air flow around the units . Thats mental ! Where do you get this info ??? I bet its from the people that sell that putty on a roll ::)

agbagb · Post by **agbagb** » Tue Oct 07, 2008 10:26 pm

Jacob, Thats one hell of a slim meeting rail, it would be nice to get DG in to frames that elegent.

JFC, I got the information from TRADA I think it was passed on to them from Moses "thou shalt have at least 0.01 cubits around thine units"

mrgrimsdale · Post by **mrgrimsdale** » Wed Oct 08, 2008 6:50 am

Thats how they were in the window I was measuring. In fact quite a common size for a small to medium Victorian sash. Things are often thinner than you think esp when you have removed thick layers of old lead paint. I've seen glazing bars at 12.5 mm. 14 0r 15mm common.
BTW old trad sashes and DG are completely incompatible. Not worth even attempting. You can have old windows or copies, or modern windows with DG. There is no middle ground!

cheers
Jacob

Post by **jfc** » Wed Oct 08, 2008 7:15 am

BTW old trad sashes and DG are completely incompatible. Not worth even attempting. You can have old windows or copies, or modern windows with DG. There is no middle ground!

Please explain

;D

woodsmith · Post by **woodsmith** » Wed Oct 08, 2008 7:21 am

In the 80s I used to fit dg units with "flexible" mastic, a bit like putty and the units were fully bedded. It was supposed to be permanently flexible but set like concrete after a few years. Then I went on to using glazing silicon, fully bedding the units; which I still do when glazing doors. For windows I have gone on to leaving a gap the edge of the unit and fitting drainage to the bottom.

To be honest I haven't seen any appreciable difference in dg unit life with any of the different methods of bedding dg units and I think it is mostly down to how well the unit is made and their choice of sealant.

I live in a terrace of four houses, they were all converted to double glazing 10 years ago by the same firm. 3 of the houses have had the odd unit fail, whereas the last house to be converted has has over 50% of its units fail.

IMHO dg manufacturers want to pass the blame of failed units onto the fitters and so they produce convoluted fitting instructions which are hard to follow; you need a deep rebate to leave a 5mm gap and ensure the alloy spacer is housed within the frame.

If an air gap protects the unit, how come I see so many plastic windows with failed units :-/

Jason, sorry to join the happy band of thread hijackers ::)

Post by **jfc** » Wed Oct 08, 2008 9:39 am

Im with keith there . I normally leave 4 - 6mm and have never had a failed unit .

lynx · Post by **lynx** » Wed Oct 08, 2008 9:52 am

Could be sticking my neck out here but this is what i understand.

You need an air gap around the unit. I usually minus 6mm from tight rebate for DGU size. You also need the bottom beads drained and vented. I don't see the point of the air gap if the window is not vented properly.

This could be why UPVC have minimal unit failures due to the plastic extrusions being drilled for drainage and ventilation.

Post by **jfc** » Wed Oct 08, 2008 4:58 pm

I started on the sashes today so all the timber moulded up and cut to size , marked up for mortice and tenons .

Cut the tenons on the bandsaw

and the cheeks on the chop saw , ive fitted a toggle clamp to my saw because i got fed up of having to fiddle with the depth gauge all the time . Now i can clamp a bit of timber under the depth gauge and i only have to make a few turns to get it where i want it .

Tenons cut

Cutting the haunches , i cut my wedges out of the was haunch material as it is the correct width .

Mitre the moulding and cut away the waste , i had my cutters made with the moulding the same depth as the rebate to save messing around with stepped tenon shoulders . You need to remember you are removing the moulding when you are marking out your joints . In this case everything is minus 15mm , the depth of the rebate and moulding .

I then use a japanese pull saw to tweak the mitre .

agbagb · Post by **agbagb** » Thu Oct 09, 2008 10:24 am

Looking neat. Do you hand cut the mortise? Will the wedges be both sides? ... I'm looking forward to the next instalment.
Andy

Post by **jfc** » Thu Oct 09, 2008 12:57 pm

Noooooo , i have a mortice machine . I cut out for the wedges by hand if that counts ;D

Post by **jfc** » Thu Oct 09, 2008 5:59 pm

Onto the horns this morning , i was sent a template of them and the cutters i bought off the shelf where not quite right . The customer is very serious about the horn so i sparked up the Legacy

in the attempt to give the customers the right horn ;D

mock up for the customers to decide on .

Then on to glue up of the sashes .

Check for square and wind and then onto the clamping wall ( did i mention how handy this wall is

engineerone · Post by **engineerone** » Thu Oct 09, 2008 6:52 pm

so it is no longer a panel saw then jason? ::)

looks good, but be careful with those horns ;D

paul

Post by **jfc** » Thu Oct 09, 2008 7:04 pm

Yup you can see the clamp and guide above the window . It works as a wall saw and a clamping wall

mooretoolsplease · Post by **mooretoolsplease** » Thu Oct 09, 2008 7:54 pm

nice write up Jason,
Rather than chiselling away the mitres where your rail and stile meet, can you not set up your mitre saw with a depth stop and length stop?
I've just made a set of 5, and one place I was able to save soem time on was by making the tenon the overall length of the window, not having it protrude from the stile.
This only meant cutting off the wedges and a quick sand instead of cutting all 3 parts and a longer time sanding.
sounds quite trivial, but with 8 per window and 5 windows, thats 40 lots of time I saved

Post by **jfc** » Thu Oct 09, 2008 8:09 pm

Chiseling out the mould takes no time at all really and is quite a nice break from the boring bits . Same as leaving the tenons long , if your cutting the wedges then you may aswell cut the waste off the tenon . You need to sharpen your saw mate ;D
As for sanding , i go over the whole window with a belt sander with a worn out 120 grit belt , so a proud tenon is easy .

thallow · Post by **thallow** » Fri Oct 10, 2008 3:12 pm

agbagb wrote: ... I'm looking forward to the next instalment.
Andy

Yep ditto - fabulous thread

thanks for taking the time!

PS agdagb - enjoying your blog too....

Post by **jfc** » Fri Oct 10, 2008 6:53 pm

Onto the top sahes now we have the horn sorted ;D

Mortice them first so the timber supports itself on the machine bed . If i did the horn first the timber would want to flip up when i machined the back of the mortice .

Then onto the horns , i needed to remove the chamfer as it was fouling the router shank but not to much hassle as all of this part is removed anyway .

Then onto the Legacy

I did them in pairs but if i had bothered to make a new bed i could have done about 6 or 7 at a time .

All the horns done and a few wedged bottom sashes . I wedge after glue up as i like to make everything spot on square and clamp it .

A few top sashes ready for clamp up and some wedged bottom sashes .

scott · Post by **scott** » Fri Oct 10, 2008 9:47 pm

Some words on painting here

http://www.buildingconservation.com/articles/sashwindows/sashwindows.htm

mrgrimsdale · Post by **mrgrimsdale** » Sat Oct 11, 2008 7:56 am

Scott wrote:Some words on painting here

http://www.buildingconservation.com/articles/sashwindows/sashwindows.htm

Good article. I agree with most of it. Doubtful about the plastic brushes - it's building in obsolescence, and unnecessary anyway - a trad window properly set up is draught proof enough for most purposes.
Also I'm getting keen on trad paint. I used to think it lasted so well because of the lead and was thus a lost cause. Now I'm wondering if it is the linseed oil which gives it the quality.
What is certain is that modern paints are crap - most of the time. You hear anecdotal evidence of long lasting paint jobs but the evidence I see is of repeated failure.
There's some sash window stuff on my site here. Sorry it's in need of up-dating I'll get around to it one day soon!

cheers
Jacob

Post by **jfc** » Sat Oct 11, 2008 9:13 am

Thanks for reminding me , i need to route in the plastic brush carriers on the meeting rail before i glue up the top sashes .

telos · Post by **telos** » Sat Oct 11, 2008 9:26 am

mrgrimsdale wrote:...What is certain is that modern paints are crap - most of the time...

Talking utter sh*te again, I see... The modern world must be a really scary place for you, huh Grandad?

If the average cretin doesn't know how to prepare wood properly for painting, this is not the fault of the manufacturer.