Lead window tray and bottom fixing detail for Upvc window into timber frame…

[jonnyhifi]

I’m putting in some triple glazed Upvc windows -
2.4 metres wide into our new dormer room -
Which is timber frame, sheathed in OSB, tyvek, battens and plastic cladding. My building surveyor says building control will expect a lead window tray , to create a flashing over the cladding under the cill. I’m worried if we mechanically fix the window at the bottom (Ie screw it down) as my window guy wants to do - the screws will perforate the lead tray which seems silly. The window guy says he’ll put a big blob of CT1 or sikafix where the screws will go through (where the handle on the opening panels will be to resist the thump and wobble on closure) so it can’t ever leak there.

I wonder if we can simply fix the window at the sides and through the header, so as to preserve the waterproofness of the lead tray. Anyone know?

I’ve asked the manufacturers of the window (formula one) who say they have no idea / recommendations on how the windows should be fixed / fitted !?!?! I’m guessing this must be a not too uncommon problem - how to mechanically fix a window without perforating weathering detail beneath it …

I’ve found online some references which seem to imply it should be mechanically fixed (at most at 600 centres), and to complicate things - if the lead tray is over 1.5m wide (it is) to have multiple bits of lead overlapping (100mm or 150mm can’t quite remember), which again seems to me to defeat the object of having a water proof barrier beneath the window, given the framing the lead sits on will be notionally level with no run off.

Any suggestions how to resolve this detail, to allow secure fixing yet guaranteed waterproofness and water run off …?

 

[Mr Chibs]

Is there any option to fix the bottom of the window to the internal side, and lead tray for the external?
What about some powder coated steel/aluminium cill flashing??

Lead flashing has be be multiple pieces when around 1m long, due to expansion and contraction through the year.

My patio door is around that size and has no fixings in the bottom.

 

[jonnyhifi]

Interesting. There is simply one 4x2 beneath the window, so in terms of structure there’s no real inside /
Outside distinction - or to put it another way -
No cavity. In regards inside /
Outside the lead - again no -
That’s not possible as the detail being asked for, includes the lead having an upturn inside of the window, so all of the window sits within the lead.

An interesting idea to not use lead at all. No one has suggested that. Given lead is a standard material for this problem, however, I feel there must be a standard set of specs to maximise benefit form the tray / fixings arrangements out there !

I do get it that the requirement for the lead to be in sections is not a “funny” constraint, but for good reason to allow thermal expansion/ contraction.

I wonder if for the lower lead section , I should bend the end over on itself to form a welt - so any water that does run along would hit the welt and go no further….

 

[Mr Chibs]

I get what you are saying

Wont' the lead have to be nailed down to the stud work?
If so, let your window man use his fixings, a good covering of CT1 (or suitable product) will probably suffice.

I will watch this thread and see how the pro's deal with it.

 

[jonnyhifi]

Ok … well here’s the conclusion from the tradesmen involved … the purpose for he lead is to act as flashing to the external cladding - so the upturn inside seen on commercially available trays would be for a different situation to mine, so for mine we may well fabricate the tray without it.

Additional detail will be a bead of lead sealant (Ie flexible -) between the underside of the plastic window ledge and the top of the lead to ensure no water comes inboard between the lead and the plastic cill running full width of the window, and up the side of the upturn of the lead at the sides.

For belt and braces we will probably put a bead of sealant beneath the lead to the membrane too. In case there’s any condensation outside beneath the lead, but even as I write this I’m not convinced - more like the justification feels like for air tightness from inside to outside.

The screws fixing the bottom of the window will penetrate the lead but will be inside of the bead of lead sealant hence it doesn’t matter the screws penetrate the lead.

Another wrinkle ice become aware of is ensuring there’s no thermal bridge through the wood frame.

The web seems to imply I need to hit a u value if .7 or better through the wood framing and window reveals.

Some calculations implies I need 20mm or more of celotex to achieve this. I have some 10mm celotex around m, and there’s space to put this between the plastic window and wood framing so we intend to put that in all round, the membrane lapping over that - the lead on top of that (thermally irritating as clearly that’s a massive thermal bridge) at the bottom of the window.
Butting up to the internal face of the plastic window frame for the reveal I’ll use a thermal laminate board - which has 20mm (possibly 25 I haven’t got it here with me) bonded to plasterboard of kooltherm (no I’m not made of money but the supplier supplied cool therm boards at no extra cost as it’s what they had in when I ordered from a company that Sells seconds boards from wales - way cheaper than from local merchants). Interestingly the tradesmen, architect and building inspector haven’t picked up that I need to eliminate the thermal bridge there, necessitated by regulation - and keep saying “ you won’t get thermal bridging through wood”. Well you do- just not as bad as through steel ! It’s interesting the calc for the u value with only 1 cm of celotex on the wood plate beneath the window isn’t sufficient - yet from the tradeswomen’s standpoint what I had intended to do with 1cm if celotex (room side of the plastic frame) they’ve never done that before and - they’ve always plasterboard straight into the wood- including the window fitters. Thank god I’m not relying on FENSA certification of the window guys as a display of knowledge - l if I want to hit building reg requirements! (Indeed I’d like to exceed them and shoot for 2025 building regs for new build or better as we intend to live here a long time - and energy is the cheapest now it will ever be).

 

[jonnyhifi]

I should also say I looked up alpha for lead and reckoned that over this span the lead could change length by 1-2mm (I forget the exact figure sorry!) due to a temp change of 20 centigrade, so it’s clear this does need to be accommodated. I presume the theory is that the lead sealant is squidgy enough to bend preferentially when the lead has a shimmy on ! And hence doing it in two pieces, with some lead sealer between the pieces.

 

[jonnyhifi]

Well - nearly at the end now.

I’ve sealed the Upvc window frame to the tyvek returning behind it, to the head and sides, with a 4cm plastic capping I’m expecting to glue as a decorative finish to the sides, which will cover that glue / sealant line (and willl be the main / first defence against rain ingress down the sides) returning and sealed to the window frame, and at the front of this bonded to a simple right angle (in section) plastic trim again spray activated CA glued onto the cladding a lt the front, forming an external corner for the return.

That seems to work well for the sides, but I’m left finally with the lead of the window tray …
To bond or not to bond to the under side of the cil at the outside.
I can see arguments for and against. For - stops wind driving rain under there (though it shouldn’t matter - as the lead has an upstanding inside on my small window and on the large window is flat inside but sealed against water ingress with soudal fixall to the Upvc frame, and a long bead of lead silicone going left to right under the cill, between the cill and the lead, so water shouldn’t be able to get through there.

Against - if water does somehow get in there - it won’t then get out.

Note - the window has hidden drainage at the bottom where it would drain onto the Upvc cill. I’m not siliconing that gap between the Upvc window frame and Upvc cill ( obviously), but the underside of the Upvc cill has no drainage holes of itself.

I’m thinking perhaps I should use the lead silicone between the lead and the cill but leave a weep hole of a cm or so at either end and maybe one in the middle “just.in case” and ensure the top of the lead tray up stand is sealed so water can’t run down there.
I will also reseal the lead tray upstand at the front where the “expert” cladding people cut through it vertically with a multi tool so they could put the plastic edge trim through it “to be neat” before cutting off the rest of the lead - before I stopped them - and we parted. They seem to be unaware the lead was functional and there was a reason I’d gone to great efforts to put a weld there. Jeez …

So what do people think about siliconing between the underside of the cill and the lead flashing, beneath the window ?