Bay windows for complete beginners

[MortarThePoint]

I'm specifying the windows for our bay window and it's raising a lot of questions that I'm sure the wiser among us have come across before. Below is an extract from the elevations drawings and a dimensions drawing of what the masonry and window elements look like in the bay. Questions:

1. Window tolerances: Across the front to the rectangular bay the dimensions are in the hands of the window manufacturer as it's not like it needs to fit with a reveal that is slightly off nominal or slightly off plumb. So there I presume I specify the actual dimension I get by measuring on site and subtracting twice the desired amount the windows are to be set back from brickwork (twice because one set back at each end of the run). That leaves the dimension between the pillar and the brickwork wall. Here we are subject to variations in plumb etc. For my windows in reveals I have allowed a tolerance of 5mm to the cavity closer and lintel on all four sides. Do I just allow a tolerance of 5mm on these side dimensions? If so that yields left: 695mm - 2.5mm(cavity closer) - 5mm(tolerance) = 687mm and right: 705mm - 2.5mm(cavity closer) - 5mm(tolerance) = 697mm. Is that correct or am I missing something or being too tight with the tolerances?
2. Roof support vs pillars: I'm pretty sure all the windows quotes I have had have said their pillars aren't structural, so I need to work out how best to support the hipped bay roof. Making it self supporting off the wall could be a big ask as it will put a lateral force on the wall that will try to pull out any fixing used at the roof's pitch abutment*. The rightangle bay posts in the window range I have gone with appears to include a metal corner post and jack arrangement, but no information about how much weight is allowed. The roof is around 1100mm x 4200mm at a 45 degree pitch (1.5m pitch abutment length) so that works out as 1.1m*((1.5+4.2m)/2 + 2*(0.5*1.1m))*1.41 * 77kg/m2 = 572kg is just tiles. The load is mostly on the wall side, but I can easily see >100kg required on each pillar support and that's before adding any snow load etc. I guess a factored load would be more like 300kg. [*] Force calc: taking moments about the bottom left corner and just considering tile weight: F_fix*600mm = 77kg/m2*(1100mm*600mm*1.41) * (1100mm/2)  --> F_fix = 66kgf = 0.7kN (first 600mm is the vertical distance to the fixing from bottom of 'truss', second 600mm is fixing spacing). There are other weights to be considered so probably more like F_fix = 2kN, perhaps 1kN if using one fixing every 300mm instead. Most of the blockwork is a low panel beneath the window so no real weight to the wall there.
3. Abutment lead vs render: The render above the bay roof needs to go on before the roof is in place (scaffolding in the way). This means what would normally be supporting the lead isn't there yet. I think I understand the approach here. The roofer puts in flashing flat to the wall which gets mostly rendered over. The roofers then tuck their soakers under this flashing when the roof gets installed.
4. Above the windows: There is a short vertical distance ~450mm that needs to get framed out in timber. This is then to receive 12mm painted plywood. Across the top of the windows 2no. 50x200mm timbers. No real need for soffit, but will need sprocket, fascia and guttering.

 

[PeterW]

1 hour ago, MortarThePoint said:

The rightangle bay posts in the window range I have gone with appears to include a metal corner post and jack arrangement, but no information about how much weight is allowed.

Just use bay pole jacks - you drill through the cill and they rest on the brickwork below. Pretty standard detail that any FENSA manufacturer/installer will specify.  

[PeterW]

1 hour ago, MortarThePoint said:

The roofer puts in flashing flat to the wall which gets mostly rendered over. The roofers then tuck their soakers under this flashing when the roof gets installed.

That will leak. Get the scaffold moved, get the windows in and roof on then render. 

[MortarThePoint]

14 minutes ago, PeterW said:

That will leak. Get the scaffold moved, get the windows in and roof on then render. 

 

Why will it leak? Won't any water striking the flashing drain onto the soakers that are tucked under it. 

[PeterW]

Sloping edges need doing properly - and if that’s not done right it leaks.  

[MortarThePoint]

Any other options? The render is booked in for just over a week's time. He'll need the scaffold to stand on too

[PeterW]

How are you rendering prior to windows ..??

[MortarThePoint]

I pre ordered the windows for the rendered sections and they are now installed

[MortarThePoint]

2 minutes ago, PeterW said:

How are you rendering prior to windows ..??

 

I pre ordered the windows for the rendered sections and they are now installed

[MortarThePoint]

 

14 hours ago, PeterW said:

Sloping edges need doing properly - and if that’s not done right it leaks.  

 

Do you mean these bits where it's a sloped roof abutment rather than ridge abutment?

 

I could tuck some DPC underneath where the lead would go that could then get folded onto the roofing membrane. In that way any water that got through the lead would be guided on to the roofing membrane by the DPC. That makes for a bit of a stack up under the render though which could be a problem (render cracking).

[MortarThePoint]

16 hours ago, MortarThePoint said:

The roofer puts in flashing flat to the wall which gets mostly rendered over. The roofers then tuck their soakers under this flashing when the roof gets installed.

 

15 hours ago, PeterW said:

That will leak. Get the scaffold moved, get the windows in and roof on then render. 

 

Other options that come to mind:

1. Add some stub rafters (~400mm long that are enough for the first two courses of roof tiles and the battens beneath. Tile top two courses of tiles and dress with lead etc. Problems: What to do about roofing membrane? Would get in the way/knocked.
2. Not render this area. hold the render back around the bay window roof and then later once the roof is in place feather in a second job of rendering. Problems: could create a visible join, could create cracking weak spot at join.
3. Weld the lead line. Weld each soaker that is added to the flashing and to the soaker beneath it.
4. Dress lead over the render. Not aesthetically what we want.
5. Handle the leaks. e.g. use DPC dressed on wall and tucked over roofing membrane Problems: extra material to squeeze under the render. Could cut it the DPC into the blockwork below the stop line of the render but above the bottom of the lead flashing. Could put excessive amounts of water on to the roofing membrane.
6. Handle the leaks 2. let the water go down onto DPM over the ceiling below. Problem: could have pooling and humidity/mold.

 

[ETC]

Cavity trays plus flashing required at the roof/wall junction.

Keystone lintel and post system to support the roof.

[MortarThePoint]

2 hours ago, ETC said:

Cavity trays plus flashing required at the roof/wall junction.

Keystone lintel and post system to support the roof.

 

Thanks. Cavity trays are in and there is a lintel in the wall (so not a pile of rubble ? ) Just kidding, I presume you mean keystone across the bay not in the wall.

 

I think I've been overestimating how far the render goes over the lead. Render says 30-50mm. That should leave lots of room to shift the lead slightly to tuck soakers under shouldn't it?

 

[ETC]

Cavity trays plus flashing required at the roof/wall junction.

Keystone lintel and post system to support the roof.

 

Yes a Keystone bay window lintel.

[ETC]

Take a look at the Lead Sheet Association details.

[MortarThePoint]

1 hour ago, ETC said:

Take a look at the Lead Sheet Association details.

 

Couldn't find their PDF but found another organisations Craftsman's Guide. they show that the render bead does go far over the lead flashing. Unless the lead is trimmed a bit, it may only just reach the nails holding the lead flashing in place.

 

They haven't chased the lead in. Should it be chased in?

 

 

[MortarThePoint]

I'm sure I've seen a lot of houses with render down much closer to the bay window tiles. Have they rendered over the lead or got tiny lead flashing upstand? 

 

Render tight up to roof:

Close:

 

Render quite far from roof:

 

[MortarThePoint]

On 10/09/2021 at 20:16, PeterW said:

Just use bay pole jacks - you drill through the cill and they rest on the brickwork below. Pretty standard detail that any FENSA manufacturer/installer will specify.  

 

Bay pole jacks supporting corner posts look to be the correct solution and rated (most) to 2t so plenty of capacity.

    https://www.windowwidgets.co.uk/corner-posts/

The challenge now is how to mount it on the brickwork as the exterior leaf corbels out by two courses (32mm) at the top of the short wall of the bay. I can lay some cut bricks to fill in behind the corbelled bricks, but it would be nice to have a better footing. I guess an L-shaped (in plan) plate laid on top of the brickwork could act as a spreader. Would be nice to bridge the cavity with a square plate, but that would create a cold bridge. [mild steel 50W/Km, 10mm x 200mm x 200mm(bridge) --> 50*0.010*0.200/0.100=1W/K]

Edited September 13, 2021 by MortarThePoint

[PeterW]

On 11/09/2021 at 22:47, MortarThePoint said:

 

Couldn't find their PDF but found another organisations Craftsman's Guide. they show that the render bead does go far over the lead flashing. Unless the lead is trimmed a bit, it may only just reach the nails holding the lead flashing in place.

 

They haven't chased the lead in. Should it be chased in?

 

 

 

That’s crap. That will leak. 
 

As a minimum it needs cutting into the wall with a 25-30mm lip and sealing. 

[MortarThePoint]

18 hours ago, PeterW said:

That’s crap. That will leak. 
 

As a minimum it needs cutting into the wall with a 25-30mm lip and sealing. 

 

I'm confused as to why this will leak. There is a stepped cavity tray above to guide water down the cavity away from the abutment and there is sufficient lead upstand to cope with roof water splash.

 

Unless it's cut in to the wall immediately below a cavity tray, the lip won't do anything.

[PeterW]

22 minutes ago, MortarThePoint said:

 

I'm confused as to why this will leak. There is a stepped cavity tray above to guide water down the cavity away from the abutment and there is sufficient lead upstand to cope with roof water splash.

 

Unless it's cut in to the wall immediately below a cavity tray, the lip won't do anything.

Water behind the render will come down the wall, behind the lead (which is a crap install from that photo) and then into the ceiling. Water coming down the cavity would hit the step tray and come out of the weeps (where are they..?) and then over the flashing. 

[MortarThePoint]

 -

Edited September 14, 2021 by MortarThePoint

[MortarThePoint]

10 minutes ago, PeterW said:

Water behind the render will come down the wall, behind the lead (which is a crap install from that photo) and then into the ceiling. Water coming down the cavity would hit the step tray and come out of the weeps (where are they..?) and then over the flashing. 

Any water that penetrates the render will penetrate the blockwork too. There isn't a continuous void between the render and the blockwork.

 

You don't have a weep on each step in render do you as that would cause staining. There is a weep at the end of the stepped cavity tray 'staircase'

Edited September 14, 2021 by MortarThePoint

[PeterW]

2 hours ago, MortarThePoint said:

You don't have a weep on each step in render do you as that would cause staining. There is a weep at the end of the stepped cavity tray 'staircase'

For a flat section you should have a weep vent at minimum 450mm centres over the flat section. 
 

2 hours ago, MortarThePoint said:

Any water that penetrates the render will penetrate the blockwork too. There isn't a continuous void between the render and the blockwork.

And when render blows, you end up with a track down the wall between the render and the blockwork. 
 

3 hours ago, MortarThePoint said:

Unless it's cut in to the wall immediately below a cavity tray, the lip won't do anything.

And that is how it should be done ….! 

[MortarThePoint]

1 hour ago, PeterW said:

For a flat section you should have a weep vent at minimum 450mm centres over the flat section. 

 

You can't have a flat roof section with render below  ?

1 hour ago, PeterW said:

And when render blows, you end up with a track down the wall between the render and the blockwork. 
 

4 hours ago, MortarThePoint said:

Unless it's cut in to the wall immediately below a cavity tray, the lip won't do anything.

And that is how it should be done ….! 

 

So you would always have a stepped cavity tray even in a blockwork elevation. Would you use coursing blocks to avoid making the flashing steps very high (which would use excessive amounts of lead and cause issues of the render spanning too far across the lead)?