BCO & Window fitting!

[HammerHappy]

Hi.
Asking this question here as well as window section as it's a building & regulation issue.

I have new upvc windows & doors to fit in my new extension with 150mm deep external sills. The BCO has said he wants them fitted in the middle of insulated cavity closer ( screwed into thin plastic?!! ). Obviously if I fit them there the external sill will not over hang and be sitting on the external leaf. Cavity wall is 100mm block and 100mm cavity. I have looked at window sills and the deepest I can find is 180mm even with this it's at best going to overhang 10mm which would be subsequently covered by the render.

The only solution I can think of is to by some 9mm upvc window board and sit the window on that creating 2 sills and maybe problems with rain trying to penetrate between the 2 sills?

This must be a common building regs issue? Any ideas? Thanks



Read more: https://www.diynot.com/diy/threads/deepest-upvc-external-sill.494431/#ixzz51VfqMXOG

 

[Mr Chibs]

Pretty sure I was offered a cill as large as 225mm before, so perhaps it's just your supplier.

Your window fixings will/can hang off the window towards the inside and will be secured onto the inner leaf of bricks.

Out of curiosity, why did he request them fitting in the middle of the closer?

 

[John D v2.0]

Normally if you want them there you'd pack them off some plates bridging the bottom cavity and then strap them back to the inner leaf all round.
It is better to have continuous insulation as it reduces thermal bridging.

 

[^woody^]

You fit the frame where you like as long as there is a vDPC up the reveal and if need be insulation to prevent local condensation at the reveal.

A BCO can't insist that something is done a certain way, only advise what criteria needs to be met.

 

[freddiemercurystwin]

Normally they're set about 25mm back from the outside face of the outer skin and fixed with straps back into the inner skin as mentioned, you specify a cill to suit the situation, not the other way around.

 

[pilsbury]

Normally they're set about 25mm back from the outside face of the outer skin and fixed with straps back into the inner skin as mentioned, you specify a cill to suit the situation, not the other way around.

What are these straps?

 

[Notch7]

Thats a building control officer being a Numpty! -I have heard BCOs say they want windows in the cavity.

In theory he is correct, but its not a set requirement, just his interpretation of minimising thermal bridging.

Galvanised building strap is available from screwfix and similar.

 

[pilsbury]

Galvanised building strap is available from screwfix and similar.

Say 200mm strap, screwed to the inner leaf and also to outer leaf with the same screws going through the frame?

 

[noseall]

Say 200mm strap, screwed to the inner leaf and also to outer leaf with the same screws going through the frame?

Brackets that fit into the window jamb profile.......
here

 

[Notch7]

Say 200mm strap, screwed to the inner leaf and also to outer leaf with the same screws going through the frame?

Usually if a window is fitted with straps, they are screwed to the outside face of the window frame, before its in the hole, then when in place, the straps are screwed to the inner skin reveals. Strap length depends on position of window.

In new builds, like extensions its a pretty standard method of construction, its quick and the straps get plastered over after.

There are special window straps that are available, but not in a builders merchant. There are special cavity closures available that are designed for quick fitting of windows.

 

[pilsbury]

And this is just for extra fitting strength?

 

[noseall]

And this is just for extra fitting strength?

No.
It is so that it completely negates any risk of cold bridging via any of the outer leaf components.

 

[pilsbury]

No.
It is so that it completely negates any risk of cold bridging via any of the outer leaf components.

So if I’m intending to fit in the outer skin, they are not needed?

 

[noseall]

Below are a couple of images of why window fitters should fit the windows far enough back, look closely at the lintel....

Crap Fitting2

• noseall
• 17 Dec 2017

Crap Fitting1

• noseall
• 17 Dec 2017

 

[noseall]

So if I’m intending to fit in the outer skin, they are not needed?

No, they are not.