Structural opening dimensions for window frames

[Moonshine]

I have seen a few different recommended differences between window frame sizes and structural openings, one being 12mm from frame to structure on each side of the frame, and one being 10mm difference for Both sides (i.e. 5mm each side).

The build i am looking at is timber inner leaf, masonry external, and metal fixing straps holding the window in place.

Does anyone know if there is a more definitive gap size, or what you used?

Edited January 26, 2020 by Moonshine

[nod]

I allowed 5 mil either side 

and used fixing straps 

 

[Russell griffiths]

I have recently had my windows measured and the surveyor said that they now require 30mm above any large openings for sliding or bi fold doors as they are getting too many call backs due to the openings settling. 

We have left 15mm in the width, 7.5 mm both sides and 15mm in the height. These are for windows the company are fitting, I have allowed less for the ones I am fitting as I am happy with a tighter tolerance. 

[Roundtuit]

I think we allowed something like 11mm each side, but a bit more top to bottom as the frames needed a 20mm batten on the bottom to attach the cill.  I think the allowance is a bit arbitrary to be honest, depending on how you're going to fix them; you'll need more of a gap if you use a compriband-type product rather running a bead of sealant around it. Our fixing straps were cranked to compensate for the gap, but I ended up hammering them flat as the crank was too severe, and then fine-tuning the fit by how deep you drive the screw in to the timber frame.  If you're planning window frames that can't be altered  (shaved a little if necessary) like alu-clad, then you might want to not make it too hard for yourself!

[Oz07]

Depends on colour with pvc I went 15 overall with grey

[nod]

1 hour ago, Russell griffiths said:

I have recently had my windows measured and the surveyor said that they now require 30mm above any large openings for sliding or bi fold doors as they are getting too many call backs due to the openings settling. 

We have left 15mm in the width, 7.5 mm both sides and 15mm in the height. These are for windows the company are fitting, I have allowed less for the ones I am fitting as I am happy with a tighter tolerance. 

I used a steel  hit and miss above mine for that very reason Great to fix to also 

Our SE stated he would never span an opening larger than 2000 with a catnic 

and with the low price of steel

He has a point 

 

 

[Moonshine]

Interesting, and it see that it varies at lot,

 

The attached is details from Velfac of the type of construction i am likely to do, with 12mm either side, 12mm at the head plus a settlement allowance, and 30mm at the cill plus a settlement allowance.

 

the settlement figure is on the drawings, and is dependant on if solid timber or engineered I joists are used.

 

For this set of drawings i think that i am going to use this guidance, but with a note that final construction opening tbc

Velfac masonry-timber head detail.pdf Velfac masonry-timber jamb detail.pdf Velfac masonry-timber cill detail.pdf

[LA3222]

Windows are made to order - I sent the structural opening sizes out for quotes and let the window companies decide what size the window needs to be.  Saves me worrying about opening sizes - just make sure they know what your opening size is and they should adjust the window size according to their specs.