Gap between new pvc window and brick surround,wall

[riclad]

I,m taking out an old wooden sash, window , 42inch x 35inch.
When i,m getting a new pvc window should i leave 3 or 4mm all round, or just a the top , or the bottom of the window.To make space for silicone and the 3 metal pieces that are used to secure the window to the wall at the 2 sides.Theres already an old concrete window sill there.
when i,m going to the supplier should i ask for windows slightly smaller than 42x35inches .
i,ll have an installer to help me put in the window.
i put in 4 windows with a friend ,5 years ago, i think
We used spacers .a few inches long,underneath the largest window 5ft x5ft .on the inside, at the bottom and filled in all round with silicone.
ie i think there was a few cm space left on the inside for silicone,
if i remember corrrectly.

 

[wingcoax]

Give your supplier the size of opening and they will make allowances.

 

[riclad]

I found on another website, leave 5mm space white windows, 7mm black,dark windows.

 

[wingcoax]

That will be total, not all round. Good luck.