Hi, 

Is it okey to build dormer face made of 100x47 on a wall plate which is only 75 mm wide? 

Many thanks 

Why is the dormer face only 100mm thick?  Surely that won't contain enough insulation by modern standards.

 

I would be asking your builder why is the wall plate so small?

This is how it was designed by structural engineer. I belive the insulation between the studs will be 100mm with 25mm or 50mm under the plasterboard in order to prevent thermal bridging which would bring it to 125 - 150mm. That should be enough. 

The wall plate is 40 year old but in very good condition. However it's narrow. I'm wondering if it's acceptable to erect wall which is wider on it. 

 

If the wall plate were 100mm wider but the dormer face wall would be 147 that would still cause the same issue. 

Is it necessary for wall plates to match width of the walls build on top of it or is that 25% overhang acceptable if not practical to change wall plate? 

 

Just now, DanieldeLorsh said:

The wall plate is 40 year old but in very good condition. However it's narrow. I'm wondering if it's acceptable to erect wall which is wider on it. 

Yes fine - what’s the outer face of the dormer going to be ..?

 

Front face will be covered with OSB, counter battens, battens, hanging concrete tiles, some lead flashing. 

I wouldn't worry. Pack it out with batten if you feel the need