Window return query for flat roof extension

[pkn]

After looking at a couple of 3D kitchen CAD designs for our extension, my wife would like to relocate the French doors from the position in the existing layout to the preferred layout (pictures attached). The doors will have windows either side and a window (1100mm) will be placed on the side wall instead, i.e. we are still keeping the same amount of glazing as the current plan, just changing the position of the French doors. This would result in a larger opening along the back, where the corner window return would be 845mm externally and 545mm internally. Would this be acceptable for building control? The extension is a flat roof, as per the elevation attachment. I'd ask my architect, but he's on holiday for a few weeks and my builder is keen to get the windows ordered this week. 

 

Many thanks for any advice on this. 

[Conor]

This is a question for your structural engineer, nobody else. Most likely OK, but you've no other way of knowing unless somebody does the calcs.

Edited July 18 by Conor

[Dave Jones]

bog standard catnic lintel be fine. no need blow cash on an SE

[kandgmitchell]

Can't see an issue subject to changing the lintel due to the wider opening - is the flat roof at window/door head height? If so what was the original design lintel?

[pkn]

Many thanks for the replies, much appreciated. @kandgmitchell, the flat roof will be at window / door head height. In the specification document, my architect has the following detail in the "Opening" section:

 

Keylite galvanised lintels (insulated) to be designed and fixed in accordance with manufacturers requirements. Lintels to have DPC stepped over. Cills to be precast concrete to suit opening as detailed. 

[kandgmitchell]

Intrigued to see the detail of how the roof joists interact with a Keylite lintel. Who is going to choose the correct lintel for the situation?

[pkn]

@kandgmitchell, that's a good question 😊 I'm not exactly sure on that and the builder hasn't queried it. The architect provided this section diagram but don't think that gives any insight on what type of lintel to use? 

[kandgmitchell]

Well how does that work then? If the door head is at ceiling level as per the elevation, the lintel must be in the depth of the joists. That throws up two queries. 1) I doubt if a 150mm deep lintel will span the enlarged opening and 2) how do the bits of joist on the outside of the lintel and the fascia get fixed?

 

I suspect the roof isn't at door head head and there is enough depth to get the lintel in over the door and for the joists to sit ontop of it. Someone needs to take responsibility for chosing the lintel though.

[DevilDamo]

Do you realise the ceiling height in the extension is going to be approx. 2.1m and I assume the existing is approx. 2.4m?

 

And btw, 665mm on the external return is a minimum dimension which you would meet. But the return dimension is also dependant on the overall length of wall, openings, etc… Your engineer should be checking the slight re-configuration. Or to play it safe, reduce the overall width of the French doors with sidelights to 3455mm, which would leave you with the same return as before.

Edited July 20 by DevilDamo