What size Catnic lintel for extension

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I have an extension 3.1 m wide and 5 long. Rafters will span back to front flat roof.

Will a 3 m catnic lintel be sufficient with 300mm returns on both sides and the rest for a French door.

The flat roof will be on the front wall above lintel but it’s a single story.
 
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Will a 3 m catnic lintel be sufficient with 300mm returns on both sides and the rest for a French door.

The flat roof will be on the front wall above lintel but it’s a single story.

No. 300mm returns won't get past building control without SE input.
 
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No. 300mm returns won't get past building control with SE input.

If it is 300 measured externally, I doubt any SE would be able to prove adequacy, unless there were suitable ties between the flank wall and roof.
 
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Returns will need to be at least 675mm each side, measured on the outside. The document reference is Approved Document Part A, diagram 14.
Download it for free here: https://www.planningportal.co.uk/info/200135/approved_documents/62/part_a_-_structure

For a 3m span generally it's best to go with a structural engineer to provide you with calculations to submit to building control, especially if you are installing bi-fold doors, as the beam/lintel can be designed to the manufacturer's deflection criteria to stop the door sticking.
 
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My philosophy is that they would not make 3m lintels if you couldn't use them. :rolleyes:
 
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Lintels are made to have a quantity of masonry over the top to spread the load. For a single storey flat roof that is generally not the case. Secondly, and this is why I mentioned it, lintel manufacturers won't tell you how much it will deflect, so you could end up with problems with binding on bi-fold or french doors. Thirdly, even if you go for a standard lintel, someone has to work out the loading.

Edit: Case in point today, I just rang Keystone to get the I-value (stiffness) of their standard cavity lintel, to see if it would go over a 4m bi-fold door. While it's advertised as going up to 4.8m long, the technical department said their charts didn't go up to a 4m lintel and they would have to "Get back to me". I have a few friends who used to work there, and one of them said that they tended to do a special for anything that long - which is no use to most builders as they want to get them off the shelf and not have to order it.
 
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