Measuring french door aperture

Hi,

I have a dodgy old small extension at the back of my house (a low sloping roof added, where at some point they removed a large amount of the back wall of the house to add extra space - see photo attached). I want to demolish the sloping roof and put a french door there instead. The door will sit just the other side if the light fitting where the angle changes.

I've got a builder booked in soon to do some other work and he's agreed to do this at the same time, but wants me to order the door now so it will be delivered in time.

The problem is, I don't know how wide to spec the door or how much width is needed either side for piers. It it likely that the timber beam that is there currently will need replacing with a rsj.

Is there a standard width that should be left either side, or does it depend on the building above? I'm guessing the latter but if anyone has ideas on how to measure the aperture I'd be really grateful. I'd like the door as large as possible.

Thanks
Dave

If you are making alterations to the aperture ( brick pier, new lintel or whatever ) then don't order the door until it is completed, you are just guessing dimensions until then. Speak to the builder & explain your concerns, you will be stuck with the door if it does not fit, not the builder.

OP,
none of what you say adds up - why would someone whose supposed to be a "builder" send you off willy nilly to order
"a door"?
What kind of door, what size door, what material etc etc?
Any self-employed carrying on like that would soon be unemployed.