Loft conversion, floors and span...

1 Feb 2022
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North Yorkshire
United Kingdom
Hi all,

This is my first post on here after reading A LOT of useful threads.

I have designs for a loft conversion where the joists (47x145) have been drawn to span just shy of 3m at 40ctrs. That span is from a steel beam in the middle of the house (dining room x kitchen knock-through) out to another steel, about 70cm off the external wall.

1) My first question is, if I do away with that second steel, the span would become 3.65m, too long for 47x145 at 40ctrs but would reducing that to 30ctrs, or even 25 or 20ctrs, be adequate?

My options for this seem to be either beef up the joists, use the steel or reduce the ctrs. Beefing up is not a great options as its a loft conversion and headroom is a premium, using the steel (in its designed position) means placing it onto a party wall with a wall plate and onto another internal masonry supporting wall right where an existing masonry purlin post is, meaning i would have to cut a hole into that purlin post. Hence i prefer the reduce ctrs option to increase the span without steel.

2) My second question is, IF i were to reduce to 20/25/30ctrs and that be adequate to solve the span of 3.65m issue, would those joists then form a strong enough structure upon which to run 2 sole plates (4x2s) across and build purlin support posts and wall to hold up the purlin of either side of a dormer?

The room is about 3m wide, with a purlin holding up 8 rafters. I plan to cut the purlin and 4 of the rafters out to form a gable ended, pitched roof dormer out to as close to the external wall as permitted dev would allow.

Id welcome anyones thoughts or experience with something similar.

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