Dormer windows & position of purlin

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15 Apr 2005
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Leeds
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Hi all, i'm thinking of putting in dormer windows front & back of my terraced house. There's loads of them on my street, so no problems with planning. In terms of width of dormer, I'm gona copy the biggest on my street. But the problem i've got is with the roof height of the dormers.

I've got fairly low sloping attic ceilings at the moment, and the purlin is at an awkward position if we wanted dormers put in. I've had two builders in:

- one says your gona have to leave the purlin where it is & therefore your
dormer will be a bit smaller (lower celing) than we want.

- another is saying you can move the purlin easily (further up towards the ridge) & therefore get a higher dormer window.

Obviously the higher the dormer ceiling the better, but why would one builder contradict the other???? when i told him what the other guy had said he shrugged his shoulders?????

Can the purlin be moved, & if so by how much? Any advice would be greatly received - thanks in advance.
 
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purlins are there to lend support to the rafters, they are usually set at centre span, or 1/3 span, 1/4 span, etc, depending on rafter length. should the purlin be moved higher up the roof, (quite an undertaking in itself!) without beefing up the rafters or adding another purlin lower down, then you will compromise the structural integrity of the roof. plus the fact that if the rafters have been birds mouthed onto the purlin you will be left with a weak spot. a structural engineer or good architect would be a good investment.
 

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