Extending hip roof to gable

[jh3141]

I have a 1930s semi with a reasonably traditional purlin & rafter hip roof. I am extending it at the side, and have permission to convert the hip to a gable. My understanding is that it should be possible to leave the existing structure intact (it is apparently in excellent condition) and build the extended roof by joining to the existing purlins, but I'm not quite sure of the mechanics. What kind of joint would I need to use to attach my new purlins to the existing ones?

Also, I'm looking at the building control guidance note for this style roof, and there's a statement that reads "if h is greater than 16t provide restraint here at not greater than 2 metre centres." It looks as though this will be needed in my design, but I'm not sure what kind of restraint would satisfy the requirement. Would ceiling joists running between my new gable wall (with appropriate hangers) and the existing wall (probably attached to the existing wall plate with nail plates or similar) be appropriate restraint? Or is it some other kind of restraint it's after?

 

[noseall]

I am going to convert the hip to a gable. My understanding is that it should be possible to leave the existing structure intact

largely yes.

Removing any individual part of a cut roof can leave the opposite elevation vulnerable to spread etc.

You can either drop a layboard on top of the existing jack rafters, i.e. directly above the existing hip and in-line with it, then drop your new compound cut jacks onto this layboard.

Or.....try and wangle your jacks so that they land onto the existing hip blade, in between the existing jacks (again with a compound cut on the jack end), although this may involve a certain amount of cutting of the existing jacks.

" Or is it some other kind of restraint it's after?

Likely to be gable masonry lateral restraint he is referring to?
You will need to ask for clarification.

 

[jh3141]

You can either drop a layboard on top of the existing jack rafters, i.e. directly above the existing hip and in-line with it, then drop your new compound cut jacks onto this layboard.

That's about what I was thinking, yes. I'm assuming nailplates at regular intervals would be an appropriate way of attaching the new structure to the old?

" Or is it some other kind of restraint it's after?

Likely to be gable masonry lateral restraint he is referring to?
You will need to ask for clarification.[/quote]

I was refering to the standard guidance note, not a specific comment on my requirements. It's in the document that's available at www.haringey.gov.uk/note18_-_purlin_and_rafter_sizes.pdf‎ (for example; I have seen the same document distributed by several councils).

 

[noseall]

Highly likely that it is gable restraint.

As for purlins, they don't necessarily require connecting from new to existing unless the structural alterations demands it. You could off-set the new purlin birds-mouth cut outs slightly so as to allow the new purlin to pass along side the existing purlin, then bolt them together.

This may only be possible provided that if when you off-set the new purlin i.e. eccentrically position the new purlin (rather than centralise it) that the larger span of rafter remaining is not beyond its span capability.

....and that the new purlin can pass along side the old without striking the existing rafters. Your new rafters are likely to be deeper in any case, yes...?