Extension roof design problems on 1950's semi

[garyo]

Hi Guys,

I've started talking to BCO and an SE about my roof design. The SE is scratching his head on how I can link my new extension roof to my existing roof without, pretty much, replacing my entire existing house roof.

The crux of the matter is that my existing house roof, although only ~20 years old, seems to have been replaced before regs were particularly stringent - it's inadequate by modern standards and based on some test calcs done by the SE, most members in the roof fail tests to even support it's own current weight/load.

The problem is where the proposed extension roof overlaps with the house roof. The plan here was to run the new rafters up the existing house roof rafters in ever decreasing triangles. However, even when doubled up, the existing 50mm x 100mm rafters @ 400centres are inadequate, as would the existing hip rafter.

We have thought about installed a beefy hip rafter and valley rafter, but these would need to be 300mm deep, and due to the existing roof purlin and wall plate positions, would cause problems with the new roof height matching the existing roof. It also raises questions about the king post that this would all bear on to.

I'm getting to the point where I'm designing all kinds of elaborate steel work in my head, but it seems OTT and I wonder what the norm is in this kind of situation - it must be fairly common?

Untitled

• garyo
• 14 Mar 2012
• 3

Thanks

Gary

 

[noseall]

my existing house roof, although only ~20 years old, seems to have been replaced before regs were particularly stringent - it's inadequate by modern standards and based on some test calcs done by the SE, most members in the roof fail tests to even support it's own current weight/load.

Your entire roof was replaced 20 years ago and did not satisfy the b'regs of the day????

It looks a fairly typical roof design whereby the new roof is cut into the existing roof by means of lay boards. Simples.

Why are the SE and BC making things complicated? Is the existing roof showing signs of distress?

 

[garyo]

Your entire roof was replaced 20 years ago and did not satisfy the b'regs of the day????

I can't be sure - the roof covering certainly looks new compared to the neighbours too, and the previous owners told me it was less than 15 years old. Now you question it though, I guess the carcass probably wasn't replaced, and it was just felt, battens, tiles. The existing roof carcass looks very good to me though, so I have no concerns there.

The BC only actually queried:

1) my beam design to support the old roof wall plate (I'm knocking through so there's no longer a wall supporting the wall plate) The SE has since addressed this by spec'ing an RSJ instead.

2) that my new extension rafters were under-spec'd. I've gone to 400 centres on these and the SE has now supplied calcs for new extension joists and rafters.

3) What the existing house hip rafter dimensions are (They are 50mm x 220 but not stated on my drawings) There is where things got complicated. I'd predicted that the question was leading towards BC stating that it was inadequate to take the weight of the new roof's 'overlap' portion, so asked the SE to validate what I'm doing. That's where things have fallen apart.

I'm temped to give BC exactly what he asked for, and no more, and see where it leads.

Can you explain what a lay-board is? I was picturing a solution similar to the cross section below. It's reassuring to know that you things are being made complicated! :

Cheers

Gary

Untitled

• garyo
• 14 Mar 2012
• 1

 

[garyo]

Hi Nose -

I'm just reading a thread with a good description of cutting a new roof in to an existing one - it seems similar to what I was planning, but I was also planning on having 'joists' connecting the legs of my jack rafters in the overlap.

FYI the thread for reference //www.diynot.com/forums/viewtopic.php?t=14934

Gary

 

[noseall]

You wouldn't normally have joists connecting the jacks on the lay-roof as spread is controlled by the layboard which is fixed to the existing rafters, unless your designer thinks there is a need of course.

What Chappers is describing in the earlier post is two differing valley designs.

One is a simple lay roof valley where the new roof jacks are fixed to the existing roof via a lay board.

The other is a clear valley where the valley board is like the inversion of a hip and where there are no supporting rafters to carry a layboard, i.e. the valley blade carries the jacks similar to a hip blade or rafter.

 

[garyo]

I don't have a need for a clear valley, so a lay board is the way to go.

Are lay boards fitted like a wall plate in that the the jacks are birds mouthed onto it, or does it tend to be a wide timber and the jacks have flat feet sitting directly on to it - in the same way that a rafter would meet a hip rafter?

 

[noseall]

Are lay boards fitted like a wall plate in that the the jacks are birds mouthed onto it, or does it tend to be a wide timber and the jacks have flat feet sitting directly on to it - in the same way that a rafter would meet a hip rafter?

A lay board is a flat piece of broad timber fixed diagonally from the ridge down towards the rafter bottoms.

You would fix all the commons right up to the last full rafter with the ridge in place above all the way up to the peak of the adjoining ridge.

Your lay board top edge then lines up with the ridge down to the bottom of the rafters. If you had an ultra long piece of straight timber and you laid it across the commons, the straight edge should just brush the edge of the layboard as you slide it up and down the commons.

The bottom cut of the jacks is a compound cut that mimics the pitch of the roof and the acute angle of the layboard.

The layboard needs to collect the whole of this cut so it may mean either a broad piece of timber or another fixed along side.