Roof structure question - perpendicular ceiling joists

[kirky1176]

Hi. My house is a detached, rectangular property with what I think is called a truncated hip roof. Effectively, all four sides of the roof slope towards each other, meeting a large, rectangular flat section instead of meeting at an apex.

The house was built in the 1950s and then extended before we purchased in 2007. As part of the extension work, the previous owners replaced most of the roof, but left some of the old roof intact. See pictures below of what they did.

I've never been terribly happy with this, but it hasn't caused any problems for the past 18 years so haven't worried about it. Anyhow, directly beneath where the old section meets the new section of roof is my sons bedroom and over the past 12 months, a crack at the junction of the wall and ceiling has appeared and gradually gotten worse. Not terrible, but there's clearly something going on.

I had a local roofer come to have a look and his concern was the unsupported cut-off purlin. He also didn't like the old 4" x 2" timbers and has suggested a remediation plan of strengthening all of the old timbers by bolting new timbers to them, essentially doubling up each rafter. This all sounds sensible to me, but having had another look at the roof, I remain concerned. In the second picture, you can see that the ceiling joists run perpendicular to the rafters and therein't any visible tie between the ends of the old rafters, and the ceiling joists. I'm no expert, but I'd imaging that this could be a partial cause of the cracking? Interestingly, even some of the new rafters are also not tied to joists, although most do as the builders installed new joists to tie everything together.

Bottom line is, I'm looking for some advice on what I should do. Would simply doubling up the old rafters help? Is it a problem that the (old) rafters aren't tied to anything at their base?

Appreciate any views. Thanks!

 

[ree]

OP,
You might have something called a Snub Hip roof?

The roof light should have been trimmed with doubled trimming rafters.
Cutting in the roof light seems to have caused possible structural problems?
The goal post prop appears to be recent?
Pics showing the outside of the roof planes would help?
If you cleared away the insulation & other obstacles in the loft, and then took some shots of your concerns, & up where the rafters meet at the hip then that would also help?

Its possible that the new roof construction sort of ended at the beginnings of the old hips because whoever tried to do it didn't understand roof framing?

 

[catlad]

Not easy to make sense of those pictures, in the first picture, the new props land a lot closer than the old purlin props, which begs the question, why would the original carpenters not have used that option? The second picture of the flying purlin! why was that even cut out?

 

[Dereekoo]

Well structurally by not supporting the new rafters with the purlin basically means, everything being equal, the bending moment and hence stresses in the wood are 4 times greater than they originally were and no doubt the deflection is somewhat greater. Having said that there are several safety factors used in timber design so its not the be all and end all but bear in mind if you ever get that snow storm that deposits the standard snow load of 75kg/m2 things may happen. (of course this is current building regs what they designed for in 1950s????) Replace/add to that that purlin. Re rafters and ceiling joists not being tied together, this is a fundamental part of ensuring the stability of any sloping roof unless somehow the original designers deigned that the flat roof section acted as some sort of roof beam to take the loads from the rafters which as it has stood for plus years may well be the case but it is difficult to visualise how that can be. In the short term try and tie the new rafters into the ceiling joists and make good that purlin and any braces that it had. The spragging around the roof light area should prevent any issues with not doubling up the joists

 

[ree]

Poster #4,
google tells me that "spragging" is a sometimes used word for sagging.
How would sagging "around the roof light area prevent any issues with not doubling up the joists"?
The goal post has been installed precisely to deal with the effects of cutting in the roof light.
All trimming requires doubled timbers.

 

[kirky1176]

Thanks for the replies, and apologies that it might not be too clear from the pictures what's going on! It's hard enough to fathom, and I can see it first hand! I managed to find a 'before' picture of the house before the extension. The 'old' section of the roof is the eastern slope of the roof you can see in the eastern elevation. So I think the gap in the rafters covered by the white(ish) material is where the old chimney came up.

Below is roughly what it looks like now. However, a couple of notes. There's no chimney at all, the previous owners didn't install one. And the roofline isn't flared as is suggested by these images. But it's otherwise what the house now looks like.

From looking at the Northern Elevation, the ceiling joists are all new and run north to south and are bolted to the end of each rafter from about the front door to the western side of the house. Beyond that, they run east to west.

 

[Dereekoo]

google tells me that "spragging" is a sometimes used word for sagging.

Suggest you re visit google and try and get normal definition of sprag

The goal post has been installed precisely to deal with the effects of cutting in the roof light.

I thought I said that what the spragged plank is for

All trimming requires doubled timbers.

Not if you support rafters (thus reducing effective span and hence bending moments and hence timber stresses) with secondary purlin spragged off underlying supports. But if if short of structural nous double up timbers and forget about spragging

 

[ree]

Poster #7,
"But if if short of structural nous" reads like you are a bit short of nous yourself?
Just saying FWIW but you do seem to be having a bit of difficulty with your sentence construction - "that what" "deigned" "????"

My google gave your word "spragging" as another word for sagging - it now seems that you wish to change the playing field?

However, the OP wishes to change things because of possible structural faults/cracking - doing what the roofer & myself suggested are necessary & might help.
Clear views of the wall plates will also help with the OP's other issues - & also with investigating any roof spread.
There are simple ways for supporting any & all purlins - more advice after my above.

 

[Dereekoo]

"that what"

lost me there, please explain in detail

"deigned"

Guilty as charged, profound apologies

My google gave your word "spragging" as another word for sagging - it now seems that you wish to change the playing field?

No change (have you a you have a thing about playing fields and goalposts) spragging is spragging as any northern builder will know without asking Google, please visit the Oxford Dictionary

 

[kirky1176]

Poster #9,
1. read your own post - you wrote the muddled sentence for what its worth?
2. How camp.
3a. Goalpost is a constant term used by experienced builders especially when erecting steel frame goalposts - but hopefully you will use your nous to understand why it was used?
3b. Level playing fields is a simple cliche to help simple minded people to understand simple things
3c. I once worked across northern UK - never heard the word.

The OP might be baffled by your spragging & OED's so maybe its time to return to the OP?

Thanks, Ree Appreciate the passion over language and terminology here folks, but would appreciate any further comments on the structure.

I'll see if I can get some pictures of the wall plates where the rafters aren't tied to ceiling joists. I can see at the new end of the roof, the builder has used what I think are called jack joists(?) to attach to the ends of the jack rafters(?), so this all appears to be as expected. It's just where the new meets the old where concern is.

Are there standard methods of fixing rafters to the wall plate to resist the lateral thrust when there isn't a ceiling joist to attach to? In the second of my two pictures (I've reposted below with a markup), there is a timber member running across the roof. Now, it's not attached to anything, but there is a second one under all of the insulation which is attached to one of the old rafters. Would these have been an attempt to resist spread in the old roof?

 

[Dereekoo]

Poster #9,
1. read your own post - you wrote the muddled sentence for what its worth?

Still cannot find those 2 words together, please help

2. How camp.

Nice of you to say so sweetie

3a. Goalpost is a constant term used by experienced builders especially when erecting steel frame goalposts - but hopefully you will use your nous to understand why it was used?

And I thought it was one used when they erected a bit of scaffolding and bunting to warn of over head cables

3b. Level playing fields is a simple cliche to help simple minded people to understand simple things

Thats why you use it a lot I assume?

3c. I once worked across northern UK - never heard the word.

Drank with the wrong people sweetie; stick to California

The OP might be baffled by your spragging & OED's so maybe its time to return to the OP?

Possibly as he is a southerner but only he knows but I feel sure he has heard of the OED

so maybe its time to return to the OP?

I guess so and feel sure he is in safe hands

 

[Dereekoo]

Poster #13,
Camp means an exaggerated style - but you choose to reveal your gutter style of sniggering prejudice wrongly believing that, wrong like everything else you have written, it was a term of identity.
While unable to decipher your own posts you even threw in drinking & California as though they carried some significance.

Since I showed how wrong your bewildered assumptions are you've dropped a long way from your earlier hopeful pretence to be seen as an educated writer - pretention & prejudice, so sad.

I dont want to talk with a bigot so I'll leave you to reek & stew in your bigoted ignorance.

Game set set and and match to me me I think

PS definition of sweetie OED - a very pleasant or kind person and what is a dont

I'd best go and make some ash (that will be totally lost on you and most folk to be fair unless they're from up north)