cracked purlin structural failure

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Hello DIYers,

3 bedroom mid sized terrace with slated, relatively shallow pitches circa 1900, 2 purlins per pitch. One purlin has been bowed for many years and seemed stable, now a severe crack both vertically at the lowest point of the purlin and also longitudinally has appeared.
Ridge has cracked and there is roof sagging at the ridge and at the cracked purlin.
The purlin depicted is the upper of the two on this pitch.
My intention is to remove the roof entirely and replace all structural elements, 4 purlins, ridge, all rafters, slate battens, new felt, lead, warm roof insulation, re-use slates.

Q. I understand that a NFRC competent roofer is able to specify the dimensions of the structural elements without relying on external input (costs) Is this correct ?

Q. Is it mandatory to replace timber in such a case for steel RSJ ?

Q. If not what are the pros and cons of timber over steel (specifically wrt to loss of "head height" under the RSJ )

Q. I believe this work is urgent and so a temporary roof will be needed, How long do you estimate it would take to build the scaffolding 1 day or 3-4 days ?

Thanks in anticipation,
KW
 

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1. Yes if he knows about timber, not just tiles.
2. No
3. N/a
4. Depends on the job and the number of blokes.
 
Do you definitely have slates, not tiles, bearing down on the roof framing?
Maybe, to give more room space and head height, you've removed previous supporting braces and collars in the loft?
 
Maybe there's dry rot ?:rolleyes: Maybe the o.p knows the difference between a tile and a slate ted? Maybe a previous owner removed part of the roof structure.
 
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Thanks for your responses.
.@tel yes i have slates. welsh ones.
I am not sure what you refer to with "you've removed previous supporting braces" the question regarding head height was essentially is an RSJ replacement for timber dimensionally bigger than, smaller than or the same size as the timber it replaces. If it is significantly deeper than timber then the amount of steel protruding into the roof void would effectively reduce the head height. I mean here really the ability to store becomes more restricted if one has to crawl under the purlin rather than stoop under it.
The original roof had thin struts attached to the rafters which supported the frame from which the bedroom ceilings were attached. Since then a load bearing loft floor is installed so the struts are removed
@nige i do not believe there is dry rot, yes i know what tiles and slates are, the roof structure is original.
 
@ woody - the reason I asked about steel vs timber is that one of the quotes I have quotes to use timber the other steel, I thought that perhaps building regs would now require steel if the entire roof structure is replaced.
irrespective of whether it is mandatory or not what do you believe the pros and cons are of the two materials ?
The reason I ask about the scaffolding time is that i have one quote which claims to be able to do the entire job in a week, ie install temp roof remove all roof cover, remove all structural elements replace all structure, replace the covering etc the other quote anticipates closer to 4 weeks. 1 week seems to my untutored eye wildly optimistic. In addition to the main pitched roof at the back is a two storey outrigger kitchen bathroom which also has a pitched slated roof and is include in the quote. I attach another pic to give an idea of the size
 
@ woody - the reason I asked about steel vs timber is that one of the quotes I have quotes to use timber the other steel, I thought that perhaps building regs would now require steel if the entire roof structure is replaced.
irrespective of whether it is mandatory or not what do you believe the pros and cons are of the two materials ?
The reason I ask about the scaffolding time is that i have one quote which claims to be able to do the entire job in a week, ie install temp roof remove all roof cover, remove all structural elements replace all structure, replace the covering etc the other quote anticipates closer to 4 weeks. 1 week seems to my untutored eye wildly optimistic. In addition to the main pitched roof at the back is a two storey outrigger kitchen bathroom which also has a pitched slated roof and is include in the quote. I attach another pic to give an idea of the size
Timber is better as its generally easier to handle and work with.

There is nothing inherently wrong with either timber or steel, just difference in costs, and working with on site - which itself could potentially incur costs. And there is certainly no requirement for one or the other.

In terms of time for scaffold, surely if you are getting a price for the job it does not matter? But yes, one week is optimistic, I'd say two weeks minimum.

Presumably you know that building regs will need to be involved, and they will confirm the roofers selection of timber sizes. It might be best if you ask for the roofer to do this before ordering and installing, rather than wait for the inspection.

BTW have you had the reason for failure investigated? Including checks for any damage/movement to the front and rear walls.

Is it worth considering allowing for a loft conversion if feasible?
 
i asked about the times just as a benchmark, because the scaffolding is a relatively straightforward and calculable action compared to the actual roof work itself, to assess the two widely different timescales
I have been given and 1 week seemed overly optimistic. I do know that building regs will be involved but my understanding is that by using a "Competent Roofer" it was simply a case of informing them that the works will commence and they will inspect at the end. My understanding is that involvement of Council and Structural engineers may add up to 1000 GBP to the price so if he is competent to do the design then why involve the others.
That is one of the reasons for hiring someone recognised by the scheme. I believe the reason is that the timbers are 120yrs old and at some time in its life there has been some subsidence causing additional load on a perhaps imperfect timber. It was bowed when i bought it in 1989 and no concern was raised by the mortgage / survey people. As can be seen from the pic the split is new no dirt in the crack.

I think the walls are easier to assess when the roof is off, unless it is dramatically bowed how does one assess the state of the walls simply from street level ?

By loft conversion you mean making it a habitable space ? I did the loft floor in order to realise some valuable storage space, it has full electrics and heat zone but no radiators. I wanted to use it as a store / hobby room but since then I have been working abroad so no i will not fit windows and go down true conversion route. Perhaps it may be worth considering making the structural amendments to accommodate a window in each pitch but just slating over so that a potential buyer could with minimal effort do a conversion. Or do you mean some kind of dormer ?
 
Maybe there's dry rot ?:rolleyes: Maybe the o.p knows the difference between a tile and a slate ted? Maybe a previous owner removed part of the roof structure

Image ending 531 (ridge is concerning.. whats that white stuff? Just the paper from the underlay or something more sinister?

Load bearing floor ? what about supporting the roof?

BTW have you had the reason for failure investigated? Including checks for any damage/movement to the front and rear walls.

Bit of spread going on? I think I would get it properly checked out.

I do not think I have seen a slate roof with purlins that size in that shape for a long time.
 
i asked about the times just as a benchmark, because the scaffolding is a relatively straightforward and calculable action compared to the actual roof work itself, to assess the two widely different timescales
I have been given and 1 week seemed overly optimistic. I do know that building regs will be involved but my understanding is that by using a "Competent Roofer" it was simply a case of informing them that the works will commence and they will inspect at the end. My understanding is that involvement of Council and Structural engineers may add up to 1000 GBP to the price so if he is competent to do the design then why involve the others.
That is one of the reasons for hiring someone recognised by the scheme. I believe the reason is that the timbers are 120yrs old and at some time in its life there has been some subsidence causing additional load on a perhaps imperfect timber. It was bowed when i bought it in 1989 and no concern was raised by the mortgage / survey people. As can be seen from the pic the split is new no dirt in the crack.

I think the walls are easier to assess when the roof is off, unless it is dramatically bowed how does one assess the state of the walls simply from street level ?

By loft conversion you mean making it a habitable space ? I did the loft floor in order to realise some valuable storage space, it has full electrics and heat zone but no radiators. I wanted to use it as a store / hobby room but since then I have been working abroad so no i will not fit windows and go down true conversion route. Perhaps it may be worth considering making the structural amendments to accommodate a window in each pitch but just slating over so that a potential buyer could with minimal effort do a conversion. Or do you mean some kind of dormer ?
Roofers work with the tiles and felt, not the timber rafters and joists - which is carpentry.

The competent roof scheme is typically for roof coverings and the like not the structure. Some "like for like" replacement is included in the scheme for things such as a rotten joist or two, but I don't think this extends to a complete roof structure replacement - even if it is like for like. I would check that with the roofing federation that administers the scheme, as it may well need a formal building regulation application.

If that purlin has snapped, it could be because of a defect or because it was undersized - in which case it's not a like for like replacement anyway, and you need to be sure that a suitable section of beam/purlin is put back in. That's why it needs assessing by a person who knows about structures.

In the same context, when the purlin snapped and the ridge dropped, something happened to the rafter ends on the wall. They either pushed over the wall or they stayed connected and pushed the wall. So you need to be sure that any required remedial work is done to the wall, and the whole thing suitably braced. Again, a job for someone who knows about structures, not a roofer or anybody just looking for any cracks.

If all of the roof structure is going to be replaced, then it may be worthwhile just increasing the timber sizes slightly so that the loft can be converted at some stage - either now or by someone in the future. That's definitely a selling point. Whether you do a dormer conversion at this time is worth considering ... even if its to say that it's not worth doing. There is a lot of money going into that scaffold and work, so if additional work to create additional value is going to be done, then now is the time.
 
Image ending 531 (ridge is concerning.. whats that white stuff? Just the paper from the underlay or something more sinister?

Load bearing floor ? what about supporting the roof?



Bit of spread going on? I think I would get it properly checked out.

I do not think I have seen a slate roof with purlins that size in that shape for a long time.
Yes its a cracker. The OP should organise some coach tours.
 
@Data - you mean the staining on the rafters, I do not know its on all the rafters on both pitches or do you mean the white area at the top of the pic, that's just a trick of the light it looks like a big hole and the sun is streaming through in reality it was ****ing down when this was taken
Load bearing floor ? - in that the original joists/ceiling support timbers are removed and replaced with i think 5 or 6 inch certified timbers in order to board out.
what about supporting the roof? - can you elaborate ?
As I suggested above the whole roof structure will be replaced, I do not think its worth messing about to save a few bob replace it all and be done with, consequently when it is off spread on the walls can be assessed although the initial assessment is that the walls seem ok and that the wall plate may have moved forward.
@woody I did ask these questions by mail at the competent roofer scheme, never got an answer. I will discuss it with the roofer who does the job.
If work were done to accommodate a conversion later on which timbers would need to be adjusted, are you referring to a dormer or my suggestion to make the apetures in the roof pitches for a sloping (velux) type window and then simply slating over the apetures. I must admit the idea of conversion had not really occurred to me so i have not given it much thought
 
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