Roof structure advice

[madworthy]

Opinions please. We are buying an old farmhouse in Devon. That bit sounds wonderful, unfortunately it has been “maintained” but not loved. So lots of concrete render, modern plaster, polystyrene sheet and emulsion paint over cob. The damp I can understand and see ways forward, thanks to lurking on this forum.

Now for the area that is causing me some concern. The roof structure. As you would expect the house has been extended and altered over time, but in more recent times there have been major repairs (bodges I feel). I think the pictures will explain my concerns better then words. I’d better say the external roof covering is currently a mix of modern fake slate and, probably asbestos, fake slate. ( I do have more pics of the roof showing some of the lovely intact A-frames as well)

 

[Lime]

Well it's a bit late to do much about it, isn't it?
Or are you suggesting the structure is now unsafe?

The damage/repair/replacement has happened and, if it is safe, any retro re-installing of old timbers will only cause further problems and more work.
It depends on how much it bothers you.
If it were mine and I wanted to replace the tiles I might be be tempted to replace the new timber for something more inkeeping with the old roof.
Cost and the availability of old/appropriate timber would be major considerations in my decision.

 

[AMc]

I'm no expert but that work looks quite recent.
Can the vendor provide details inc. structural engineers reports for the timber used?
Can the vendor prove building control signed off on it?

 

[CliffordPope]

The old timbers in those pictures form a structure exactly like that in my cottage, and appear to me to be similarly designed to take a thatch roof not slate. The purlins look rather insubstantial to be supporting those secondary rafters.

 

[Lime]

I agree with AMc & CliffordPope.

 

[middi]

The " repairs" seem ok, albeit it appears to be done with softwood rafters of smaller dimensions that I would have prefered. This should not be to much of a problem though, as long as the purlings are sound and properly supported the load from the tiles will be spread more evenly in the centre of the roof.

That black roof felt will tend to condense water vapour on the inner face if adequate air circulation and ventilation is not provided within the roof void. If any insulation has to be done, make sure it is as thick as possible ( up to 30cm) and spread across the top of the upper floor joists.
The reason for this thickness is to counteract the ventilation that is needed above the insulation in the roof void therefore avoiding condensation droplets dripping in the loft.

Middi

 

[madworthy]

Thanks for your opinions so far, Midi I had forgotten to check the felt and you are right it is too tight. Thanks for the reassurance. As for building control sign off AMc , I suspect that the previous owners view was "it's our house we'll do what we want and not tell anyone".

The plan is to replace the covering, eventually, we both like the idea of returning the house to thatch which it was once, so it is nice to here CliffordPope that is how your structure looks. We will make that decision closer to the time, and probably seeking more advise here. Lots of other jobs first.

The previous pictures do show the least attractive part of the structure. Some of the A-frames are gorgeous with wonderful pegged joints. Also what looks like wattle and daub panels between original rafters in some places. here are a couple more pictures.

 

[andrew gayton]

it would appear to me that the new 2 x 4 (100 x 50) common rafters have been laid on thier back as opposed to thier edge, so they are therefore in a manner that will allow for an increased deflection - this was probably done to ease nailing to the purlin (prior to the 19th c, most timbers were laid on thier back, but the overall sectional size was generally greater. laying on the edge allowed for the use of timbers which were smaller in section). laying the commons this way need not be such a bad thing if they are at close enough centres and the purlin support is adequate. i would have advocated the retention of the original 'failed' commons and simply partnering up with the new common rafters and maybe adding an additional tier of purin support. the use of modern cut timber against the historic carpentry is fine and indeed an 'honest' approach to the repair of the roof carpentry. the use of underslaters felt is ok provided the space is ventilated over the wall plate and over the insulation to avoid condensation. if this is not possible, use a batten at the felt lap to allow ventilation or cut horizontally at the ridge piece.

 

[Roger H]

Hi Andrew, when you say 'cut horizontally at the ridge' are you meaning cut through the sarking? I've an issue with lack of ventilation and was pondering more intrusive ventilation methods. is cutting the sarking an option? (sorry for marginal hijack...)

 

[andrew gayton]

yes; cutting the sarking felt at the ridge board will allow an increase in ventilation as will 'gapping' the sarking overlay with a section of batten sawn through to form a wooden peg to slip onto the inner section of felt.

case study! - a neighbour asked me to look at thier roof as a builder has suggested that it was leaking and needed resarking. I inspected it and noticed that the insulation had been topped up over the ceiling (a cold roof situation) with no continuous ventilation over the insulation at the wallplate. as a result, the roof was literally running with condensation. I simply cut the felt at the ridgeboard all the way along between each rafter - remember that this will not leak as it has the ridge tile over it - and the ventilation provided by this was enough to allow the roof to dry and remain ventilated.

this is the only option in certain situations where the existing carpentry will not allow the use of a tray retainer or where the eaves detail cannot be adapted (aesthetically or practically) to allow a continuous 10mm gap with 50mm over the insulation.

 

[Roger H]

Andrew,

That is exactly my situation - wet sarking following insulation top-up. I'll try that. My additional plan is to insert a 'owl hole' in a gable with some mesh insect protection to get some additional air in, but your solution will hopefully get me the through-ventilation I need. Useful.

 

[agbagb]

yes; cutting the sarking felt at the ridge board will allow an increase in ventilation as will 'gapping' the sarking overlay with a section of batten sawn through to form a wooden peg to slip onto the inner section of felt.

It's good to know I'm knot the first to do similar, I used 4" bits of 15mm copper pipe. It certainly makes a difference, extra blanket time as we've been sleeping the attic room, brrr.
I had thought about cutting the sarking all the way down, a couple of inches either side of the rafters and then stapling in breathable felt. But the pipe or wedges is an easier option.

madworthy, you've got some lovely old timbers there but it looks like it'll need carefull assesment and reinforcing before replacing the fakes with potentially heavier real slates.

 

[Penners]

Madworthy - I'd go along with everything that's been said above. I've seen roof structures that are considerably worse than yours, and yet still doing their job!

The only things that I particularly notice are those new, overlapped rafters (your photo shows two, but maybe there are more?). That really is a lazy lash-up. However, the joins have the support of the purlins, and just as long as you're happy that the purlins are sufficiently strong and/or well supported, this should be OK. However, if/when you do strip the roof to revert to thatch, I would replace those with full-length rafters. I would also let the thatcher have a look at the structure (he'll want to anyway) and take his advice on any other repairs.

 

[madworthy]

Penners, thanks for the comments. There are more then two overlap rafters these were probably my biggest worry, the purlins do have some extra support, but not a lot. I'll aim to install additional "temporary" bracing, until the roof is stripped, as it is mostly dry bar a couple of ridge tiles need reseating. (I do like the term temporary as it does cover a multitude of sins)

 

[andrew gayton]

regarding the lapped common rafters -

one has a cut for sitting on the plate? so, it is either someone 'using up' off-cuts, or the rafters have been lapped here to form a 'new' deflection to marry in with the remainder of the carpentry. imagine, in section, the 'chinese hat' of the existing settled roof, then lining through a new straight timber from wall plate to ridge....... it can often simply not work, hence the use of two timbers lapped at the purlin. as mentioned, i would look to put in another 'partner' (or 'cripple' if you read older sources) puring and look at the c/c of the commons. if the space is not to be used then the addition of some additional ties - to effectively form A frames - would do no harm.

panic not.