Collar Ties in loft of thatched property

[MacUtopia]

Hi,

We are getting a lot of cracks appearing in vertical corners and across ceiling lath and plaster in our thatched place. Upon looking into the loft space I noticed one of the main A-Frames had cracked and that other areas had moved around 1-2cm.
We had a carpenter around today and he said it looked like a classic case of too much weight on the thatch (due to a build up of moss and the terrible rain over the last few months) and this had caused too much stress to this frame. However we can strengthen this timber no problem, but there also may be the additional issue of spread. He pointed out this spread may have been the thing that actually caused the crack in the first place as there simply wasnt enough strength in the structure. And he noted we didnt have any collars at all, this would help strengthen the whole length and certainly help alleviate any issues due to spread.
Any people care to put in their point of view?

Image of broken frame and view of loft in question included.

 

[Kearn]

What’s the building, history, size, context?
Any external images?
Failed timber looks pretty fragile it’s own right, presumably been subject to a decent amount of soaking in its life when the thatch may have been substandard/building derelict. Odd to have no collars, I assume the logic being its a fairly small roof and the tie beam and PRs are adequate triangulation, but that’s a mere guess. However, your first image appears to have a tie beam at the lowest section, into which your principal rafter is jointed, then the single pegged section near to the crack, I would call that horizontal member a collar, but why in that very low location I don’t know. Possibly a historic attempt to stop the flanks spreading and pull them inwards by adding a crude collar - collars are in tension. I say crude, as it doesn’t appear to be jointed, merely lapped over behind the PR, which is why I’d think a later repair than original build. However, I’d expect the peg to fail before the PR.

Equally, depending on what it was originally built for it may have just been the easiest option to throw something up.
Our house became a dwelling in the 1600s, but was built as a large 5 bay, 2.5 storey wool barn - a big tall structure, however, it was built without any cross frames and only has teasel tenon joints at the four corners instead of every bay. It shouldn’t really still be standing, maybe it wasn’t designed to last back then.
All we have at our intermediate cross frames is a tie between eaves plates, dovetail lapped over the eaves - these always shrink as the green oak dries, but only in one direction of the the grain so the joint loosens and allows the eaves plates to pull outwards.

Which of the PRs has cracked? I ask, as the left side of the roof has been altered from original to give a shallower pitch than the usual 55 for thatch and the original triangular roof truss is no longer taking the load as designed. Equally, depending on how that arrangement now meets the wall plate, chances are its load may not be transferred vertically any longer and could be trying to roll your wall outwards.

Personally I wouldn’t buy the ‘wet thatch being too heavy’ argument - most thatched roofs are now tiled which massively increases their load, often without failure. And that’s before the extra load of insulation and boarding/plaster internally.
Maybe the fact that it is now lying in a more shallow plane than it should be means more rain is being absorbed and held rather than shed. But that’s me clutching at straws.

I’d start by understanding the structural changes to the original roof structure. Why did they happen? Were they ever structurally sound and transferring load correctly? If they weren’t, then this was eventually likely to happen. If they were, then something else is likely to have changed to cause this more recently. But from the limited pics with no context, it looks like historic changes, failures and attempts to remedy may have run their course.

But then, your framer has presumably been up in the roof and should be coming to his own evolutionary conclusions. Differing joint styles, ways of cutting timber, carpenters marks, actually species used - these are all part of reading the changes over time. If your framer can’t read and explain the roof, then I’d find another or persevere as is, but you will need to bring in a structural engineer versed in timber to direct the repairs. Internally, you can get away with metal work if you so desire, which can hold the line on the structure and add support where it is historically or currently missing, without major intervention to the historic structure.

 

[jamest]

It looks to me as if there's a ridge beam there, not a very big one though, perhaps I'm just not able to see it properly. Ridge beam roofs don't spread unless the ridge beam sags, because the rafters tend to swing inwards from the ridge, not outwards. If the ridge beam is undersized (which it looks as if it might be) or is sagging (which I can't tell), then collar ties will help. Obviously, if the supporting A frame members crack, there is a major risk of ridge beam sag (or collapse) and consequently of roof spread. Anyway, can't really see if there's definitely a structural ridge beam there, so any comment is speculation!

 

[MacUtopia]

What’s the building, history, size, context?
Any external images?
Failed timber looks pretty fragile it’s own right, presumably been subject to a decent amount of soaking in its life when the thatch may have been substandard/building derelict. Odd to have no collars, I assume the logic being its a fairly small roof and the tie beam and PRs are adequate triangulation, but that’s a mere guess. However, your first image appears to have a tie beam at the lowest section, into which your principal rafter is jointed, then the single pegged section near to the crack, I would call that horizontal member a collar, but why in that very low location I don’t know. Possibly a historic attempt to stop the flanks spreading and pull them inwards by adding a crude collar - collars are in tension. I say crude, as it doesn’t appear to be jointed, merely lapped over behind the PR, which is why I’d think a later repair than original build. However, I’d expect the peg to fail before the PR.

Equally, depending on what it was originally built for it may have just been the easiest option to throw something up.
Our house became a dwelling in the 1600s, but was built as a large 5 bay, 2.5 storey wool barn - a big tall structure, however, it was built without any cross frames and only has teasel tenon joints at the four corners instead of every bay. It shouldn’t really still be standing, maybe it wasn’t designed to last back then.
All we have at our intermediate cross frames is a tie between eaves plates, dovetail lapped over the eaves - these always shrink as the green oak dries, but only in one direction of the the grain so the joint loosens and allows the eaves plates to pull outwards.

Which of the PRs has cracked? I ask, as the left side of the roof has been altered from original to give a shallower pitch than the usual 55 for thatch and the original triangular roof truss is no longer taking the load as designed. Equally, depending on how that arrangement now meets the wall plate, chances are its load may not be transferred vertically any longer and could be trying to roll your wall outwards.

Personally I wouldn’t buy the ‘wet thatch being too heavy’ argument - most thatched roofs are now tiled which massively increases their load, often without failure. And that’s before the extra load of insulation and boarding/plaster internally.
Maybe the fact that it is now lying in a more shallow plane than it should be means more rain is being absorbed and held rather than shed. But that’s me clutching at straws.

I’d start by understanding the structural changes to the original roof structure. Why did they happen? Were they ever structurally sound and transferring load correctly? If they weren’t, then this was eventually likely to happen. If they were, then something else is likely to have changed to cause this more recently. But from the limited pics with no context, it looks like historic changes, failures and attempts to remedy may have run their course.

But then, your framer has presumably been up in the roof and should be coming to his own evolutionary conclusions. Differing joint styles, ways of cutting timber, carpenters marks, actually species used - these are all part of reading the changes over time. If your framer can’t read and explain the roof, then I’d find another or persevere as is, but you will need to bring in a structural engineer versed in timber to direct the repairs. Internally, you can get away with metal work if you so desire, which can hold the line on the structure and add support where it is historically or currently missing, without major intervention to the historic structure.

Thanks for the reply.

The main and oldest part of the building is circa 1500 and is formerly an Tudor Open Hall House. This is the part thatched and it GII listed.

I think there has indeed been many adjustments and as you point out on the now marked up attacehd photo, the pitch change may very well have caused an issue two fold:
1. The lesser pitch means water takes longer to run off, hence the now buildup of moss, as opposed to any other part of the house.
2. With the pitch shallower on the left side, does this then mean it's potentially putting more strain on the right side as they are not ballanced?

If this is the case, then surely collaring would assist in strengthening this whole section. And as also pointed out, the ridge beam is pretty puny. Further down in the next bay, the ridge beam is much more substantial. So I suppose we could add an additional ridge beam too, but I would have to be cautious as to not add too much additional weight.

I'm going to check where the rafters ar poking out under the eaves tomorrow to see if we have issues externally.