Structural Correction of old roof

[wizbongre]

Hi folks,
We're hopefully going to be completing on a purchase and move to a big old Grade 2 listed house sometime this year. We've had a very extensive heritage survey completed (which was better than expected!) and one of the major issues it has highlighted is for one of the roofs on a 2-storey outrigger from the main house.

Its a pretty standard single ridge roof with a gable end, comprising timber structure and slate tiles. There are a couple of connected issues:

* There is no wall-plate so the rafters are simply resting on the top of the wall
* There is nothing tying the roof to the walls
* There is no cross-bracing to stop the roof from spreading

This has all led to the top of one of the walls being pushed out slightly - its not clear if this is historic or has been caused by some recent repair work to the roof, where the slates have been replaced. Regardless, its high on the list of things for us to rectify.

Our surveyor has suggested we get a structural engineer to design some sort of scheme to rectify the issue by both tying the roof to the walls and providing some structural mechanism to stop further spread. We can then take measures to rectify the spread at the top of the wall.

Any ideas on what options or recommendations might be appropriate in this case? We have access to inside the roof via a loft so should we be able to do something without removing the whole roof?

 

[^woody^]

If this is an old house, then then points mentioned - plate, ties, bracing are modern things that would not have existed at the time it was built. A typical surveyor will most likely proclaim that they 'are not there', but not explain if they should be. Cross bracing for instance relates to trussed rafters, not traditional roofs.

So whether they are actually necessary or not would be something for a structural engineer. More correctly, a structural engineer should advise on the problem and the remedies - which may not be the same things the typical surveyor has mentioned. Specifically, you would want to know if any movement is progressive, or any lack of restraint causing an actual defect.

In context, old houses may have bowing roofs and walls and all sorts of issues that are not perfect, but that does not automatically mean they have problems and require rectification. Likewise for the lack of things more modern houses may have, it does not mean they are needed.

On the flip side, significant restraint works in roofs can be costly, and if listed, may require specific methods of repair. You'd be wise to get this assessed (including if an engineer's recommendation can actually be done in terms of the listing) before your complete.

 

[wizbongre]

Thanks Woody.
To be fair, our surveyor is pretty clued up on old houses and his report pretty much says as you have commented - that the lack of wall plate, ties, etc. is not uncommon and not necessarily an issue given the age and design of the roof. He has commented that some of the rafters already have timber connecting them to the ceiling joists below but they may not be adequate in extreme conditions as they are not uniform across the entire roof. And that the spread may be historic or it may not - as ever he is just covering every possibility.
His recommendation is "rafters are connected by collars as near to the rafter feet as possible...using timber or wire rope".
To me, the collar ties are what you refer to as cross bracing, to tie the bottom ends of the rafters together forming a triangle? Something like the image below:

 

[Notch7]

Whatever you decide to do, it’s best to write to the conservation officer.

The work you describe could be deemed emergency repairs which don’t need consent, but it’s best to let them know - especially if you intent to extend do alterations necessitating a site visit

I’ve been involved in a project in Sussex on a 9 bed mansion built in 1726 and the conservation officers have been over it like a rash

I got that particular project as my carpenter mate knows this conservation surveyor:

Dr Duncan Philips | Listed Building Surveys

listedbuildingsurveys.co.uk

 

[^woody^]

To me, the collar ties are what you refer to as cross bracing,

Cross bracing or diagonal cross bracing would run across a run of trusses to brace them all together. The only equivalent in a traditional rafter roof would be a binder at 90° to ceiling joists.

 

[datarebal]

There is no cross-bracing to stop the roof from spreading, (collars) so where's the ceiling?
As you say wall plates were not always used.
Have any timbers already been removed before you moved in?
It shouldn't be to difficult to sort once the roof is stripped off and everything is properly exposed.

 

[wizbongre]

We, luckily, don't need to extend or alter any of the structure of the house - its one of its appeals. So we're very much in the realms of maintenance and fixing.

I need to get up into the loft and have a proper look as/when we get in and get an engineer in to review beforehand.

@datarebal Hard to tell from what I can see but think the ceiling is attached to joists that are sitting in the wall but NOT directly connected to the roof structure. Also not clear if there is anything that ties ceiling, roof and walls together.
I'm hoping its something we can address from underneath/inside the roof, as the slate covering itself has not long been replaced (in fact, our surveyor question if this has contributed to the wall spread.)

 

[kp00110]

maybe some pole plate type timbers laid in the corners to tie the ceiling joists and rafters together? so get your triangle that way?

 

[datarebal]

Any pictures ?
The ceiling joists may have been bricked around but still could attached to a wall plate that you can't see yet .. obviously guess not having seen the property .