Stabilising a Spreading Roof

[malcolm]

I have a barn built around 1850. There is historical spreading of the roof most visible in the outside wall. You can see differential movement between two parts of the building, and between the wall and doors and windows.

I think the doors and windows were installed about 10 years ago when the barn was refurbished. The windows are perfectly vertical but the door frame in the middle of the wall is about 8mm out of true. Assuming it was fitted straight it suggests further movement in the middle of the building.

Some attempt was made to stabilise the roof when the barn was refurbished. The tie beams were screwed to the wall with brackets. One of the brackets has pulled it's fixing out and one tie beam has a crack. Both suggest they are doing too much work. Though there is no cracking in the internal decoration so it might not be moving much at the moment.

I had a structural engineer around. He suggested building a few metal roof trusses and supporting the ridge with a steel beam. That would certainly work but is a little involved.

I can't see any reason why steel gussets attached to the rafters and tie beams with steel tie rods running beside the tie beams wouldn't take the load off the existing tie beams and prevent further spread. Victorians did much the same thing when they were repairing overloaded roofs.

Anyone have any thoughts or experience about this sort of thing?

 

[Zebra]

I can offer little help except to point you in the direction of the following thread, where some similar issues were discussed
http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=13859
Your plan sounds OK but won't Building Control have to OK it?

 

[Lime]

I have a barn built around 1850. There is historical spreading of the roof most visible in the outside wall. You can see differential movement between two parts of the building, and between the wall and doors and windows.
I think the doors and windows were installed about 10 years ago when the barn was refurbished. The windows are perfectly vertical but the door frame in the middle of the wall is about 8mm out of true. Assuming it was fitted straight it suggests further movement in the middle of the building.

Some attempt was made to stabilise the roof when the barn was refurbished. The roof is lined with 12mm ply and the tie beams were screwed to the wall with brackets. One of the brackets has pulled it's fixing out and one tie beam has a crack. Both suggest they are doing too much work. Though there is no cracking in the internal decoration so it might not be moving much at the moment.

My first thoughts.
Most roofs that spread don't have the benefit of construction that includes strong tie beams. Do you know how the roof and tie beams attach to each other?
I wonder what the barn was used for. If it held heavy material that was prone to spreading that might have forced the walls out of true.
Which of the two brick walls is more near the vertical, the one with the window in it or the one with the door?
Adding the 12mm ply may have stabilised twist, warp etc. but the extra weight did nothing to help stop the apparent spread.
I'd want to see what is going on under the tiles where the tie beams are positioned.

 

[malcolm]

I think the barn was once used as a beer cellar. The edges of the door frame and brick corners have become very rounded, presumably after years of beer barrel rolling.

The piece before the window leans the most. That has a rafter sitting on top and has been rebuilt at some point in cement.

What is the best way to get in there to look at the connection between tie beam and roof without causing too much destruction? The roof is felted and battened on the outside, and is plasterboarded on the inside. There is celotex in the cavity between.

 

[88v8]

How are those struts attached to the tie beams? Does the connection suggest that the struts are supposed to support the tie beams in tension, or the rafters in compression?

I think your solution could be a good one. If you go that route, to affix the gussets you'll need to expose the junction of tie and rafter, so may as well go at the exploration from the inside.

However, it looks to me as if those are compression struts, and the tie beams are carrying excess vertical load. Excess, that is, for their cross-section. So I would simply increase the cross-section by thickening them up in the middle. This could be done elegantly by screwing a curve to the bottom, or using the strut arrangement sometimes seen beneath the chassis of Edwardian cars (been looking for a pic, can't find). No need to attached the reinforcement to the rafters, although I would wish to check that there is a sound connection between rafters and tie beams.

Ivor

 

[malcolm]

That was a very good question. It looks like the struts are nailed on. I rather suspect if the whole roof has been assembled with the same level of craftsmanship I might find the cause of the roof spread. I'll wait for a sunny day and pull the lower few layers of tiles off and see what horrors await.

Edit - I used a hole saw to look at how one of the tie beams was joined in. It looks like the inside is plasterboard only, and there are bits of wood each side of the rafter which might have been added to support the joint or to fix the plasterboard. I have the technology to replace plasterboard so will hack away and find the construction.

 

[Penners]

Is it an optical illusion in the top photograph, or are the tie beams sagging in the middle? The way that the struts have been joined to the beams strongly suggests that they were planned as compression members to stop the rafters from sagging, and if the tie beams are indeed sagging, this would seem to support that argument.

Forgive me if this is blindingly obvious, but each of those tie beams should be part of a stable truss structure, with the ends of the ties firmly secured to the lower ends of the rafters. If this were the case, then the weight of the roof on the rafters should place the tie beams in tension, and that tension should be sufficient to stop them from sagging under the weight of the rafter support struts. Each of these trusses should then be mounted on a wall plate, securely attached to the tops of the walls, with bird's-mouth joints in the rafter ends preventing outward movement of the wall plate.

But this seems not to be happening. Something within that truss structure seems to be allowing (a) the walls to spread, and (b) the tie beams to sag (unless this is a camera lens illusion). To me, the most probable cause of this is some sort of failure in the joint between the ends of the tie beams and the bottoms of the rafters, allowing these to pull apart.

Please note that I am NOT an engineer - but for better or worse, that's where I would start looking.

 

[malcolm]

The struts are nearly vertical and I reckon the bend in the tie beams was caused by the ridge dropping as the roof spread. If the struts met in the middle of the beam they could offer some resistance to wind loading and support the rafters a bit, but as they are I think they are mostly decorative. I think you are right - it must be the rafters have a better connection to the wall plate than the tie beams. Which means they aren't joined together properly.

 

[88v8]

It looks like the struts are nailed on.

If the struts are meant to be in compression then nailing is OK, not ideal but OK.
Agree with Penners that the connection betwixt tie beams and rafters needs investigating. If it's just nailed, then some bolts n washers might do the trick.
However, I'd still beef up the tie beams that have sagged as they look somewhat in distress.

Ivor

 

[malcolm]

Some time later I've exposed the join between the tie beam and rafter to find annoying things.

Traditionally this kind of roof would have had a number of thick A-frames at maybe 10 foot spacing, solid perlins connecting them, then thin rafters on top (like the one here: http://www.antiquebuildings.co.uk/sold%20buildings/stonehurst/stonehurst.htm )

Mine has been re-roofed. The thick A-frames have been removed so I'm missing structure. The tie beams have been secured only to the wall plate using steel straps running down from the top of the beams at a 45 degree angle. That's not going to tie the wall plate in. The 3" by 2" rafters are undersized for their current purpose and are nailed to the wall plate.

The roof felt appears to be the non-breathable type yet there is no eves ventilation. That should be sortable by removing the lower few tiles and adding ventilation.

I'm not quite sure what to do just yet. It might be sensible to strip all the plasterboard to see what else has been botched and whether I can add back some structure. I'd rather stabilise than replace the roof as the walls are far from straight and anyone sensible would say don't build on them. Alternatively the wall plate is 3" by 6" and it might be possible to add ties to that.

 

[88v8]

How annoying.
As you say, the straps useless at that angle.

Can you bolt the rafters to the tie beams, even just with coach bolts & plate washers?
And could you double up the rafters with another 3x2" alongside?

That would minimise the disturbance, no need to take the roof off, even though two 3x2s don't equal the 6x2 that might have been there.

Lot of faffing though.

Ivor

 

[malcolm]

I guess I'll need to take the plasterboard off as building regs want thicker insulation which would put the plasterboard on the cold side. That'll make the job bigger than I had hoped but will expose the roof for a better look. I'd imagine some combination of additional steel tie rods, plywood lining on the inside of the roof, and thicker doubled rafters should sort the structure without having to remove tiles.

 

[Penners]

It's difficult to understand the whole structure from your close-up photo, but rather than having to install steel tie rods, I wonder whether the addition of a purlin along each face might achieve your aim. You would still, of course, have to improve the fixing of the joists to the rafters.

 

[88v8]

Mmmm, it's a long run for a purlin, have to be strutted off... the tie beams which are already sagging

In my previous house, the roof was undersize. When the chap built it in 1920, he told us, he was very hard up and used what came to hand. The rafters were 4x2", the joists ditto, and over the intervening 60 years it had sagged.
We fixed that by doubling up the joists (it was a collar roof) and nailing Anbrac metal strapping along the underside of the rafters. The Anbrac being in tension beefed them up adequately, and doubling the joists - well, not quite, I think I used 3x2" to avpid disturbing the plaster - prevented them distorting any more in compression. No further cracking or separation of the plasterwork in the bedrooms 30 years later told me it had worked OK.
Sometimes it takes a long while for undersize scantlings to become apparent.

I reckon, as far as one can see from the pics, that bolting the tie beams to the rafters with half inch coach bolts, and doubling the rafters, would suffice. It would prevent any further sagging of the rafters, and put the tie beams back in tension, instead of the compression they are currently not appreciating.
Depends crucially on the ends of the tie beams being in a fit state to bolt to, of course.

Now, looking back at the original pics, is that some sort of attic up there? What are those struts hanging off? And is there in fact a purlin up there already, where the slope joins the horizontal ceiling? A purlin supported by the 'attic joists' ??
Bit more investigation perhaps.

When you've got the whole picture, you might want to draw one of the trusses in cross-section to help you see how the stresses are and should be distributed.
Best keep Building Control out of it, btw, or they'll have steels in there and heaven knows what.

Ivor

 

[middi]

As already pointed out by other posters, you need to investigate how the roof rafters are attached to the wall plates ( if at all there !!!) and how the tie beams are attached to the wall plates.

Looking at the photos there should not be anything wrong with roof load that may create a "spreading" problem, provided that ;

1) there are wall plates in situ
2) the tie beams are well secured above the wall plate ( this to eliminate side walls spread)
3) roof rafters are secured above wall plate.

I am very surprised that your structural engineer has no explained to you exactly what is at fault with your roof.
After all he is the one that has seen it's method of construction.
good luck

Middi

 

[malcolm]

Thanks all. I've started stripping the plasterboard off. I'll post some photos (and get the structural engineer back in) when the roof is properly exposed. It looks like much of the roof spread occurred before the barn was re-roofed, but the standard of re-roofing was very poor and isn't controlling the spread.

 

[malcolm]

It was well worth stripping off the plasterboard (there is more to remove). There doesn't seem to be a lot holding up the roof.

The principal rafters are missing completely. The bend in the tie beams is due to them trying to hold up the sagging purlin.

The roof structure is mostly the 3 by 2 rafters. The lack of tying in has caused so much roof spread that the original tie beams are not long enough any more. One overlaps the wall plate by an inch at most. Not safe at all.

I'm going to have to be careful with restoration creep (where the job gets bigger all the time). My guess (before consulting anyone sensible) is that it would make a whole lot of sense to strip the roof and rebuild it with traditional oak trusses which don't have bits missing. If I am to keep the original tie beams the top of the walls will need to be closer together. The walls have moved a good 100mm in places and it seems daft not to rebuild them straight and sort the horrible concrete lintels at the same time. Mostly they are still in lime mortar so I should be able to use the original bricks. Gables are reasonable. Does this sound daft?

 

[88v8]

Urrgh, not ideal.

Daft? Not if you will live forever, and have nothing else to do.

Errmm, get some steel plates made to extend the ties if required, then bolt them to the ties and to the rafters. Composite wood/iron roofs figure quite highly in my 1910 Building Construction book, nothing inherently 'wrong' about it.
Leave the walls, they ain't goin no place.

Have the rafters bent?
Screw another 3X2 under each rafter. That will give more room for insulation, and bringing the plasterboard lower will hide the 'join'.

The purlins look too high up the roof, but heyho.
That rickety structure up top could be turned into a cross-brace for the rafters if you intend to retain the current flat top to the ceiling.

Bodge it up, move on. Making this building perfect is [probably] not what you were put on earth for.

Ivor

 

[malcolm]

Thanks Ivor. That sounds much more sensible. I'm really bad at restoration creep when I'm working with cars and it does extend the project timing somewhat.

I suppose if the walls haven't fallen down yet then they won't if they aren't allowed to move further. I could put a few steel tie rods in to keep everything stable then extend the existing tie beams so they sit on something solid and tie them in to the wall plate a bit better. Some new principal rafters to support the purlins would be sensible to stop the tie beams bending further. A little extra to reinforce the rafters wouldn't go amiss and there's not much extra weight involved in any of that compared to the weight of the tiles.

 

[middi]

If expense is not a limiting factor you can certainly install a brand new oak roof but, as suggested by Ivor, you would be better off ( financially, anyway) to stabilise and strengthen the roof with steel straps and rafters added supports at a fraction of the cost of a new roof.
Just ask your structural engineer to help you on this aspect if you are unsure of dimension of components required to achieve a satisfactory roof stability.

Middi