Repairing a Victorian truss

[chuckey]

My stone built Victorian barn has a huge truss across its centre. Because the roof is made from limestone flags, varying from 2" thick at the bottom to 3/4" thick at the top, there are two purlins a side hanging off the truss. The way the purlins were fixed to the truss is that a hole about 2" deep and the width of a purlin was cut through the truss and the end of the purlin (cut down so it was the 2" deep) poked through the hole and fixed with a peg. The area where the purling sat against the truss was relieved by about 3/4", so the majority of the purlin's depth sat inside these steps.
The problem is that over the years the purlins have gone a bit banana shaped pulling the body of the purlin of the step, so leaving it hanging of the top 2" or so, so some of the purlins have split along their length at this point.
The problem is compounded by the trusses being sistered, so the hole in the truss is actually about 8" long by 2" deep in a 14" X 4" section of the truss in two places each side.
What I was thinking of doing was to have some proper steel joist hangers for the purlins. But to fill this hole with mild steel that is "glued" in to add a bit of strength back into the beam, seems OTT, but I doubt if a piece of wood could be made to fit any better. As the hole is towards the top of the truss, the "joint" would be in compression, so I suppose that could help. Looks don't matter. Any ideas?
Frank

 

[blackrodd]

Hello and welcome, Some pics would be best as you're question is a bit involved.
Regards Rodders.

 

[Limey Lurker]

First glance at the heading I thought, "Blimey, surely he could get a modern one on the National Health".

 

[chuckey]

Pictures, :-

Frank

 

[blackrodd]

Nice pics! one of the complete A frame front would possibly be helpful.
These "A" frames are usually 6 feet apart, and are designed as one way to support the purlins
As you probably realise, if you were able to "straighten" the purlins and roof in general, then the rag slate wouldn't fit any more.
So as you are aware the best solution would be to consolidate what you have, this has been started by whoever cut and fixed the white painted compression "Strut" under the purlin in the first pic.
It appears, in the slight background that pic, this "Strut" suport is not fixed to that "A" frame.
I suggest you use an acrow prop, and a tilting "fillet" so the acrow is on the same plane and angle as the roof, and gently take the tension off the "A"frame, about 4,or 5mm and cut fix a compression strut, carefully cut to the correct angles, screw or nail in place, do all of them, and make sure you use really DRY timber we don't want any shrinkage!
This method will also raise the split purlins at each end and I would be inclined to get some really large gang nail plates, such as made and sold by EXPAMET, these are about 10"x6" and full of holes for you to nail up with 50mm galvanised nails 1 each side!
I've repaired several "A" frames this way and had good results.If they're too long, bend in a vice and "hook" over the top, or get some made.
Be careful with the acrows, nail the tops and drop a prop between the base plate and closest wall to help stop any
"unscheduled" movement, and have some "look outs" watching for any problems, slates "cocking", slates coming adrift, parked cars peoples gardens etc, etc.
HTH Regards Rodders

 

[Billy Flitch]

Hi Frank, what a difference a pic makes, you lost me in the first post but with the pic`s I can see what your talking about.
For future reference if it helps you at all that joint is called a wedged tusk tenon. I think who ever did the repair job to hold every thing in position did a first class job, the strut supports the the truss and the soldiers supports the purlins. The foot of the strut is secured by a batten and the head by a block. I like it, he has every thing covered but nothing in stress.It looks like it would be an excellent repair for the rest of the purlins.

Now back to the joint, the weight of the purlins and any thing that's sits on them is not designed to be taken by the tenon underneath the tenon is a notch cut out of the end of the purlin this is the tusk and the tusk sits on a ledge cut into the truss and it seams the purlin has moved out of the joint and the tusk come away from the ledge placing any weight on the tenon and causing a stress fracture under the tenon. OK at the moment the purlin will not fall down because the soldier is supporting it. If I have got it right what is worrying you at the moment is the notch in the truss that is unsupported by the purlin moving out.

So I have to qualify any thing I say now by the fact that I´m here and you are the man at the sharp end, firstI think that there is more of the purlin in the joint than you think but why if the purlin is moving is the wedge still in place in the tenon?
Is it possible to pull the purlin back into the joint, yes but what would be happening at the other end of the purlin does it sit in the gable or is there another truss there? Would this cause damage to the roof covering ?The possibility of just repeating the repair with the strut to the other three purlins still remains open.