The wall is not flat, it is divided into approximate thirds
__________ _________/ * \_________
with the recess holding the remains of a Jacobean pargeted panel which I am of course very keen to preserve. Unfortunately it has later had a prop plonked right in the middle at position * which is where I came in ....
Anna
The message from snipped-for-privacy@freeukisp.co.uk (Anna Kettle) contains these words:
I am a bit bothered by the notion that the bottom rail of a truss hangs. Now I am more than a bit rusty on the subject but it seems to me that while the tie beam would be in tension all the other components should be in compression.
If you attempt to hang the centre of the ceiling support beam from the apex then, additional rafters not withstanding, you may be putting an unacceptable spreading load on the pair of rafters you tie the load to.
As I said in a previous message if there was never a king post in the construction then the prop you now have could have been put in to provide stability to the slightly curved beam which, without the prop, may have attempted to rotate 180 degrees either during construction or afterwards.
Looking back again at your drawing another possibility crossed my mind. That the prop is there to take most of the weight of the ceiling support beam. It appears from the drawing that the ends of the CSB are attached to the rafters. If there is no other support at those points there is a very real possibility the if you took away the centre support the the CSB wouldn't fail but the rafter attachment or the rafters themselves would.
I think you really need to get an expert in ancient building construction to actually visit.
I am surprised that the the usual suspects haven't pointed out that Building Control should be involved in structual alterations of this nature.
I have been up into the attic and have done a more detailed drawing
-There is no obvious weakness that I can see in any of the connections but they are covered in dust. The older connections are made with ?3inch? handmade iron nails, the 1960 connections with machine made nails. Is there any more I can do to tell if there is weakness apart from eyeballing it? Nothing is moving. I have only inspected the ridge connections from the attic floor
- The inserted ceiling tiebeam is nailed into the old front rafters with at least two old iron spikes. At the back of the house the ceiling tiebeam was probably a bit short because it is nailed to a timber extension, which is nailed to the replacement rafters with old nails (so the replacement has been there for some time)
- In 1960 a new set of rafters was put on top of the old ones and if I go for the 'hanging' option then might make sense to hang the ceiling tiebeam between two of these (the ceiling tiebeam is centrally placed between two new rafters. I cant see the connection between rafters and wallplate
- There is yet another inserted tiebeam at waist height in the attic. I guess that it was put in at the same time as the 'old' rafters as it uses the same old nails and reused timber. It would be possible to put struts between this and the ceiling tiebeam I suppose
- The space between the original tiebeam and the ceiling tiebeam is spanned by six verticals. They are three inch diameter and semi circular in cross section ie half a small tree. Although I expect they give some support to the ceiling tie beam I think their main purpose is to carry the lath and plaster. It would be possible to beef these up with extra timbers at the 1/3 and 2/3 points
- And just so you dont get bored ... there is yet another option. The ceiling tiebeam runs only about one foot from the side of the chimney stack so it might be possible to tie it to the stack somehow?
Anna
The message from snipped-for-privacy@home.co.uk (Anna Kettle) contains these words:
Nothing in this world is certain but ISTM that your safest bet is to go the 1/3 / 2/3 route. Two new supports on a par with the central one and inserted before the central one is cut out should give minimal movement. (But don't blame me if your house comes tumbling down around your ears. Some old structures are still standing only from habit. :-))
Cantilever loads require a firm structure to attach to. I wouldn't be happy using an ancient brick chimney for that.
Dear Anna Before you tie anything to anything let alone try to support a tie beam from the ridge of rafters you need to check and probably beef up the connections at the base of the rafters and at the top. I do not like this idea on practical risk grounds despite its elegance. I think (as a man who makes his living from working with old timber frame houses) that you would be well well advised to bit the bullet and find an engineer who is used to such buildings. Ask in the SPAB for a local engineer that they know. Philip Venning is the Hon Sec and a good man who can direct you to a source within spab that can help. Dont use any old engineer. Ask them how many TF refurbs they have done in the last 5 years and check them out! Chris
Yes I'm starting to think that would be the best way to go too, it changes the supports of the building the least
Anna
OK that idea is ditched
I know a couple of structural engineers who regularly work on old timber frames and I am planning to get one (his name is David Hargreaves, you might know him?) to come and visit to look at another structural problem so maybe the thing to do is to hold fire on this project and ask him for his advice before I start pulling the house around
Thanks for the help you and Roger have given me. I have a much better idea of the issues now which has got to be a good thing
Happy Christmas Anna
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