reinforcing beams

[Anonymous]

I have just bought an 18th Century cottage with ground floor ceiling beams believed to be made from elm. This was renovated in the 70s with as much of the original house retained. Cracks have appeared in the ground floor internal render and further investigation (by removing foorboards upstairs) indicates that two of these beams are deteriorating at the point of entry with the external wall. Possibly- it is only the render that is holding these beams up! What is the normal/ best course for action?
A third beam has a "sag" in it. (probably pre-1970 renovation.) Is there a normal course for reinforcing/ strengthening such a beam?
Julie and Dan

 

[Anonymous]

The only way you are going to sort this one is to acrow the beams and hack off around their bearings to have a look. I have got over such problems in all manner of ways but until you know what is going on there is no way that a solution can be suggested.
Nasty little worms of all sorts seem to really like elm especially in slighty damp conditions. So, if your place was done in the 70s I suspect that cement plasters and or pointing were used. These materials almost always encourage dampness and this may well have accelerated the problem.
For a sagging beam we have often used a 12mm steel plate the same dimensions as the top of the beam. Holes are pre drilled in the sleel at about 400mm centres and then the whole thing resined and coach screwed to the top surface of the beam. There are plenty of other solutions as well but this perhaps gives you an idea of what is possible.
If you want to be really safe and sure (as we do!) then hire a structural engineer who is familiar with traditional buildings. Mike.

 

[Anonymous]

Not so long ago I repaired some oak beams by having heavy duty hangers made for the ends, bolted to the walls. Worked very well. Important to remove the decayed ends though.

 

[Anonymous]

As Mike said, get an engineer to take a look - but also have a look at the render on the inside and outside of the building. If it's cement-based, it will have tended to 'focus' damp on the holes where the beams rest, accelerating the decay. If that's the case, get it off and let the walls breathe with some lime-based render.
We had a similar problem in our place, and solved it with 8mm steel fishplates to reinforce a scarf joining new beam-ends to the old beams. And we soaked the new timber in nasty chemicals first to prevent any further rot and kill off an spores that might be lurking in the holes (though, had I known more at the time, I would have used a borax-based preservative).

 

[Anonymous]

There are lots of ways of repairing what you describe with the appropriate one depending on the extent of degradation the beams have suffered. Most of them have been mentioned and generally invlolve providing new support with a steel hanger or by reinforcing a longer length of the beam with a steel plate.

Also also as pointed out by others it is important to address the source of the moisture as well so you are not just curing the symptons. If a relatively vapour impermeable render has been applied inside then this could be an issue.

From what you describe you should have an appropriately experienced and qualified person inspect (this probably means a structural engineer with experience of older buildings). A certain amount of opening up (eg: removal of ceiling/finishes) will be required for this.

Regards

Toby Maclean

 

[Anonymous]

Re: Julie's sagging beams - I seem to remember being told that there are tables that give values for the permissible sag in beams (depending on length, cross section, type of timber etc. ) Anyone throw any light on this ?

 

[Anonymous]

A long answer to a short question I'm afraid:

There are indeed limits given in British Standards for permissible deflections in beams (generally maximum deflection (under full working load) = 0.003L where L is the clear span of the beam). This is limited to a maximum of 14mm to avoid possible issues with vibration etc. Note that the full working load that should be designed for is rarely, if ever seen in practice and so deflections would normally be less than this figure.

However, this only applies to beams that are in good condition and appropriately installed. That is deflection due to the bending of the beam itself. If the deflection is due to defective support conditions or caused by deterioration of the beam then this guide is not applicable.

Actually how much a beam deflects is, in itself, not important. As long as it is strong enough; the defections do nor cause problems with cracking of other finishes; the dynamic repsonse (vibration under footfall) is acceptable; and the whole thing isn't too alarming when you walk on it.

In Julie's case, the beams with concerns about the condition of the ends should be investigated and repaired as necessary. The third "sagging" beam may or may not be a problem depending on whether it meets the criteria above.

TM

 

[Anonymous]

The stated sag limits are to do with legal requirements for new builds, and have nothing at all to do with structural integrity, safety, or period buildings. The acceptable sags for period buildings are very much greater.

NT

 

[Anonymous]

NT: I agree - as per my third paragraph. Deflections, per se, are actually irrelevant unless they are a sympton of something more serious, or are causing a problem because your plaster keeps cracking.

The deflection limits for design however answer John F's question.

Toby

 

[Anonymous]

Point taken, I thought you were answering the OP

NT