Rot or Fire?

[Flyfisher]

At first glance this will appear to be a daft question (indeed, it may turn out to be a daft question).

I'm repairing an outbuilding roof that's in poor condition and has been leaking in some places for quite a while and, unsurprisingly, has caused the roof timbers to decay in a few places. Not too difficult a job to actually brace/replace the affected timbers, but all the rigmarole of working on a roof.

Anyway, after stripping a section of pantiles, I've found that a number of the timbers look as if they've been in a fire - as per the pictures below - except this doesn't really make any sense because the charred look is very localised with adjacent rafters being unaffected. There is no burnt smell that I can detect.

So my question for the wood rot experts out there (Matt? :wink: ) is, could this damage have been caused by some form of wood rot following years of localised water ingress? I seem to recall photos of wood rot with a similar physical patterning, but nothing that has also turned the wood black.

Any ideas?

 

[Matt Green]

It's charring rather than rot but it is an interesting situation. The reason it looks a bit like (brown) rot is that the processes have a few similarities. Brown rots preferentially remove the cellulose and hemicellulose from wood leaving a brittle matrix of lignin behind that snaps across the grain when the wood dries. Heat has a similar effect: the cellulose burns out first leaving the less flammable lignin that contracts and snaps giving a similar looking cross cracking appearance.

So why would it be burned in such a localised way? It could be deliberate. Taking a blow torch to rotten timbers, particularly those afflicted by Serpula lacrymans (dry rot) was a known/proscribed way of dealing with the problem. Extreme as it may sound it dries the timber rapidly, kills the fungus and leaves the timber not much worse off and a bit more resistant to future attacks owing to the fact that a lot of the easily available nutrients have been burned away.

Just a guess but it seems plausible.


Matt

 

[Flyfisher]

Thanks for that Matt - it makes a lot of sense. There are quite a few earlier repairs to the roof surrounding the charred rafters I've found so far.

I'd never heard of the blowtorch treatment but it would certainly explain the localised charring - it couldn't possibly have been a roof fire. So it makes perfect sense that the previous repairers used a blowtorch to 'treat' the affected rafters as well as fixing a new one alongside.

I'll try to take another photo of the exposed part of the roof tomorrow to give a better idea of the localised charring in situ, but I reckon you've solved the mystery so case closed.

Many thanks!

 

[MdB]

I've heard of fence posts being deliberately charred before being inserted into the ground to improve their resistance to rot - likely for the reasons Matt said. This doesn't sound appropriate for your roof but does link in.

 

[FamilyWiggs]

Ah. That might explain some charred timbers I have. Didn't look like a "proper" fire. Thanks.

 

[CliffordPope]

Could they be salvaged timbers from another building, used to repair rotten wood in yours?

 

[andrew gayton]

if there are renewed timbers around this charred timber, could it not be that the worst tibers were removed and the sound charred ones left in place following a fire? is this an old laundry room?

 

[Flyfisher]

Here's a photo of the rafters in situ. I can't imagine how a fire in the roof could have caused this burning because it looks as if the fire has been applied from above rather than from below, which is what I'd expect if there had been a roof fire.

The outbuilding was once used as part of a chicken hatchery, so a fire is certainly not out of the question, but again it doesn't look convincing to me (not that I have any experience of roof fires - thankfully!).

I wondered about CliffordPope's point about the burned rafters being reclaimed but I find that hard to believe - there are not enough of them to have made the savings (if any) worthwhile, I'd have thought.

If I were a betting man, my money would be on Matt's blowtorch suggestion. It's the sort of charring that a blowtorch would inflict, it seems to have been applied from above and repairs are adjacent to the charring so it all ties in with some long-ago roof maintenance.

 

[Penners]

In our last house, when I was renovating an elderly and poorly constructed shower, I removed the tiles from the masonry wall and discovered some traces of rot on the surface of the plaster behind. Being unsure of the type of rot, I reckoned that death by fire was probably the safest solution - so I applied a blowtorch flame liberally to the surface of the plaster over the affected area and beyond. It was done on a hunch, but it's interesting to know that the technique is a recognised one.