Possible dry rot to roof timbers... or wet?

[AxPop]

I'm in the process of buying a property and the surveyor's report has highlighted the possibility of dry rot to a number of rafters and purlins (he couldn't get access close enough to check properly). This roof forms part of a "newer" extension dating from around 1820-40 to the house. He advised me to get a damp specialist to investigate further, however having been stung by cowboy dpc companies long ago, I'm hesitant to jump straight in without thinking it through first.

The timbers do appear to have some white residue - although whether it's wet (white) or dry rot I'm unsure. It also looks like someone's had a fight with the insulation, I wonder if this mess has covered the high-level air bricks causing poor ventilation up there. Elsewhere there doesn't appear to be any evidence of damp or leaks in the rooms below. I'd welcome some good advice before I move forward.

Photos attached

 

[stuart45]

Are you sure they aren't spider webs.

 

[a twig]

Assuming you mean the white staining on the rafters - more likely to be evidence of a previous water ingress than a current problem in my experience

If you can, get up there yourself with a screwdriver and a decent brand moisture meter and give the rafters a poke and a measure. If they are solid and have a reading of 15% or under I personally wouldn’t be too worried.

All the above is just my opinion of course

 

[malcolm]

I think it was Peter Ward who mentioned that dry rot needs a certain water content in wood to survive and it dies if the wood is dry. It's the damp companies that make a big deal of it.

You do need good ventilation in the roof space after a membrane has been fitted. The original method of ventilation was wind blowing through the gaps between the tiles. I had a breathable membrane fitted and also installed air bricks every 1m around the roof to add back in some of the ventilation I had lost. Often the membrane is added without replacing the ventilation and that can cause the roof space to become damp and provide the conditions for rot.

I found whitish patches on the top of my 1921 rafters (to the left in the photo). The roof had no membrane and the tiles were falling off so the timbers got quite wet. I imagine older timbers will have had more time to go more white. The out of focus beam at the top of the second photo looks the worst but it's tricky to make out what is going on there. I'd not be too concermed about the rest of the roof - it looks solid to me. But I would make sure it is properly ventilated. And I would remove the insulation that's been thrown in and replace it with 300mm rockwool (or equivalent) fitted properly.

 

[Lacan07]

Looks like salts from historic leaks to me. I've got all manner of staining in the timbers in my lofts from previous leaks, the tone of the wood play tricks on you. Some of it is black and looks for all the world as if it were wet but is in fact bone dry. And much of it is white.

If its dry its not a problem, the fungus relies on water to survive or reactivate. As malcolm says a lot of the fear mongering surrounding dry rot is whipped up by the damp companies.

 

[Matt Green]

That's not 'dry rot' or even fungi, it's magnesium salt staining from the old slates.

 

[AxPop]

Thanks for everyone's advice on this. I think the key is to get the insulation laid properly, and create/reopen ventilation at the eaves and ridge levels.

 

[Athers1971]

Just jumping in here...if the roof felt is non-breathable, and you were converting a loft/attic to proper living accommodation with insulation on the rafters and at the dwarf wall, how would you deal with ventilation?

 

[Lacan07]

Just jumping in here...if the roof felt is non-breathable, and you were converting a loft/attic to proper living accommodation with insulation on the rafters and at the dwarf wall, how would you deal with ventilation?

When insulating rafters there's rules about how much gap you need to leave between the felt and the insulation, when its the non breathable bitumen felt its 50mm I think, and if its the modern breathable stuff you can leave less of a gap, something like 20mm, but don't quote me on that.

For current building regs you'd have to insulate between and on top of the rafters to meet the necessary U value required.

Edit: Also, when insulating rafters don't forget it will be a 'cold roof' installation so absolutely no gaps allowed, warm, moist air from inside cannot be allowed to penetrate to the cold side of the insulation where it would condense and rot the timbers. Detailing is important, all gaps and borders / junctions must be filled and taped, and all fixings must be taped too.