Anyone identify the type of rot from picture?

If the white filaments on the joist are cobwebs then with the state of the roof I would suggest wet rot. If the filaments were exposed when the rotten wood was pulled away then you have dry rot which is the worst to deal with as you have to cut back to sound wood plus 2 foot and disinfect walls and nearby sound wood to kill off the spores.

Richard

Looks like traditional wet rot to me... no cuboid cracking or fruiting bodies, and localised to the source of the water.

Me too.

To the OP:

Don't panic - remove the source of moisture and ensure some ventilation to the area and that will cure the root problem.

Replace or augment the wood with new as required to repair the strength.

You could throw some chemicals at it, but the main thing is to get it dry first - in which case the rot should stop spreading.

And don't panic - this is not some sort of ebola where one speck left will destroy your house - it's there because the conditions are right to support it currently.

I *would* have said there was dry rot there, from the transverse cracking and shrinkage in the boards.

It also looks that in the past ivy or some other climbing plant and been growing through the roof probably causing a leak that allowed the wood to get and remain wet.

Yes cos its really a wood digesting fungi. If its been damp then dry that is perfect for so called Dry rot. Ug. Brian

From the photos I would say wet rot only.

I would suggest, as others have indicated, replace the affected timbers and make sure any water ingress is stopped and there is adequate ventilation.

Even dry rot doesn't spread in dry timbers, so ventilation is critical.

Thanks for all the replies. I will try and get up there again on Monday or Tuesday and have a closer look at the filaments and the cracking on the sarking. The loose piece lying on the floor was not knocked off by me so either fell off or was done by beastie(s). Will update if anything changes. Next step will be for the tenants to consult lease and insurance...