Well, overall, I'm happy to go with the majority vote that it's wet rot.

I've exposed the wall plate in two more locations, 1m and 2m to the left of
the rot centre and the plate is 100% sound.

I've pulled a couple of the short ceiling joists out which means I can see
right to the far end wall (0.5m to the right of the rot) and although the
plate is knackered there, surprisingly the roof rafter seems mostly fine.

No hairy stuff anywhere.

Reckon I can cut, remove and slide a new plate in, if I lift the roof 1/2"
with acrow props. I'll leave it open for now - it's no longer an urgent
job. Let it get baked out a bit in summer then do it.

Thanks for all your opinions

Cheers

Tim

[Context if needed...]

Tim S coughed up some electrons that declared:

Hi all,

Lots of work at the Bungalow today. All went well, until this, though:

http://photos.dionic.net/v/public/bungalow/mushroom/

Wonder if the phots are enough for someone to be able to cast an opinion
on whether it's dry or wet rot?

These are the timbers underneath a lead flat roof, on the side of the
building (which is otherwise hipped roofed).

Looks more like wet rot to me - but how can I be sure it's not dry rot?

If it's wet rot, then I can replace the visible section of wall plate and
one flat roof rafter (which is separate to the main roof rafters) and one
ceiling joist.

If it's dry rot, then I'm scared... Presumably that means new flat roof
time, due to having to cut back 2-3 feet from the affected areas?


More background:

I didn't see any "cotton wool" nor fruiting bodies. Not sure what the
white staining is, but it's not hairy.

The neighbour told me that the roof used to be felted, and was leaded
10-15 years back - presumably there could have been a serious leak then.
It's generally dry now.

The rafter has rotted completely at the end, and is apparantly all right,
otherwise *except* that the core has rotted for about another 8" (needed
to jab a chisel in to discover that).

The wall plate is totally gone a foot either side, but neighbouring
rafters seem fine. The wall plate has bottom half rot for all the visible
parts, but on the far left, it's petered down to perhaps only 1/8" is
dodgey.

Looks like the water got into the inner wall leaf, then rotted the wall
plate from beneath.

The planks supporting the lead seem solid and as you can see, not problems
at all at the other end, where the main hipped roof rafters sit.

Thanks *very* much in advance

Cheers

Tim