A further update....
In addition to the three damp and timber companies that have visited (details in previous post), we've had four more:
(4) Kenwoods. Said it was definitely dry rot. Thought the white whispy growth was salts. He was scaremongering... got to do it right, get a guarantee, otherwise it will just come back etc etc. He said he would half the living room area and the corridor. Haven't received quote yet.
(5) Timberwise. He said it was definitely Mine Rot (Fibroporia Vaillantii). Their report says:
Some flooring had been lifted to reveal a localised infection of mine rot (Fibroporia vaillantii), affecting the floor, joists and bearer plates. The probable cause for the infection is a lack of subfloor ventilation and timbers being in direct contact with damp sub-floor brickwork; the over site has no suitable isolation of timbers from brickwork.
The remaining floor area has the same problem (no isolation of timber bearer plates where brick piers are situated). The clients own builder should lower over site where feasible and install a suitable felt damp proof membrane. The fire place hearth also has built in timbers around and inside the damp affected hearth; client should remove or again isolate these timbers from masonry or brickwork.
Timberwise propose to remove excess over site to the front section of the front room where our works are to be carried out; remove loose brick piers and affected flooring, joists and bearer plates. New two course brick piers will be installed as necessary and dependent on site conditions will be cemented into place. The over site will be treated with a suitable fungicidal solution and newly treated flooring, joists and bearer plates installed, isolating from the brickwork with a suitable damp proof membrane.
Client should increase sub-floor ventilation by installing 1 new 225mm x 150mm air vent to the left hand side of the bay and a periscopic vent to right side of the bay as indicated on the sketch plan.
They've quoted:
Brick piers as specified £376.73 + vat
Spray treatment £207.54 + vat
Timber renewal £779.05 + vat
Removal of rubbish £209.58 + vat
(6) Rentokill. Wet rot. No need for chemicals. Just remove affected timbers, make sure wood is isolated from brickwork with damp proof membrane. (not trying to sell us anything hooray!)
(7) Peter Cox. Said he can’t properly diagnose it as the area has been tampered with (new skirtings added, floorboards been up a week etc). Said thinks it’s more likely wet rot than dry. Said (with an air of 'why am i wasting my time with you') we look like DIYers so just buy the chemicals and spray ourselves. They’d charge £400-500 just to spray.
Sooo 7 opinions in and the counts are:
Wet rot: 3
Dry rot: 2
Mine fungus: 2
What confusion!
Only 1 of the 7 thought the white whispy growth was dry rot, the rest thought it was more likely to be salts/resin from the new plaster. Also our friend checked the floorboards in room above and they were dry and fungus free. So we're going to hedge our bets here and not remove plaster. Also we made bigger holes in the chimeny breasts for vents and stuck our hands in - seems free of rubble downstairs and you can feel airflow in the upstairs room.
So for the floor area, the consensus seems to be remove the timber in affected area, clear the rubble and add more sub-floor ventilation. We are already over budget and want to do this as cheaply as possible, so we will use our builders who are still on site to do the work replacing timbers. The question is whether to treat with fungacide or not. A friend of a friend advised us to buy the wood treatment for £18 from Toolstation (protects from wet and dry rot and wood boring insects apparently) and paint the the timber ourselves. Can anyone advise on the effectiveness of this or whether we should fork out for a company to do it?
Another question I have relates to the issue of 'isolating' the sub-floor timber from the bricks, which a few people have recommended. From what I understand this means adding some kind of damp proof membrane between the bricks that lie underneath the wooden beam that supports the joists (sorry I don't know the correct terminology). Is that right? And does this need to be done throughout the whole room? I can see how this can be done in the area where we're replacing the timber anyway but not sure how we'd get the membrane in place in the rest of the room where we're not replacing anything. Any clarification appreciated!