Identify this rot

[Lower]

I appear to have some rot in the joists under my lounge.

The sub floor ventilation had been blocked in one corner by the addition of a conservatory and i am in the process of putting a duct through the conservatory floor to reinstate the ventilation. Holding a smoke match to the hole i've created for the duct shows good airflow under the floor.

However, there is little or no rot in that corner of the room though there is some dampness. There is however some rot in two other joists and some floor boards but in an area which is quite close to unblocked air bricks.

See photos below:

Rotten floorboard

Rot in joist

Where i pushed in a screwdriver

This doesn't look like dry rot to me as there is no sign of fungus or white roots. What is odd is that the joists seem to have rotted internally leaving an undamaged outer skin.

Apart from the corner where the conservatory has been installed the subfloor is very well ventilated so i am confused as to why i have this rot and what i can do to prevent it reoccurring.

Confirmation i don't have dry rot would be appreciated as would any other suggestions as to what is going on.

The house is of 1920's construction and flower beds/borders abut the areas where the airbricks are so it is possible that the airflow was blocked by these in the past and that was the cause of dampness which has caused the rot.

 

[AronSearle]

Confirmation i don't have dry rot would be appreciated as would any other suggestions as to what is going on.

I can't tell from those pictures, but does it matter?

You have a moisture issue, wet or dry rot, you still need moisture for decay.

(dry rot can transport moisture, but still needs a source of moisture in the timber to start, and it is unlikely to have transported moisture along the timber from the masonry walls.)

What is odd is that the joists seem to have rotted internally leaving an undamaged outer skin.

They were probably preservative treated, likely in-situ, which does not really penetrate into the timber.

This is why I generally advise against treating timbers in such uses, as it only hides the decay, it does not stop it (ventilation does that).

Apart from the corner where the conservatory has been installed the subfloor is very well ventilated so i am confused as to why i have this rot and what i can do to prevent it reoccurring.

Do you have vents to both sides of the house, so that it blows through?

Maybe the conservatory blocks this "wind tunnel" effect?

Move the flowerbeds away from the vents.

Assuming there is no water leak (which you would spot) this really is just an issue of lack of ventilation, the floor isn't well ventilated, you need to prod and poke about to find out why, blockages etc.

 

[Lower]

It matters because it affects the amount of corrective work i have to do.

What started as a simple floorboard replacement and ventilation improvement (though blue bricks are a b@stard to drill though and chop out) has resulted me finding this rot where i didn't expect it.

Over the weekend i've replaced the floor boards showing significant rot and sistered the joists. I think it's wet rot that has subsequently dried out as the ventilation is now pretty good with the exception of the corner where i've just put in another vent. If its dry rot then i've got a load more work to do to eradicate it.

The ventilation under the floor seems pretty good to me. The room has unobstructed subfloor air brick ventilation on 3 sides and a 4" cast iron pipe running ventilating the 4th side (the side that abuts a solid floor) through to the other side of the house. A smoke match held to the vent that i've just created shows good flow under the floor. I can hold the smoke match approx 6" from the vent and the smoke gets drawn in its entirety into the vent. I also checked the flow through the cast iron pipe with a smoke match and that also showed a strong current of air.

I suspect that in the past plants have been allowed to obstruct the airbricks and that is why the dampness and rot occurred.

If that's true and the rot is wet rot, the work i've done is sufficient. If its dry rot i've got a lot more work to do.

 

[Burnerman]

I think its wet rot that has dried out over time...no apparent sign of fungus or hyphae there.
I'd still chemically treat the timbers as if there were, though just to be on the safe side.
John

 

[gregers]

id hedge its wet rot,but looking at pic 2 had me a bit worried,the way it has shrunk like it does for dry rot.