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A joist under a supporting wall doesn't sound right.
Water coming though an air brick doesn't sound right either - And if it did, it should still be nowhere near the joist.
Whereabouts is the air brick in relation to your DPC / floor level?
(Sorry - no builder recommendations there, I'm afraid)
NotThamesWater-
And @motman
A joist under a supporting wall doesn't sound right.
Most likely (depending on age of the house) a bearer that has been fixed to a load bearing wall.
Water coming though an air brick doesn't sound right either
Its a fairly common problem. When houses are built air bricks tend to sit a decent way above ground level which means water cannot run into them. Over time the ground level is built up and people either don't realise or don't care that water can now enter the house through the air bricks.
The worst example I've ever seen of this was when I had to put right a floor structure that had rotten because the client had decided to lay a new patio himself on top of his old patio. Somehow he mistook an airbrick for a drain and the fall on his new patio directed ALL of the rainwater that fell onto said patio into the airbrick. Its fair to say his wife was a little upset with him.
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I might be wrong about it being under a supporting wall but there are definetely bricks sitting above it on part of its length.
There is quite a gap from the air brick to to the joist and it sits above the DPS but as Bobbo mentioned over the years there has been a build up of dirt etc meaning that when there is heavy rain it can run down and drip over that particular joist. Hard to explain without photos.
Bobbo
motman
A few weeks back I noticed that the gap between the skirting board and the floorboards in one corner of our living room has suddenly increased by about a cm or so. Crawling down underneath the floor I can see that one of the joists has rotted, probably from rain water running down from one of the air bricks. The joist that sits on top and is holding that part of the floor up has dropped slightly as the rotted one has compressed below.
I’m thinking some of the floorboards will need to come up and then the rotten joist replaced but it will be awkward given its under a supporting wall. So definitely beyond my DIY skill level.
Can anyone recommend a suitable builder around SE London?