What does dry rot look like?

A damp, mushroomy smell in a room with no obvious cause is worth taking seriously. It is not proof of anything on its own, but it is often the first sign, and it tends to be strongest near the floor: under stairs, in a cellar, along a skirting on an outside wall.

If you can smell it and you cannot see it, something wet is hidden. That is the point to look properly rather than to redecorate.

Early growth shows as fine strands spreading across timber and onto plaster. The Property Care Association describes it as "fine grey strands develop, similar to cobwebs, which enables the dry rot to spread from the infected timber onto the surface of the wall."

This is the stage most people paint over. It is also the cheapest stage to deal with.

In still, humid conditions the fungus produces what the PCA calls "a mass of cotton wool-like growth with bright lemon-yellow patches." Water droplets form on its surface. That is where the name comes from: lacrymans is Latin for tears.

In drier spots the same growth appears instead as a thin silken grey skin, sometimes with a lilac tinge. Same fungus, different conditions.

This is the sign most people picture, and the one worth memorising. As the fungus feeds, the wood darkens and splits into a grid of squares, across the grain and along it. The PCA calls it "the characteristic cuboidal cracked appearance." Eventually the timber crumbles to a dry brown powder.

The catch: timber can lose real strength while still looking intact, because the fungus takes the sugars out of the cellulose before the structure of the wood visibly gives way. Prod suspect timber with something sharp. If the point sinks in, the wood is gone whatever it looks like.

Patches of fine brown or orange dust are spores. Where you find them, look for what produced them: a fruiting body, usually pancake or bracket shaped, rust-red in the middle with a paler edge.

A fruiting body means the fungus has run out of easy food and is trying to spread elsewhere. By this point the damage behind it is normally well beyond what is on show.

Both are caused by the same thing: timber that is wet. The PCA's distinction is spread. Wet rot decay "is usually limited to the areas that are regularly wet," while dry rot "can spread relatively quickly in the right conditions and can even grow over masonry in order to colonise susceptible wood that may be close by."

That matters because the two need different treatment, and treating dry rot as though it were wet rot is, in the PCA's words, "less certain of success." If you are not sure which you have, get someone in who does this for a living.

Find the water and stop it. Everything else is secondary. The PCA is blunt about it: "Fungal decay always arises because the wood has become wet, in excess of 20% moisture content. Finding the source of dampness and eliminating the ingress of moisture, whilst promoting drying, is always necessary."

So the sequence on a real job is: track the leak, fix it, open up enough of the building to see how far the fungus has gone, cut out and replace what has failed, isolate sound timber from damp walls, and get air moving through the space. Chemical treatment has its place as a control measure. It is not the cure, and the same guidance warns that preservatives "should not be relied upon to provide long term protection against dry rot in conditions where timbers are persistently wet."

Anyone who quotes you for a spray and nothing else has not found your leak.

Dry rot is not a decorating problem. It attacks joists, wall plates, lintels, floors and roof timbers, and it moves through the wall to find fresh wood. The cost curve is steep and it runs one way: a leak you fix this month is a plumbing bill, and the same leak in three years is a structural one.

If you are buying, this is one to settle before you exchange, not after.