Dry Rot vs Wet Rot in Window Joinery

You pushed a screwdriver into the bottom rail and it went in like cake. Now you want to know whether this is the ordinary kind of rot that a joiner cuts out in an afternoon, or the notorious kind that eats a house. The distinction is real and it changes the price by an order of magnitude, so it is worth getting right before anybody quotes you.

The short version: in a sash window, the overwhelming majority of decay is wet rot, and wet rot is a joinery problem. Dry rot is a building problem. Telling them apart takes about five minutes with a torch and a penknife.

The names are backwards and always have been

Both fungi need water. Neither of them will touch dry timber. "Dry rot" got its name because the decayed wood ends up dry and crumbling, not because the fungus grows in dry conditions.

What actually distinguishes them is plumbing, not moisture level. Wet rot fungi can only live where the wood itself is wet. Take the water away and they stop, full stop. Dry rot — Serpula lacrymans — grows cord-like structures called rhizomorphs that carry water from a damp source into timber that would otherwise be too dry to attack. That is the whole difference, and it is why one is a repair and the other is an investigation.

The second difference is habitat. Serpula wants still, dark, unventilated, humid air. It needs somewhere around 90% relative humidity to get going, and it establishes on timber at a moisture content around 28 to 30%. Once it has a hold it can keep working at anything above roughly 20%. Below about 20% moisture content, decay stops.

A sash window hanging in an outside wall is windy, bright and exposed. It is close to the worst environment Serpula could pick. That is why nine times out of ten what you are looking at is wet rot.

There is one exception, and it is the reason to keep reading: a box sash frame contains a sealed cavity. The weight pockets either side are dark, still and unventilated, and their bottoms sit on the masonry. Water running down inside the box — from a failed sill, a blocked hopper, a leaking parapet gutter above — puts standing damp into exactly the conditions Serpula is looking for. If dry rot turns up in a window, that is where it starts.

Telling them apart

Two more clues worth knowing. Cuboidal cracking — timber split into small cubes both along and across the grain — means a brown rot has eaten the cellulose and left the lignin behind. Dry rot always does this, and so do several wet rots, so cubes alone do not prove dry rot. Fibrous, stringy, bleached timber that peels into threads means a white rot has gone for the lignin instead. White rot in a window frame is wet rot, not dry rot.

The single most reliable dry rot indicator is a rhizomorph crossing something that is not wood. If you find a grey strand running across brickwork or under plaster, that is Serpula and you need a surveyor.

A five-minute diagnostic you can do yourself

Do this on a dry day, from inside, with a torch and a bradawl or a slim screwdriver.

1. Press, do not stab. Push the point into the bottom rail of the lower sash, the cill, and the bottom 150mm of each stile, using firm thumb pressure only. Sound joinery resists. If the point sinks 5mm or more with no effort, the timber has gone.
2. Check the paint film. Rot underneath shows as blistering, a spongy give under the paint, and hairline cracking that follows the grain. Paint bridging soft timber can look perfect from two paces.
3. Look at the putty line. Failed glazing putty is the single commonest entry point for water into a sash. Where the putty has shrunk away from the glass, water runs straight into the rebate and sits on the bottom rail.
4. Open the pocket. Take off the staff bead and the parting bead, then find the pocket piece in the pulley stile — it is a short removable panel about 200mm long, usually held by one screw or a couple of nails. Get a torch into the weight box. You are looking for standing water, wet plaster dust, cotton-wool sheeting or grey strands.
5. Smell it. A faint earthy smell near a wet cill is ordinary. A pronounced mushroom smell that you notice on entering the room is not.
6. Follow the water uphill. Every case of rot has a source. Check the cill throating, the pointing above the frame, the render, the drip on the lintel and the gutter. If you cannot name where the water came in, you have not finished.

What wet rot actually needs

Almost nothing chemical. Wet rot in a window is caused by water sitting on timber, and it is fixed by removing the decayed wood, letting the rest dry, replacing what has gone with sound timber, and stopping the water getting back in.

In practice that means one of three repairs:

Cut out and splice. The correct repair for a decayed cill or bottom rail. The rotten section is cut back to sound timber on a sloping scarf, and a matching piece of well-seasoned timber is let in, glued and fixed with non-ferrous screws. Baltic redwood is the traditional choice for London sashes; accoya is increasingly used for cills because it does not move much and takes paint well. This is the repair the SPAB recommends and it lasts as long as the original. The method is set out in detail in our guide to splicing new timber into a rotten frame.

Resin repair. Two-part epoxy consolidant and filler has a place for localised decay — a small area on a moulding, a decayed screw fixing, the corner of a glazing bar. It has real limits. Epoxy is rigid and timber moves; over years the bond can open at the edge and let water back in behind a repair that looks sound. It is also useless if the timber underneath is still wet, so the moisture content needs to be below about 18% before you start. Used to save a mouth-shaped hole in a bottom rail it is fine. Used to fill half a cill it is a cosmetic delay.

Replace the component. When a bottom rail has gone through and the tenons in the joint are soft, splicing costs more than making a new rail. When more than roughly a third of a sash is decayed, a new sash made to match is usually the honest answer — the reasoning is covered in what can be saved and what genuinely needs replacing.

Note what is not on that list. Spraying a whole window with fungicide does not repair anything and does not remove the water. Chemical treatment is a secondary measure, appropriate where damp genuinely cannot be eliminated, and it is not a substitute for finding the leak.

When it really is dry rot

The approach changes completely, because you are no longer treating a window.

The first job is to establish where the water is and stop it. That is usually not the window itself: it is a defective gutter, a cracked hopper, raised external ground levels, a blocked sub-floor vent or a leaking pipe. Until that is fixed, nothing else matters.

The second job is drying and ventilation. Serpula cannot tolerate airflow and cannot tolerate wood below about 20% moisture content. Opening voids up, restoring ventilation and letting the fabric dry does most of the work.

The third job is repair, and this is where you should push back on over-specification. The old convention of cutting sound timber back 0.5m to a metre beyond the last visible decay, and hacking off wide bands of plaster to "sterilise" walls, is rarely justified in a historic building. The SPAB is explicit about that. Removing more original fabric than the decay requires is a permanent loss, and it is often sold as prudence.

Be wary of any survey that arrives free from a company that also does the remedial work. Guarantees written by remedial contractors are worth what the company is worth in ten years' time.

Stopping it coming back

Rot is a maintenance failure with a fungus attached. Sash windows lasted a hundred and fifty years in London because somebody painted them.

• Keep the paint intact on the horizontals. Cills and bottom rails take the water. The tops and undersides of sashes are the surfaces most often missed, because you have to slide the sash to reach them — the sequence for doing it properly is set out in our piece on painting sash windows.
• Re-putty when it shrinks. A putty line with a 1mm gap behind it is a funnel aimed at your bottom rail.
• Keep the cill drainage working. The throating, the small groove machined under the front edge of a cill, exists to break the surface tension so water drips clear rather than running back to the wall. Paint fills it. So does render. There is more on why in the window cill guide.
• Check the pockets every few years. Two screws and a torch. If the bottom of the weight box is wet, you have found a problem while it is still cheap.

Decay caught at the paint-blister stage is a couple of hours' work. The same window left three more winters is a new sash, and the same window over a wet weight pocket for a decade is a wall.

Getting a straight answer

If you are not sure what you are looking at, photograph it — the timber surface, any growth, and the surrounding wall — and get someone to look before you cut anything away. Removing the growth destroys the best evidence you have.

For wet rot in the joinery, a competent sash joiner can diagnose and price it on site in twenty minutes. Our sash window restoration work is largely this: splicing cills, renewing bottom rails, and putting right the thing that let the water in. If you are anywhere in London, our sash window repair team will tell you plainly whether what you have is a morning's work or something that needs a surveyor first, and the second answer costs you nothing.