After the waterproofing job: how long until the wall dries?

The contractor has redone the bathroom waterproofing upstairs. Two weeks later, the patch on your ceiling looks much the same. Was the repair a failure? Not necessarily. A wall or slab that has been taking on water for months holds a lot of it, and fixing the source only stops new water arriving. The water already in the plaster and masonry still has to leave.

There is no honest number of weeks we can give you. What you can do is check whether the wall is drying, which tells you far more than any estimate.

Why a wall dries slowly

Water leaves a wall mainly by evaporating from its surfaces. A thick RCC slab or a brick wall with cement plaster on both faces has a lot of material and limited surface. Paint, tiles and wallpaper slow evaporation further. And as the water moves out, it carries salts to the surface, so efflorescence often appears during drying even when the repair worked.

Drying also happens from the surface inwards. The paint can look and feel dry while the plaster behind it is still damp. That is why a wall repainted as soon as it looks dry often shows the patch again a few weeks later. A meter reading against a dry reference picks up what your hand cannot.

What affects drying time

How to tell whether it is drying

One reading after a repair shows direction, not a final answer. A wall that reads lower than it did before the repair, compared against the same dry reference wall in the same flat, is moving the right way. A wall that reads the same or higher after a few weeks suggests water is still arriving, and the source may not be fully fixed or there may be a second source.

• Readings falling, patch shrinking: the repair is likely working. Keep checking before you repaint.
• Readings steady, patch the same: too early to say, or the source is still active. Check again after some time.
• Readings rising, or the patch growing: water is still getting in. Go back to whoever did the repair, with your before and after readings.
• Wet again after rain only: if the original source was a bathroom, there may also be an external source, such as the terrace or an external wall.

What to do while you wait

1. Do not repaint yet. Paint over a wet wall peels, and it hides whether the patch is shrinking.
2. Scrape off loose, flaking paint so the surface can breathe.
3. Keep the room ventilated, and move furniture away from the wall.
4. Brush off salt deposits dry. Do not wash the wall.
5. Photograph the patch from the same spot every week or two, so you can compare.

When to check, and why it matters

Check before you pay a contractor's final instalment, before you close a complaint with a builder, before the balance is paid on a resale flat where the seller agreed to fix the damp, and before interiors go on the wall. In each case you are deciding whether the repair worked, and a measured trend is better evidence than a wall that looks a little lighter.

Readings on plaster and masonry are relative, which is why the comparison has to be made at the same marked spots, against the same dry reference. The how we inspect page explains the method.

A re-scan repeats the readings and thermal images at the same marked spots and shows the change against your original report.