Leaking roof or condensation? What’s the difference?

Last Updated on: 8th October 2021, 09:08 pm

Leaking roofs are a number one nuisance.  It can be very difficult to work out where the leak is coming from and different weather conditions can produce leaks in different places.

More confusingly, where there are horizontal roof boards or rafters the leak may be eight to 10 feet away from where you see a wet ceiling.

Top 5 places for roofs to leak

  1. The tiles themselves: cracked or missing tiles are an obvious culprit;
  2. Flashing: the lead (or lead replacement) around the edges of chimneys, windows or where the roof meets a vertical wall.  Older cracked flashing can often be the cause of leaks;
  3. Gutters: OK, not technically roofing, but a blocked gutter will cause just as much of a damp problem in internal walls.  And if the gutter is overflowing it’s worth checking the downpipe and drain – if these are blocked, the water backs up;
  4. Cracked mortar: traditional mortar bedding accounts for more leaky roofs than anything else.  If you see pieces of mortar in your garden, you could have a problem
  5. Solar panels: these are causing an increasing number of roof leaks. If you are thinking of installing solar panels on top of your tiled roof make sure the manufacturer has designed the system for your weather conditions. 


And finally – the leak that isn’t a leaking roof.  Damp on the inside of a roof can be caused by condensation due to inadequate ventilation rather than rain coming in.  

We create a lot of water vapor in our homes: showers, cooking and the weekly wash all contribute and, if your house is well insulated, then that water vapor will stay inside.

What typically happens is that warm damp air rises up through the house and ends up in the roof.  If you don’t have enough ventilation up there then it can condense on the underside of the tiles.

The problem can be really significant, with enough water to soak your insulation and cause mould or rot on the timbers.

How do you distinguish a leaking roof from condensation-caused dampness?

You will know if your damp roof is caused by condensation if you only get the problem in cold weather– so if you thought you had a leak last winter, but the roof has been fine all the way through the summer, it’s worth checking again during the first cold snap.

The cure for condensation is to increase the amount of ventilation in the roof space: we recommend both high and low level ventilation to ensure a good flow of air.  

Don’t let anyone tell you that vapor-permeable underlayments will solve the problem on their own: they won’t without proper ventilation.

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