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Roof leaks when frosty but not when raining

I have a pitched slated rood which is leaking when it gets frosty or when there's snow lying on it. Water comes in around the top of a velux window when the frost or snow is melted in the morning. However, it can be absolutely tipping down with rain and nothing comes in. I've had a look at the outside of the roof and can't see any broken or slipped slates or anything. I can't get to the inside of the roof because the room it covers was built up into the roof space (so there's plasterboard and insulating material right up against the roof). As far as I know from some work done to another section of the roof a few years ago, the roof is felted under the slates. Any suggestions what could be causing this, and what I could do to fix?

8 Answers from MyBuilder Roofers

Best Answer

Hi, the chances are is that you have a build up of condensation if the roof doest have adequate ventilation, i.e breathable membraine vented soffit and ridge lines this normaly causes condensation to appear during freezing and snow especialy if as you say the loft space is being used as a living space.


Answered 19th Feb 2011

hi this could be one of two problems. your velux window might have been installed wrong or the pitch of the roof might not be sufferient enough. If the lap of the slates isnt enough the snow will creep up the slate and down the felt .


Answered 17th Feb 2011

It could be any of the problems ace build has suggested, also we have seen some fitted minus parts of top apron missing, also they havent put an undereave under the tile, can you look out of window, and see any felt between the slates.
Could be not enough pitch, when you have a build up of snow, you may have capillary action.
Seems the velux was never fitted properly, if installed correctly they are great windows
Simple enough job.


Answered 17th Feb 2011

Most likely a ventilation problem, try fitting two roof vents about 1 third of the way up the roof, of each side.


Answered 17th Feb 2011

It would be best to contact a local roofing company who provide free, no obligation quotations as trying to give an accurate answer without a hands on investigation is just a stab in the dark


Answered 17th Feb 2011

I would agree with B J D. might be an idea to put a lead apron under the last row of slate above the velux and up onto the top of velux so any build up of snow/ice will have nowhere to go but off the roof. Sounds more complicated than it is!!, good luck.


Answered 18th Feb 2011

knock it down!lol not really i think any of these answers could be right!


Answered 28th May 2011

Blown snow can be a problem, however; if the Velux window was installed with a BDX felt collar detail with transverse drainage gutter, this would solve the insulation/condensation as it not only prevents cold bridging but allows any ingress of water externally to be easily deflected away from the Velux down onto the felt and into the gutter.

This detail is required under part L of the Building Regulations (Robust detail) although I have encountered thousands of examples of disregarding the Regulations over the years as an ex-Velux service technician. If the BDX is fitted around the Velux along with quality felt or membrane, your roof would not even leak before slates or tiles were fitted!


Answered 12th Jan 2012

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