Loft ventilation in roof with no eaves

[Mike1501]

Hello,
I live in a bungalow that has what I believe is called a "cottage" roof design. It is about 30 years old. In this type of roof there are no eaves (no soffits/fascia boards). The roof has a substantial overhang and the ends of the roof trusses are exposed. The wall extends right up to the underside of the roof. This all means that there is no ventilation at all along the bottom edges of the roof apart from any that gets in through the slates. Inside the roof there is a layer of felt under the slates. The only ventilation provided consists of two airbricks in each of the three gable walls (the bungalow is L-shaped). These airbricks are located one on each side of each gable, about half way up. Every winter I get condensation forming on the felt on the rear section of the roof i.e. the coldest section facing away from the sun. When it is really cold the water droplets freeze on the underside of the felt and then melt when it warms up and fall like rain in the loft. I am aware that it is possible to install vent slates and ridges, which might cure the problem if enough were installed but that would involve quite a lot of work. As an alternative, is it feasible to cut additional vent holes in the two opposing gable walls with some kind of electric fan system at one end which could be switched on via a hygrostat?
All advice gratefully received.
Thanks.

 

[edphill]

Hello,

Looking at your problem, I was immediately going to suggest slate vents - I have had them retrofitted before and it wasn't a major issue. They are cheap from Screwfix and as it's only a bungalow there won't be a problem with getting to them (we were fitting to a 3 story building!).

Your other suggestion is also perfectly feasible - get a 4" extractor (depending on size of loft) and that should be easy to install assuming you can run a cable to it without too much difficulty. Fans with a humidistat are about £20 and will only start when moisture reaches a certain level.

The other thing to consider is where the condensation is originating. Do you have a decent level of loft insulation? If not, companies like Npower or Tesco will come and insulate your entire loft to meet current requirements for about £100 - £150 as long as it's below 100 sq.m.

Water vapour will always condensate at the coldest point it can reach - which is apparently your loft. If you can stop the moisture getting up there, that could solve the problem.

Hope this helps.