I have an issue with my attic when the temperature plummets. Should I change the felt on the roof, which would mean removing the tiles?

January 2014

Question

I have an issue with my attic when the temperature plummets. I upgraded the loft installation two years ago and since then have noticed water droplet build-up on the felt. I have installed ventilation tiles into the roof but the issue still persists. The last solution suggested to me was that I should change the felt on the roof, which would mean removing the tiles. Have you ever heard of this?

Answer

You refer to changing the roofing felt, which I assume at present is a bituminous, impervious felt. Breathable membranes are used today but replacing the felt is a costly solution, which involves removing and reinstating roof tiles and battens.

The membrane does allow movement of water vapour through it and some manufacturers even claim that these do not need ventilation. However, these membranes have not been tried and tested over any significant period.

A ceiling covered with insulation will act as a barrier to prevent heat loss. This should be continuous but is often breached by gaps such as at access hatches, recessed lights, penetrating services and ducting extracting air from showers into the attic.

When the warm air meets the colder roofing felt it will condense and form water droplets. Bituminous felt significantly reduces the air movement and consequently reduces the amount of evaporation in the roof space. Cross flow of air is essential and in modern homes this is usually provided by preformed, perforated slots in the eaves. I have noted large gaps between these vents during inspection of some dwellings, where they should equal not less than a 10mm continuous air gap for a typical pitched roof.

You have recognised the problems and installed tile vents, which need to be closely spaced. There are proprietary ridge vent tiles but these are rarely used.

Ensure the insulation is free from the eaves. Check the amount and spacing of the soffit vents. Gables allow the opportunity to add vents. There are proprietary continuous ventilators that are placed under the front row of slates. A roofer will need to remove and refit the lower four courses. Preformed vent slots can also be placed between the insulation and felt at eaves. Check insulation is evenly distributed and placed over and not below the water tanks.

Improving the ventilation and ensuring it is evenly distributed should deal with your problem and is more cost-effective than installing a breathable membrane. A healthy roof void needs a draughty void. The type and construction may even have a bearing on the problem. If the problem persists it may be worthwhile consulting a local chartered building surveyor.

Jim Drew is a member of the Western Region Branch of the Society of Chartered Surveyors Ireland. scsi.ie