Roof ventilation is often something that a home owner doesn't really give much consideration to. However, there are many benefits of roof ventilation and as such, these need to be taken into consideration, especially in older houses or when the home owner is planning on making changes to the way the house is used, for example in loft conversion.
How it works
How it works At its most simplistic, the requirements for ventilating a cold attic area are that the air should enter the attic at the lowest possible point of one side of the roof (such as through eave vents or even ceiling vents). It should exit the attic on the opposite side of the roof at a much higher position through a roof vent. This helps to keep a steady air flow throughout the year. Benefits Over time, water vapour rises up through the ceiling and insulation and into the loft. This hits the (usually) cold air conditions of the attic, causing it to condense. This condensed water will then settle, causing damp patches and even permanent damage, if this is not sorted. With adequate ventilation, the aforementioned air flow will help to remove the water vapour, before it has the chance to condense.
Things to avoid
Vents on the same side of the roof
By having vents on the same side of the roof, the flow of air can be looked of almost like a wind tunnel, with the air flowing straight from the lower vent directly to the higher one.
This will not benefit the rest of the loft in any way, and will still lead to condensation and dampness.
Do not have more higher vents than lower vents
This will cause the air flow to act in the opposite direction, that is from top to bottom.
During warmer weather, this won't be much of a problem, but in colder months, mist and fog can be drawn into the attic.
Not only will this occur, but rain can also be drawn-in because of the opposite air flow.
Do not have vents half-way up the roof This will still leave moist air trapped above the level of the higher vent. Condensation can still form below these vents.