Roofline Roundup
For roofline installers the issue of roof
ventilation can be complex. There are different requirements
for different roof structures and getting the ventilation
wrong can lead to long term condensation and damage to the
rafters. The kind of questions I get asked are: “What’s
wrong with the old practice of installing non-ventilated
timber roofline?, “If the old timber soffit is non-vented do
I replace with vented or non-vented?” and “When I am
installing replacement PVC-UE roofline, which type of soffit
do I use, vented or non-vented?”
Before the 1960’s, most houses had
efficient ventilated roof voids. But the introduction of
felt roof linings and ceiling insulation created cold roofs,
initiating problems of damp. These damp problems were added
to by cavity wall insulation and double glazing. The
addition of central heating meant houses were drying out,
but at the same time the increase in home laundry white
goods, washing machines and tumble driers, increased the
atmospheric moisture. There was only one place for the
moisture to escape – the roof. Building regulations were
amended to protect timber roof trusses from rotting from the
extra moisture, and ventilation of the roof space was
introduced.
For roof pitches greater than 15° an
equivalent of 12.5mm continuous air gap is required. However
where roofs are flat or have a roof pitch of less than 15°,
this increases to an equivalent air gap of 25mm.
So what type of ventilation should I
use?
The simplest ways to ventilate roofline
are over fascia venting or soffit ventilation. Over fascia
vents are more aesthetically pleasing but leaves can
accumulate at the back of the gutter installation and
effectively reduce the air ventilation level. Soffit
ventilation is by far the most effective. There are three
ways to ventilate soffits:
1.
A ventilated strip that fits alongside a conventional
plain soffit. But these often come with a nylon mesh and can
get clogged up with dead insects and roof debris.
2.
Disc vents. This option requires accurate
calculations to ensure the correct air flow, depending on
the size of the discs, and the volume of the roof.
3.
By far the most effective option is pre-ventilated
soffits. The correct airflow will be achieved by using
either standard ventilated, giving an equivalent 12.5mm air
gap, or double vented, increasing the air gap to an
equivalent of 25mm. And, because they are pre-drilled the
installation time is significantly reduced.
