Loft Conversion Ventilation
Ventilation for a loft conversion is in two distinct areas for Building regulations
requirements. Te first is for trickle or mechanical ventilation of the room or rooms itself and the other is for
venting of the roof and floor voids to prevent condensation.
Venting of a habitable room such as a bedroom for a loft conversion only needs to be by trickle vents usually
within the top framing of the new windows or external doors. Rapid ventilation is also required by an opening sash
of the main window for example. It is rare to have mechanical ventilation for a habitable room.
Bathrooms and shower rooms and even WC’s require mechanical extract fans for forced venting even if they have an
The rates for all ventilation to these rooms are clearly described within the Building Regulations.
Venting of the cold roof void can be achieved in a variety of ways depending upon the design and roof coverings
installed. All cold deck pitched roofs over a loft conversion require venting of the cold side of the roof void and
a 50mm min. gap is usually required. For roofs having breathable felts installed such as Tyvek or Klober then
perimeter venting at the soffits and ridges is not required as the whole roof felt breaths.
For pitched roof loft conversions having existing bitumen based felts that are to remain then perimeter venting
will be required at the soffits of the eaves line and at the ridge line for cross venting. The amount of clear
venting gaps are clearly described within the Building Regulations.
New flat roofs for dormer roofs for example are normally warm deck roofs these days where the insulation is
placed on top of the joists and there is no cold roof void to vent. These roofs do not require cross venting. If
the insulation for the new dormer flat roof is located under the flat roof joists or between the joists or even a
combination of the two then perimeter cross venting from new eaves soffit to ridge line will also be required.