The need for ventilation has come about due to the increase in standards of insulation combined with an increase in activities within buildings that generate water vapour. Ventilation is essential in roof spaces to prevent harmful condensation and remove water vapour.
Diagram D62 - Roof ventilation
The diagram above shows the requirements for ventilation to roofs. The dimensions are per Section 2.1 of Technical Guidance Document F required for continuous ventilation openings in various roof types above and below a pitch of 15 degrees. These should be adhered to avoid long-term condensation and associated problems.
Diagram D63 - Roof ventilation - Flat roofs
Continuous ventilation openings or some equivalent method of ventilation should have the same area on opposite. This needs to be greater than a continuous ventilation strip running along the full length of the eaves. The type of roof determines the width of the strip.
Diagram D64 - Roof ventilation along pitch
Diagram D65 - Dormer roof ventilation
If above a dormer window, the roof is of pitched roof construction, a 10mm continuous strip should be provided on both sides. Recommended dimensions are for continuous openings. If spaced openings are used, ensure they are suitably located and the total area is equal to the area of equivalent continuous ventilation.
The diagram above shows a dormer roof with a flat roof dormer window, the dimensions illustrated above are for continuous ventilation openings. As previously noted if regular spaced openings are used, ensure that the openings have the same total area as the recommended continuous ventilation.
Types of Ventilation
Diagram D66 - Types of roof ventilation
1) Ventilation in eaves
The use of proprietary eave ventilators is recommended to ensure that the flow of air is not obstructed over the insulation. The diagram below shows ventilation in a pitched roof greater than 15°. Proprietary eave ventilators should incorporates a fly screen. The purpose of the fly screen is to prevent insects, birds, etc. from entering the roof void. Note that the ventilators are supported by a plywood underlay. Recommended dimensions are for continuous openings. If spaced openings are used, ensure they are suitably located and the total area is equal to the area of equivalent continuous ventilation.
Diagram D67a - Ventilation at eaves
2) Slate or tile ventilators
This type of ventilation should be used where it is not possible to ventilate the roof through the soffit, and/or at a lean-to roof abutment. Guidelines on calculating the number of vent tiles/slates are outlined below. These vent tiles/slates should always be installed in accordance with the instructions of the manufacturer.
Diagram D67b - Ventilation at eaves using vent tile
3) Lean-to abutment
When providing ventilation to a lean-to roof abutment, where the slope of the ceiling follows the slope of the roof in a lean-to roof, vent slates/tiles should not be used because they do not provide ventilation to the areas between all the rafters. In this case, continuous abutment ventilation, as detailed in Diagram D68 should be used.
Diagram D67c - Lean to ventilation at abutment
Used in cases where the form of the roof does not allow for conventional ventilation at eaves level. These can also be used at mid to high level in certain circumstances, such as in parapet walls, mono-pitched roof construction, and in mansard roofs.
The following example details how to calculate the number of slate/tile ventilators required to achieve the minimum required ventilation at eaves level.
Diagram D68 - Slate tile ventilation
The slated roof on the house has a total eaves length of 16m and a roof pitch of 25°. Technical Guidance Document F of the Building Regulations requires a 10mm continuous opening over the entire eaves length (or equivalent) for a roof with a pitch of 15° or more. The assumption made above is that the form of construction will not permit conventional ventilation at eaves, thus making slate or tile ventilators necessary.
(Total eaves length) x (continuous opening size) ÷ (Capacity of vent slate/tile) = (No. of vent slates/tiles required)
Note: A vent capacity of 20,000 mm^2 is assumed for the purpose of this example and is a common capacity for a tile/slate vent. It should be noted however, the area that can be adequately ventilated by each vent can vary depending on the manufacturer and this should be confirmed prior to undertaking any calculations.
(Convert all lengths to mm)
(16000) x (10) = 160000
(160000) ÷ (20000) = 8
Based on the above calculations a minimum of 8 vent slates are required required 4 vents should be provided on each side of the roof for cross ventilation purposes. Should an uneven number of vents be required at a minimum this should be rounded up to ensure the same level of ventilation is provided on each side of the roof.
Cold Deck Construction of a Flat Roof
Due to its relatively low life expectancy of this type of roof construction (due to the inherent risk of leaking and condensation), it should be avoided wherever possible.
However, where this type of construction is deemed necessary, either a vapour control layer (500 gauge polythene) should be tacked to the bottom of the joists to control moisture build up in the insulation, or foil-backed plasterboard should be used in the ceiling. An example of typical ventilation at abutments for a flat roof of cold deck construction has been detailed below.
Diagram D69 - Cold deck ventilation
Proprietary mushroom vents can be used as an alternative for ventilating flat roofs. The required number/frequency of these vents will be advised in the manufacturer’s brochure. In order to ensure air is not obstructed across the roof, mushroom vents should be installed along with counter battens fixed above the tapered firring pieces.
Image of a typical “mushroom” ventilator
Vapour control layers
While they reduce the amount of moisture reaching a roof void, vapour control layers are not a valid alternative to ventilation. In dormer roofs, as the ventilation runs in line with the rafters, a vapour control layer consisting of 500 gauge polythene with sealed laps or some equivalent on the warm side of the insulation is required.
A vapour control layer should be provided to the ceiling area of the roof in dormer construction as well as the vertical and sloped areas.
Roofing underlay used must be appropriately certified and the requirements of the certificate should be obeyed during installation. It may be necessary to provide a vapour control layer at ceiling level and/or extra ventilation along with certain types of roofing underlay. Check with the manufacturer in such cases.