To comply with roof construction regulations, one of the most important things you need to get right is roof ventilation.
Adequate ventilation improves thermal performance, reduces moist air (and therefore condensation and water vapour become less of a threat to the structural integrity of the roof), and offers a high level of energy efficiency.
In order to ensure the roof is well ventilated to allow proper air flow and insulation, there are a few regulations one must stick to.
To help you understand what those are, we've put together an introductory guide to eaves ventilation regulations below.
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You can find the relevant guidance in Approved Document C and BS 5250 (otherwise known as 'Code of practice for control of condensation in buildings').
Always read the guidance yourself before undertaking roof construction or the modification of a roof space.
The basic principles of the documents (Approved Document C and BS 5250), though, state that no roof should be compromised by condensation so that it affects thermal or structural performance.
Basically, you need to provide ventilation openings in your roof space and eaves to allow for proper ventilation and air flow to prevent condensation.
Below we'll show you what complying with the regulations entails for 2 main types of roofs.
Pitched roofs need ventilation openings at eaves level for cross ventilation. These openings should be at opposite sides of the roof, running the full length of the eaves. Both ventilation openings should be 10mm wide.
You can purchase components to ensure the insulation in the roof doesn't obstruct air flow where the insulation and roof meet, too.
Where the pitched roof is a single slope, the openings will need to be at eaves level and high level (either at the junction of roof and wall or through the roof covering). The opening at high level should be at least 5mm wide, and at the highest point of the roof slope.
Following this guidance will allow for proper cross ventilation, preventing condensation.
These roofs are usually flat, but you may still have pitched roofs with a pitch less than 15 degrees, but over 0.
Whether you have a pitched roof less than 15 degrees or a completely flat roof, the regulations included in Approved Document C and BS 5250 are clear.
The ventilation openings will need to be at opposite sides of the roof again (both pitched roofs and flat roofs) to promote cross ventilation. They should run the full length of the eaves and be 25mm wide.
When a roof span exceeds 10m, then more roof ventilation may be required totalling 0.6% of the roof area.
Roof voids in these cases should have a free air space of 50mm between the roof deck and insulation layer on the underside of the roof along the roof joists.
The same rule applies to roofs with pitches 15 degrees or more, where the ceiling follows the pitch of the roof. In this case, you will also need to ensure continuous ventilation along the length of the ridge too, with an opening 5mm wide.
The ridge of the roof is simply where the two sections of roof meet, and this additional vent at the ridge is important in maintaining proper ventilation, where the roof spans more than 10m.
You'll probably have heard of cold roof voids and warm decks before, and they basically are two different roof ventilation and insulation types based on the current regulations.
A cold roof is standard and is what most people have. You will need to follow the advice above to meet regulations. However, this isn't always possible, so a warm deck roof is the alternative.
A warm deck roof refers specifically to the roof void. Rather than requiring ventilation, an exception is made because of something known as a vapour control layer.
Where effective cross ventilation can't be met, the warm deck roof has insulation placed above the roof deck, but below the weatherproof membrane. Ventilation isn't necessary, because the insulation is installed along the vapour control layers, preventing the buildup of moisture along the roof and ceiling level.
This vapour permeable membrane deals with the heat and moisture in the roof void caused by the insulation, which then guarantees thermal and structural performance of the roof without ventilation.
Cold roofs have insulation laid at ceiling joist level, leaving the roof colder than the accommodation below. The ventilation helps keep it cooler - hence the name.
Warm decks don't have that same ventilation, so they're warmer than the accommodation below.
No matter the roof's construction (warm or cold, pitched roof or flat, etc.) understanding the regulations is important to prevent moisture and guarantee the structural integrity of roofs on buildings.
Stick to the advice in this article, and read the regulations in BS 5250 and Approved Document C for yourself, and any changes you make to your eaves ventilation will definitely be within the regulations.
And, of course, always seek the advice of a professional and have them carry out the work for you if you aren't a professional yourself.
