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How to Ventilate Your Flat Roofing System

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13th April 2018

Choosing the correct ventilation solution for a flat roof will play an integral role in extending a structure's lifespan.

It’s unavoidable that the thermal temperature in the roof space will bear the brunt of the heat generated in the living spaces below. Ventilation is required to allow the air to find a route outside of the property. This prevents it from becoming trapped inside, leading to condensation, damp and long-term damage.

The thermal and insulation regulations set out by the government in 2010 also make it a legal requirement for all buildings to meet certain standards. Our guide to ventilating a flat roof is intended to give you a basic overview of why ventilation is needed. It also covers what action needs to be taken to ensure airflow can travel freely through the roof structure.

What is Flat Roof Ventilation?

Flat roof ventilation allows for the circulation of air in the property, particularly for warmer rooms in residential buildings. There are two types of flat roof systems - warm and cold roofs – which affect the positioning of the ventilation.

When thermal insulation is placed below the deck, the ventilation must also be on the underside. In the instances where a warm flat roof has been laid, ventilation isn't usually required.

Why is it Important to Ventilate a Flat Roof?

To meet the requirements set out by the government for thermal insulation, roof ventilation may be a requirement in order to achieve the desired certification for the property.

This specifically relates to cold flat roofs which require ventilation because they are more susceptible to high levels of condensation developing on the decks and external tiling or cladding. Condensation occurs when warmer, moist air meets colder surfaces.

Commercial Flat Roof Ventilation - Roof Stores
Commercial Flat Roof Ventilation

In the case of a property, this would occur when warm air rises from the living spaces up into the cooler roof areas. Once it makes contact, it condenses and forms water droplets which will invariably lead to the spread of damp.

If damp is not addressed quickly it can quickly become a much larger problem. This can cause structural damage to fixtures and joists. If the damp penetrates the insulation, this will degrade the material, rendering it far less effective.

Rot can then start to appear in the woodwork and on other surfaces in the structure. Damp can also spread down into the interior of your home affecting plaster, wallpaper and paintwork. If mould begins to form then it not only looks unpleasant but can create health problems, especially for those with respiratory conditions.

How Does Ventilation Help Flat Roofs?

The installation of sufficient ventilation into a flat roof will ensure warmer air can rise from the living areas and find a way to escape outside without compromising the integrity of the structure.

In turn, this will keep the roof's temperature at a similar level to the level outside. This reduces the chances of condensation forming on the surfaces inside.

Roof Stores stock a range of Klober flat roof ventilation solutions designed specifically to meet the criteria above.

Ventilating a Flat Roof

When constructing a cold flat roof, the thermal insulation layer is positioned underneath the structural decking. For this type of roof to work, there must be an independent ceiling in place to support the insulation.

To ensure sufficient ventilation, enough space must be provided to allow for the movement of air, preventing the risk of condensation forming within the roof. Building regulations state that a ventilation gap of at least 50mm must be in place between the insulation and the underside of the flat roof deck.

For both a warm and cold flat roof, a vapour control layer must be installed.

  • On a warm flat roof, the layer is installed underneath the insulation to stop moisture interacting with it through rising thermal pressure lower down in the building.
  • For cold flat roofs, the vapour layer needs to be positioned underneath the insulation layer, on top of the room below to create an effective barrier.

Which Ventilation Solutions are Available?

In general, the installation of a cold roof is not recommended for the construction of new properties. Mainly due to the added requirement for roof void ventilation and the likelihood of cold bridging occurring. This is when a gap appears in the insulation which becomes far colder than the main areas leading to condensation issues.

If a property with an existing flat roof requires a replacement, the option of a warm roof should be given serious consideration. If that is not possible then new insulation should be added to the existing system to improve thermal performance. This will also require further thermal calculation to assess how likely the formation of condensation will be.

Klober Flat Roof Vents - Roof Stores
Klober Flat Roof Vents

Ventilation must also be provided through the eaves or roof ridges (using fascia vents) or the skylight upstand on the roof.

  • Klober Circular Soffit Vents provide eaves ventilation through the soffit and are a particularly easy option to install. They also come in four colour options.
  • Klober Breather Vents can be installed in both cold and warm flat roofs if needed, and their design enables them to ventilate between the layers. They work with multi-layer and three-layer roofing systems, helping to reduce the build-up of moisture.

Attention should be paid to BS5250 (Control of Condensation in Buildings) which details the requirements for ventilation. This takes into consideration the roof area, pitch and span. The basic rule being that the wider the span of the roof, the more airflow is required.

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Klober KG9856 In Line Flat Vent 0247 Brown

Klober KG9856 In Line Flat Vent 0247 Brown

Klober In Line Flat Vent. Brown, available for delivery across the UK.
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How Much Does Flat Roof Ventilation Cost?

For the most up to date pricing on our flat roof ventilation solutions click here.

The ventilation system required for a cold flat roof must be installed during the construction of the roof itself. In many cases, cold roofs are used in storage spaces where there is no habitation occurring underneath. This significantly reduces the amount of rising heat, and in turn, condensation.

The cost for vent fascias is very low and usually built into the total materials cost in a professional quote. This will also feature pricing for the insulation material. UK building regulation guidelines adhered to by the installation company will ensure there is sufficient ventilation space in the structure.

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