Firstly there are two types of flat roof.
Cold deck flat roof.
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.
This system is called warm deck insulation and eradicates the condensation and venting issues associated with the cold deck system.
The insulation is placed below the waterproof covering and is either mechanically fixed or bitumen bonded on to the top of the deck.
A cold flat roof insulation requires more work taking off old boards and replacing them and the roof surface.
Condensation occurs when warmer moist air meets colder surfaces.
Inverted warm deck roof.
This design dates back many years and flat roof insulation was somewhat of an afterthought with this type of roof.
Where a cold flat roof does incorporate insulation this is.
A warm roof and a cold roof.
The solution warm deck insulation.
The epdm rubber roof membrane is then installed using the contact adhesive onto the deck.
The breakdown warm roof vs cold roof.
The insulation boards in an inverted warm deck are laid over the structural deck and the waterproof covering.
Traditional cold deck flat roof.
Alternatively battens can be secured to the underside of the joists and gmw fitted between these in much the same way as between the main roof joists.
A warm roof is defined as being when the all or the majority of the insulation is above the joists or deck.
The sandwich warm deck roof is the most common type of flat roof.
Fortunately these days there is a better form of insulation available for your flat roof.
The insulation in a warm deck roof is installed immediately below the roof membrane on top of the roof deck usually with a vapour control layer underneath the insulation.
The layer of insulation is essentially placed outside of the roof.
The underside of the rafters in this case will need to be finished with a suitable vapour control layer vcl and plasterboard to provide a suitable internal finish to the roof.
A ventilation gap usually 50mm should be provided between the top of the insulation and underside of the roof covering to allow the air to flow across.
The roof itself is therefore kept warm.
The 18mm roof grade deck is then fixed on top of the rafters leaving suitable air flow between the insulation and the deck.
This is known as a cold roof because the roof deck and roof space are close to the external air temperature.
Traditionally flat roofs simply consisted of a structural timber deck with a waterproof covering directly applied to the top of the deck.
Contrasted with the cold deck roof this construction has a much lower risk of condensation as the void will be of a comparable temperature to the roof.
