In cold and very cold climates selecting a roof deck condensing surface temperature criteria of 45 f is very conservative.
Cold deck flat roof design.
In the case of a flat roof insulation is usually either a rigid board above the deck or a mineral glass wool quilt immediately above the underdrawn ceiling depending on the type of roof design and construction.
This temperature criteria can be reduced to 40 f which corresponds to interior average conditions of 70 f 35 rh where high interior moisture loads due to spas indoor swimming pools and excessive humidification are.
There are three recognised designs of flat roof construction.
The alternative roofing insulation design places the insulation layer between or under the timber rafters level with the ceiling joist.
Can be slightly more difficult to install than a cold roof design.
The waterproof layer is above the insulation and a vapour control layer is placed below the insulation.
Cold flat roof designs.
This therefore leaves the roof and timbers cold requiring a breathable membrane between the insulation and roof.
So what exactly is a cold roof.
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.
Flat roof design guide.
Hopefully this video will help explain the difference between cold flat roof construction and warm flat roof construction this videos show detailed drawings.
The sandwich warm deck roof is the most common type of flat roof.
Where a cold flat roof does incorporate insulation this is in the form of mineral wool.
Warm cold and inverted.
The recognised flat roof build ups for best design practices are.
The insulation is placed below the waterproof covering and is either mechanically fixed or bitumen bonded on to the top of the deck.
Which makes sense as this space is otherwise empty and so it s a simple low impact way to insulate a roof.
What is a cold roof.
A cold roof design usually leaves a 50mm gap between the top of the insulation and the roof deck above.
A cold roof has the insulation placed below the structural deck which thus remains cold.
This design dates back many years and flat roof insulation was somewhat of an afterthought with this type of roof.
A warm roof has the insulation above the structural deck which is thus kept warm.
There are two basic categories of roof design.
In the case of a flat roof insulation is usually in the form of either a rigid board above the deck or a fibrous quilt immediately above the ceiling depending on the type of roof construction.
Inverted warm deck roof.
