Module 4: Flat roof insulation


Flat roofs with pitches below 10 degrees and shallow pitched roofs below 20 degrees are detailed differently to steeper pitches due to the roof coverings necessary at these pitches.

Starting with pitched roofs below 20 degrees with a slate or tile covering, the roof build-up is the same as for the steeper pitches but all of the joints of the wood fibre sarking boards must be taped or sealed to prevent water penetrating the joints. Alternatively, a breather membrane with all of the joints well sealed can be used to cover the wood fibre sarking boards prior to application of the counter battens. For more traditional flat roof coverings such as single ply membranes or green roofs, the build-ups change.

Standard flat roof construction

For standard flat roof constructions covered with single ply membrane/EPDM or bituminous sheets the following build-up would be typical:-

Single Ply membrane
18mm Ply
200mm rafters @ 600mm ccs, fully filled with SteicoFlex 036 flexible wood fibre insulation
25mm counter-battens
12mm plasterboard

This construction achieves a U-value of 0.13 W/m2K.

This is an unventilated flat roof construction which can save considerable amounts of space over more traditional synthetic materials and also overcomes issues relating to ventilating flat roofs. As with all wood fibre constructions, this is a very simple, fast and economical roof construction method with excellent acoustic insulation and decrement delay. These properties make this type of construction ideal for roof dormers which are often used above bedroom areas.

The construction works by the variable vapour control layer remaining very vapour tight during the winter months, preventing significant moisture accumulation . As the temperature and humidity within the structure rises due to increased sunlight heating the roof, the variable vapour control layer becomes more permeable and allows excess humidity to pass back towards the interior of the building. The annual process ensures that there is no accumulation of moisture in the roof but is only possible because of the moisture storage ability of the wood fibre sarking board on top of the joists.

Green roofs or metal covered flat roofs

When constructing standing seam roofs or green roofs the construction above will not work for two separate reasons. Firstly, in the case of the green roof construction, the layer of plants that sits on top of the roof acts as a very good insulator. This would seem beneficial, however, this prevents the heat from the sun raising the temperature and humidity in the roof to the point that it can dry towards the interior which then raises the risk of moisture accumulation over the long term.

Secondly, in the case of the standing seam roofing, metal sheets tend to accumulate moisture under them as night time cooling allows condensation to form on them. Without adequate ventilation, this can accumulate and again cause problems.

In both of these scenarios the typical construction would be as follows:-

Green roof covering or metal roof covering
18mm Ply
50mm counter-battened ventilated void
200mm rafters @ 600mm ccs, fully filled with SteicoFlex 036 flexible wood fibre insulation
25mm counter-battens
12mm plasterboard

This construction also achieves a U-value of 0.13 W/m2K.

Please note that although these images do not show it, most flat roofs still have some pitch to the surface of them to ensure they drain correctly. This applies here too and is vital to the long term functioning of the roof. Without adequate drainage the surface will eventually fail.

Construction of flat roofs

In the dry

In terms of construction, for vented or unvented roof construction, it is important to keep the materials dry. The ability of this type of structure to remove moisture trapped in during construction is much lower than for a pitched roof and therefore greater care must be taken.

If the roof is small enough to be effectively covered with a tarpaulin or if construction is taking place during a dry period then the flexible wood fibre, SteicoFlex, can be installed first. The friction between the insulation and the joists/rafters is usually enough to keep it in place without any support from below. If the section being insulated is made up from off-cuts then temporary support from battens or OSB should be used under the joists.

Once in place the UdiTOP/Beltermo Ultra wood fibre sarking boards can be fitted, followed by the rest of the build-up. The roof structure should be protected until the waterproof covering is applied to the top surface and the structure is weather tight. If any leaks have occurred the structure must be left to dry before applying the variable vapour control layer membrane to the underside.

The Ampatex Variano variable vapour control layer membrane should be applied as a continuous layer to the underside of the joists with no penetrations, other than vent pipes, and carefully fixed with either staples or double sided tapes. The joints in the membrane and staple heads must be well taped with an acrylate adhesive tape, such as Ampacoll Flexx or Ampacoll INT, to ensure airtightness.

The perimeter of the membrane should connect in to the vapour control or airtightness layer in the walls to ensure there is no flow of moisture into the roof structure. Ampacoll Fenax tape is ideal for this whether the walls are masonry or timber frame.

In the wet

If the roof structure is likely to get wet during installation then the flexible wood fibre batts should not be installed before the sarking boards as they will soak up rainwater. The UdiTop/Beltermo Ultra boards will cope with the rain, especially if there is nothing below them absorbing rain water but should be protected from continuous rain.

The sarking board layer should be completed but should not be covered with either the breather membrane or the ply layer if the surface of the boards is wet. As a rule of thumb, try not to add layers to the roof whilst the layer below is wet.

Once the waterproof layer on the top of the roof is complete the flexible wood fibre insulation can be installed between the joists, from below. This will require the use of goggles and a dust mask as the fibres in the insulation do tend to drop out whilst pushing the batts into place. Whilst the fibres are not hazardous they will cause discomfort in the eyes and in the lungs.

The vapour control layer can be installed as per the above process and then battens should be fixed to the underside of the joists, running perpendicular to the joists, to support the insulation and prevent it from dropping out for any reason. These should be positioned at centres suitable for the ceiling boards.

If downlights are being installed in the ceiling then the battens should be sized to create the appropriate void depth. Under no circumstances should the vapour control layer membrane be cut into for lighting to recess into. This would allow uncontrolled amounts of moisture into the structure and ensure it’s eventual failure.