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What is a warm roof?

A warm flat roof is a type of flat roof that has insulation above the roofing joists or rafters, but below the weatherproof covering, which in this case would be the EPDM rubber. The idea of a warm roof is to make the entire structure warm by preventing any cold bridging.


What is a warm roof?

Why use a warm roof?

Energy efficiency: By the installation of a warm roof, the room below will benefit from being warmer in the winter and cooler in the summer. This can lower energy bills and improve carbon emissions. Additionally, current building regulations are striving to enforce the insulation upgrade as compulsory to create energy efficient roofing.

Less damp potential: Traditionally, older flat roofs which were insulated with wool type (also known as rockwool) insulation, were fitted between the joists with no damp proof membrane or ventilation. Aside from the low energy performance of this method, it can also cause damp and condensation due to warm and cold air collision in the roof void.

By the installation of a warm roof with rigid insulation sheets atop of damp proof membrane, the condensation is prevented without the need for ventilation.

Applies to current building regulations:

Current building regulations stipulate that since 2006 and the implementation of approved Part L documents (Conservation of Fuel & Power) an existing flat roof has been required by law to comply with rigorous u-value targets, based on any refurbishment of more than 50% of the existing flat roof. For new build applications it will also be necessary to ensure your flat roof is insulated to the appropriate u-values and meets the approval of your local building control.

Whilst we appreciate that this is not viable for everyone wanting a flat roof renovation, it is certainly worthwhile noting the importance placed on the insulation of your flat roof.


Warm roof vs a cold roof

Firstly, to clarify: A cold roof is a roof which has the insulation placed in between the roofing joists. This type of system rarely uses any form of damp proof membrane and so requires some form of ventilation, often through the fascia.

The disadvantage to a cold roof is that it allows for cold bridging through the joists, which is where an element allows heat transfer and the loss of energy, making it a less energy efficient method. As mentioned, it also requires ventilation, which is not always viable or aesthetic.

The benefit of a cold roof as opposed to warm roof however, is that it does not require an increase in roof height, which can be an issue in some cases. By adding between 25mm and 125 to the roof height as well as an extra layer of 18mm OSB sterling board, the flat roof height is increased significantly. This also subsequently requires deeper fascia to be installed to cover the installation.

In terms of costing, the cost for deeper fascia is often off-set by the ability to install the warm roof from above, which as opposed to a cold roof, often requires the insulation installed from below, involving additional ceiling works.


Our experience and how its done

There are many types of insulation available and also different ways to install it. However, having installed a multitude of insulated flat roofs over the past several years, wherever applicable we use the following method:

  • Installation of firings (if necessary to create a steeper roof gradient for water dispersal).
  • Installation of 18mm OSB sterling board onto the firings or joists and laid staggered for less crosshair movement.
  • Installation of damp proof membrane and subsequent Xtratherm 8ftx4ft insulation (depth dependent upon specification), laid staggered.
  • Installation of a top layer of 18mm OSB sterling board, fixed through the insulation and into the first layer of OSB.
  • Installation of Firestone EPDM rubber and finished to roof requirements.

The benefit of this system is that there is a sandwich effect on the insulation providing a structurally sound flat roof. If the weight is a potential problem (which is very rare), there is always the option of 11mm OSB also.

Many installers fit their roof covering directly onto the insulation or onto 5.5mm ply which can have the disadvantage of allowing too much contraction and expansion of the roof covering as a result of the insulation, which in the case of fiberglass and felt, can cause cracks and tears as they are not designed to allow the same amount of movement as EPDM.