Flat roofs are a common choice for a range of projects, including ground level extensions. However, on older buildings, they can often be very poorly insulated and a potentially major source of heat loss. If you are carrying out work to repair a flat roof, it’s a good idea to look at whether you can install insulation.
In this article, we look at some of the points to consider when fitting insulation over an existing flat roof, including:
- When it is possible to fit insulation directly over the existing waterproofing.
- Which insulation materials are best for flat roof refurbishments.
- What U-values you need to achieve.
Can you insulate over an existing flat roof?
In order to overlay a warm flat roof, a moisture test must be carried out using a moisture meter. If the existing insulation is wet it must be removed. All components must be in good condition - this includes the full build up. Any areas of waterproofing which are defective must be properly repaired. Remedial works must be carried out prior to overlaying. If the membrane has failed it must be removed.
Which flat roof insulation is best when refurbishing a flat roof?
This will depend on the condition of your roof and how thin the construction needs to be.
Where a suitable fall (at least 1:80) is present, our range of Thermaroof flat roof insulation boards can provide thermally efficient and cost-effective solutions which are compatible with most commonly-used waterproofing systems.
Learn more about our flat roofing insulation.
In some cases, a very thin insulation solution may be needed. For example, this can apply where there is an existing parapet wall. In this case, the minimum height of the wall should be 150 mm above finished roof level, unless it is a perimeter curb detail where the water is to flow away from the curb, in which case 50 mm is the minimum height. On a level threshold a minimum height of 75 mm must be achieved with the flow of water running away with addition overflow.
Where there is limited space available to achieve this, a Vacuum Insulation Panel (VIP) system can be the answer. Our OPTIM-R Roofing System features VIPs which have a declared thermal conductivity of just 0.007 W/mK – far lower than commonly used insulation materials. This makes it possible to achieve your U-value with a very thin insulation thickness. The OPTIM-R Roofing System also features rigid insulation infill panels of the same thickness which can be cut to fill gaps or to allow penetrations through the insulation layer.
If there are issues with water ponding on the existing flat roof, then a tapered insulation system can be a good solution. Our Thermataper range solutions are, again, compatible with most commonly-used waterproofing systems and feature angled (tapered) boards which can be used to achieve a fall and insulation in a single system. We offer a supporting design service to help to achieve both your desired U-value and the required drainage in a single system.
Learn more about tapered insulation
Do I need to fit insulation when repairing a flat roof?
This will depend on the level of work being carried out. If you are either replacing the existing membrane, or the full deck, then you will need to fit insulation as part of the work.
Where you are replacing the full deck, you should typically look to achieve the U-values for “new thermal elements” shown in the table below. If you are just fitting a new membrane, then you will typically need to meet the U-values for “replacement thermal elements” shown in the table. In all cases, no area of the roof should achieve a point U-value worse than the threshold of 0.35 W/m2K.
| Area weighted average U-value (W/m2K) | |||||
| England | Wales | Scotland | |||
Element type | Domestic | Non-domestic | Domestic | Non-domestic | Domestic | Non-domestic |
New thermal elements | 0.15 | 0.18 | 0.13 | 0.15 (domestic in character) | 0.12 | 0.16 |
Replacement thermal elements | 0.16 | 0.18 | 0.16 | 0.18 | 0.12 | 0.16 |
Should I install insulation above or below the existing roof deck?
In almost all cases, we recommend fitting the insulation above the roof deck. This is known as a warm roof construction.
BS 6229: 2018 advises that cold roof approaches (where insulation is fitted below the deck) “are not recommended”. A hybrid approach, where insulation is fitted above and below the roof deck, should not be used.
In all cases, a condensation risk analysis should be carried out alongside the U-value calculations for the new insulation layer.
How to prepare an existing flat roof for insulation.
Before installing the insulation system, all components must be in good condition including the joists, deck, vapour control layer (VCL), insulation and finished membrane. If moisture or any other issues are present, these must be effectively addressed.
If you intend to use the existing waterproofing system as a vapour control layer, you will need to carefully inspect it to ensure it is in good condition with no tears, holes or signs of bubbling which typically indicates water has penetrated below the waterproofing layer.
The deck should then be cleaned ensuring there are no projections, such as fixing heads, and that the surface is dry, ready for the insulation layer.
For specific best practice when fitting our insulation systems, look at the Sitework section of our product literature.
On site training and support
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