What is a flat roof recovery board?

A flat roof recovery board is placed directly over the existing waterproofing layer of a roof that requires refurbishment and provides a firm, flat surface to which the new roof covering can be adhered or mechanically attached.

Flat roof recovery boards act as a separating layer between the existing and new waterproofing and can add an extra layer of protection to the existing build-up of the roof. Some flat roof recovery boards have fibre glass mat facers, and these can help increase the compatibility where adhered membranes are used.

Flat roof recovery boards that offer a clean, flat, homogeneous surface compatible with the new waterproofing system help provide the optimum conditions for producing a robust and resilient waterproofing finish for roof refurbishment.

What are the most popular types of flat roof recovery boards and why?

The most popular types of flat roof recovery boards can be cementitious or gypsum core roof cover boards.

Cementitious flat roof recovery boards typically use cement to bind together either wood flakes or cellulose fibres to form a cement fibre board. They are classified as non-combustible, impact and water resistant; however, they can be heavy and difficult to cut.

The gypsum used in most gypsum core boards is a naturally occurring mineral. The Gypsum Association says, ‘ Calcium Sulfate Dihydrate (CaSO₄∙2H₂O), otherwise known as gypsum, is an inert compound containing up to 21 percent, by weight, chemically combined water. Both natural gypsum and synthetic gypsum share this inherent characteristic, which can be leveraged to provide buildings, with passive fire resistance.’

The non-combustible gypsum core flat roof recovery boards can have facers such as glass fibre and be treated to increase their water resistance capability. They are lighter than cementitious boards and tend to be easier to cut and handle.

The dense, non-combustible nature of both cementitious and gypsum core flat roof recovery boards makes them compatible with a wide range of waterproofing systems so that they can bring the most overall benefits to the refurbished roof.

The extra rigidity provided by a flat roof recovery board increases the robustness of the finished roof. The recovery board creates a firm, flat substrate to which the waterproofing membrane can be mechanically fixed or adhered. Recovery boards that feature fibreglass mat facers also improve the compatibility with the membrane and promote a more even spread of adhesives. Therefore, adding a flat roof recovery board as part of the refurbishment can provide extra resistance to the wind uplift forces that can try to lift the waterproofing surface away from the roof deck during high winds. Including a dense, flat roof recovery board can also provide a layer that can help reduce the transmission of external noises such as rainfall and increases the acoustic performance of the new roof build-up.

Choosing a flat roof recovery board that is non-combustible can provide an extra layer of fire resistance to the refurbished roof. This is useful if a torch-on built-up roofing waterproofing system is used. Additionally, non-combustible flat roof recovery boards can also help to limit flame spread across the flat roof and provide an extra layer of fire resistance to help protect the deck should the roof be subject to an external fire source.

Those flat roof recovery boards that are compatible with a variety of waterproofing systems give more freedom to the specifier. This increased compatibility can promote a better bond between the waterproofing layer and a flat roof recovery board and can provide the refurbished roof with extra resistance to damage caused by foot traffic or dropped tools during routine maintenance.

Georgia-Pacific provides a range of non-combustible, glass mat faced gypsum core flat roof recovery boards for use with a wide range of waterproofing systems. DensDeck® Roof Board is classified as A1 in accordance with EN 13501-1 and non-combustible as described and tested in accordance with ASTM E136.

If you would like to discuss how our DensDeck® Roof Boards when used as flat roof recovery boards could help with your next flat roof refurbishment project, please contact our team for advice today.