When is roof recovery a viable option?

May 28, 2013

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When the integrity of the roof membrane is no longer reliable it needs to be replaced. Roof replacement involves removing all existing roof material down to the roof deck, including the vapor barrier, insulation, membrane and flashing.  In some situations, it may be viable to leave the existing roof including the membrane in place and install a new membrane over top. This is often called a roof over roof application and is known as roof recover.  Roof recovery is promoted as being environmentally friendly and with its inherently lower cost is often chosen in place of roof replacement as a viable alternative.  However roof recovery is not always a viable long-term alternative to roof replacement and its life cycle may far exceed that of a complete roof replacement.

The performance of the membrane in a roof recover option will be only as good as the substrate. Therefore roof recover should never be undertaken where the existing roof, insulation or deck is known to be unsound.

A roof recover should not be done over an existing wet roof or roof containing wet insulation. While surface moisture can be easily detected and dried, the detection of moisture in the insulation will require the use of non-destructive testing such as infrared thermography. Wet areas should be replaced with dry insulation before a roof recover is carried out. The cost of such replacement should be included in the contractor’s price. If the amount of wet insulation is excessive, it may be cost effective to replace the whole roof.

Most recover options will require the use of insulation on the existing roof prior to installing a new membrane. This new insulation will provide a smoother base for the installation. Prior to the application of the insulation all irregularities in the existing roof should be removed. The insulation should be applied over a reasonably even surface and with enough asphalt or mechanical fasteners to prevent lateral movement. Some membrane manufacturers recommend special insulation and fastening methods for roof recover which should be used for the warranty to be valid.

Addition of new insulation and membrane creates a new plane for vapor condensation in the system. The recover option should be evaluated and any detrimental condensation should be reduced by changing the design as required.

The height of the curbs, perimeter, joints, drains and other penetrations will need to be raised to accommodate the additional material thickness on the roof. This will add cost to the roof recover option. Practical experience indicates that this is rarely done, reducing the life expectancy of the roof in many instances.

One condition that cannot be practically evaluated without removing the existing roofing is the condition of the deck. Often in places where the roof insulation has been wet, the steel deck may have rusted and may need replacing. If a roof is recovered there remains a risk that potential areas of corroded or otherwise deteriorated deck will go unnoticed and may cause future problems. Other unknowns include the extent of wet insulation, and the adherence of the existing roofing to the deck. Although the risks associated can be minimized to a practical limit they may never by eliminated completely.

Roof recovery can often be a viable and lower cost alternative to complete replacement, as long as it is properly designed.