Single layer systems, such as PolyElast Rapid-O Single Layer Overlay Bituminous Membrane, are effective for improving waterproofing on roofs and the longevity of an existing waterproofing system. However, the condition of the waterproofing and supporting substrate of the roof must be investigated first, before proceeding with the overlaying process.

Depending on the materials and conditions, the deck might need to be replaced first. Whichever material the deck is made from, it must be investigated to see if there is any damage. For example, if water ingress has made its way in and weakened a chipboard or stramit substrate, or if cracks have formed in concrete.

Another factor to take into consideration is if the roof is insulated from above, and the type and thickness of the insulation. The roof coverings should also be checked to see if any damage will need to be repaired or removed altogether. This may be necessary if there is any moisture. Signs of damage include opening of seams, tears, blistering, or physical damage.

After an external inspection is complete, a thorough internal inspection should be completed by someone who is knowledgeable about flat roofs. For example, stains on the ceiling or the underside of the deck should be investigated. Core samples must be undertaken to several areas across the whole field area, at both high and lower points of the falls. Then it can be determined if a Cold Roof Design is present, or if water ingress is present within the existing system. A Cold Roof Design could mean that there is condensation and insufficient ventilation.

If these factors are not taken into consideration and not investigated, it could cause further problems in time. Once all factors have been fully investigated (and fixed or replaced where necessary), the process of overlaying can commence.

All details are to be formed in a 2-Layer system of Underlay and Capsheet.