Spunbonded Polypropylene (PP)

In conventional roofs splits often follow insulation joints or originate at a point of weakness such as a corner or roof penetration or at joints in metal flashing to which the membrane is bonded.

Blisters are of two main types. Blueberry or pin blisters, often caused by moisture in the roof system, occur in the pour coat and may expose the felt to weather action. Additional gravel or white gravel may help or the area may need to be recoated and regravelled. Larger blisters caused by moisture and air occur between felt plies or at the membrane-insulation interface.

Ridges sometimes occur in conventional roofs, usually over insulation joints. In time, these may crack open so that there may be no alternative to replacing the roof.

Migration, in which progressive movement of the membrane takes place in one or both directions, can occur on conventional flat roofs. It contributes to ridging and splitting, piles membrane up against roof projections, and may tear flashings and counter flashings. It is probably caused by thermal forces and a poor bonding of the roof components to the deck.

Slippage of membranes on conventional, sloping roofs is evidenced by exposed bitumen on higher parts of the roof and piling up of felts on lower ones. It is produced by bitumen having too low a viscosity for the slope and is likely to occur in warm weather when roof temperatures may reach 70°C or more. Viscosity of the bitumen may increase with age enough to stabilize it. Alternatively, it may be possible by some mechanism to secure the membrane to the insulation, which is quite probably stable, or to the deck. It can also occur in protected membrane roofs if the slope and top-surface loading combine to produce a sufficiently high shear stress in the membrane, even though membrane temperatures will probably not exceed 35°C. Blocking, secured to the deck, may be required to prevent it.

Alligatoring on conventional roof membranes is caused by shrinkage cracking of the top surface bitumen. Eventually it will expose the felts to weathering damage.

Gravel scouring by high winds or water exposes conventional roofing membranes to the weather. It frequently occurs at roof corners but may take place on any area of high local wind speed. If detected at an early stage, flood coating and regravelling will correct the problem, but if the felts are damaged and curled, replacement may be necessary. On protected membrane roofs, insulation will be exposed to the sun if the gravel cover is lost. In both types of roof, larger gravel or even paving stones may be used to prevent its recurrence