Bitumen Sarking Felt

Defect and Repair

Hardening

Deterioration, or loss of the desirable properties of bitumen, normally takes the form of hardening. The net result is a decrease in adhesive and flow properties and an increase in the softening point temperature and coefficient of thermal expansion. Hardening of a bitumen results in a reduction in its ability to accommodate deformations without splitting. The actual mechanisms of deterioration are not fully understood, but it is known that photo-oxidation is a major factor and that it is most active at high temperatures under solar radiation. Water, dissolved compounds from atmospheric pollution, attack by microbiological organisms, and changes in its internal structure are other factors contributing to hardening of bitumens.

Splitting

Splitting occurs in a bituminous membrane when its ability to accommodate strain is exceeded. The stress producing strain may be caused by temperature variations, shrinkage due to aging, moisture changes, and the behaviour of the structure to which it adheres. Adhesion to a rigid substrate can cause the strain to take place uniformly over the membrane, but any differential movement in the substrate at joints or cracks causes great risk of high local strains and splitting.

The ability of a membrane to endure strain without failure varies with the type of materials used and the temperature and rate at which the strain takes place

Alligatoring

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

Note: Refer to roof system defects

For more defects, please refer to the Defect Library.