Flat roofing

System failures and detail failures

Accurate diagnosis of a defect is an essential prerequisite to specifying a suitable repair. Understanding whether or not the failure is due to system or detail will assist in the selection of a suitable and cost effective solution without necessarily overspecifying or adopting an over-optimistic approach.

A system failure is simply a failure in the roofing material that could occur at any point over the roof as a whole. For example, a defective roofing specification (unsuitable insulation, missing vapour barrier, unsound deck) could promote multiple failures. If one defective area is repaired there is every likelihood that other defects could occur elsewhere for the same cause. In the case of a system failure, replacement of the entire roof is probably the only sensible option.

On the other hand a detail failure occurs at a specific point or points and is related in some way to the detailing of the roof covering at that position. Repair would be possible, with no reason to doubt that the remaining areas would perform adequately.

Examples of system faults include:

Basic cause Materials at risk Effects
Problems due to moisture

Affects organic materials such as chipboard, plywood, strawboard, etc. Results in swelling, sagging, degeneration due to biological effects, rot.

Moisture damage to HAC roof decks - risk of corrosion to reinforcement or alkaline hydrolysis.

 Swelling can cause compressive stress in the deck, relieved by bowing. If the membrane is fully bonded, tensile failure may occur. Degeneration of the deck will result in loss of support and rupture of the roofing system.
Problems due to temperature

Can affect certain types of rigid foam, resulting in shrinkage or expansion.

Expansion movements in large concrete roof decks resulting in bowing.

 Places stresses on the membrane at board joint positions, rippling or rupture of membrane.
Problems due to a combination of moisture sensitive and temperature sensitive materials Typically affecting woodwool slabs. As above.
Problems due to poor design All. Inadequate design leading to deflection of the roof or risk of wind uplift; increased flexibility, risk of overloading, etc.

Examples of detail faults include:

Basic cause Materials and locations at risk Effects
Moisture

Localised faults in laps of BUR roofing.

Missing or defective isolation joint at perimeter or upstand details - BUR and asphalt.

Defective upstand detailing, BUR and asphalt.

Lack of provision for movement at movement joints or changes in structure.

 Localised water ingress, blistering. Repairs can be executed locally; total renewal not required.
Temperature

Slumping of asphalt at changes in level, upstands, etc.

Cracking due to brittle failure at low temperatures

 Localised water ingress, blistering. Repairs can be executed locally; total renewal not required.