Waterproofing Concrete Roofs

Waterproofing Concrete Roofs

The accurate design of falls on a flat roof is annecessary consideration in the overall drainage of water from the roof. Falls create ‘flow paths’ to channel rain-water away from the roof to suitable floor wastes and/or box gutters. To be effective, it is critical to clear surface water as rapidly as possible from the flat roof to avoid ponding or stagnation of water on the roof itself.

Water ‘ponding’ on a flat roof is a prime cause of substrate deterioration, because fluctuations in temperature between wet and dry areas of the roof can cause differential thermal movement. Together, with the accumulation of acids left by evaporating rain, this
would cause a progressive breakdown on the roof surface. In general, a minimum fall of 1 in 80 will help to prevent ponding of water.

During the construction phase, precautions must be taken to prevent excessive moisture from being trapped between the reinforced concrete roof structure and membrane. This is one of the common causes of flat roof waterproofing failure. Large amount of water vapour can evaporate from reinforced concrete or a wet screed. Once the waterproofing membrane is laid, drying out of the structural slab will mostly take place from the underside of the deck.

Cross ventilation beneath the deck is usually quite limited due to erected partition walls. Any confined moisture subjected to increased temperature from direct sun, will create vapour which will in turn exert itself directly beneath the waterproofing membrane. If this moisture vapour pressure is not released or vented sufficiently, the build-up pressure will begin to form a blister on the membrane itself, and residual dampness on the underside of the roof.

When the waterproofing membrane has suffered an irreversible stretch, subsequent cooling will not cause the air pocket to return to its original size. Therefore, if a dry surface cannot be achieved within a reasonable period of time, direct membrane adhesion should be re-scheduled, or an alternative method should be considered.

The waterproofing membrane is considered to be most important component of the roofing system, as it serves the main function of keeping water out of the building. Liquid-applied membranes are seamless, vary in degrees of flexibility, areeasy to apply, detail, maintain and repair when needed. However, supervision and adherence to manufacturer’s application recommendations during application is required, particularly in ensuring appropriate curing of concrete, consistent wet film thickness and uniform application.

One type of pre-formed membrane is the polymer-modified bitumen membranes which are applied by a heat torch or attached with an adhesive. These blended or ‘modified’ asphaltic products are bonded to a high strength fabric of polyester or fiberglass and produced into rolls. They have elongation and recovery properties, which make them suitable to protect against stresses created by wind, temperature fluctuation and normal structural expansion/contraction of the structure.

Some of these products are also modified to increase their resistance to fire, thus increasing their fire-rating. Styrene-butadiene-styrene (SBS) rolls are modified with ‘rubbers’ and compatible with petroleum products. Atactic Polypropylene Polymer (APP) rolls are modified with ‘plasticizers’, and not compatible with all petroleum-based products, grease and oils

Maintenance of residential concrete roofs are highly likely to be required during the life of the roof. There can be numerous reasons for complications to arise, such as building movement, storms, new services being installed, age, etc. For metal roofs the problems are usually isolated to specific areas. Silicone may have broken down and needs to be replaced with polyurethane. In some areas a reinforcing fabric with liquid applied membrane is required. Concrete roofs are more difficult to pin point the problem area.

Extending the life of existing metal guttering, particularly box gutters, can be achieved by applying a liquid applied membrane system. First step is to clean the gutter. When rust has occurred to the metal, remove loose rust with wire brush or mechanically to get back to the metal surface. Treat the surface with a rust converter. Then apply membrane system with reinforcing. Box gutters usually require polyurethane in all corner joints, then using strips of fibreglass matting, mould the inside shape when applying the liquid membrane. Apply at least one more coat of membrane to finish the job.

Thus the solution is often to apply a liquid applied membrane over the surface,comparable to a balcony application. Another growing trend is to create a “Green Roof”, essentially starting with waterproofing of the roof surface. ‘Green roof’ membrane systems are very specific, in order to prevent not only moisture ingress, but to prevent root systems from perforating the membrane itself.

Root resistance may be built into waterproofing membranes, either by the addition of root-inhibiting chemical additives, or because the composition of the membrane provides an impermeable barrier to root growth. Root resistant waterproofing is faster to install than separate waterproofing and root barrier layers, but can be more costly. Examples include certain types of ethylene propylene diene monomer thermosetting, thermoplastic PVC and thermoplastic polyolefin membranes; however, the root-resistance of a product must be confirmed with the manufacturer, with certification provided.

Advice should be sought from a specialist waterproofing manufacturer to find the most suitable type of waterproofing treatment for the roof structure and the proposed green roof design. The manufacturing and installation of waterproofing membranes should comply with Australian Standards.

UV degradation refers to the cracking or disintegration of materials exposed to ultraviolet radiation, most commonly due to sunlight exposure. The absorption of ultraviolet radiation energy from the sun stores enough energy within the molecular bonds of the material to cause disintegration of the intermolecular bonds holding the structure together. This is why the selection of exterior grade, liquid applied waterproofing membranes, designed specifically for roofing applications, is of paramount importance to ensure the longevity of the membrane, & concrete protection.