Waterproofing Membranes

Waterproofing membranes are the continuous waterproof layers on flat and low-slope roofs, podiums, plant decks, terraces, planters and balconies. They sit on a structural deck, usually with insulation above or below, and are often hidden under ballast, paving on pedestals, soil or tiles. The membrane turns up at parapets and walls, terminates under flashings, and is penetrated by outlets, plant supports and balustrade posts.

How insulation is arranged decides how failure plays out. In a warm roof the insulation is above the deck and under the membrane; a leak saturates the insulation, which then holds water across a wide area. In an inverted roof the insulation is above the membrane, protecting it, and is held down by ballast. In a cold roof the insulation is below the deck, and condensation in the deck or void becomes the risk.

What shortens life:

• Ponding from flat or reversed falls, deflecting decks and blocked outlets.
• Foot traffic and plant on unprotected membranes; plant installed later through the membrane without proper penetration details.
• UV exposure on membranes intended to be protected.
• Movement at upstands, deck joints and changes of material.
• Roots in planters and green roofs without a root barrier.
• Incompatible materials: PVC in contact with bitumen or polystyrene; solvents on EPDM.

Failure at details

Membranes rarely fail first in the open field. They fail at upstands where the membrane turns up a wall and is terminated, at outlets, at penetrations and at changes of level. Terminations pull out of their chases or bars; upstands are too low and water rises above them; outlets corrode or detach from the membrane.

Seam and lap failure

Single-ply seams that were under-welded or contaminated open over time; EPDM tape and adhesive seams debond; bitumen laps lift. Seam failures run in lines across the roof.

Blisters and splits

Moisture or air trapped between layers or under the membrane expands in heat and forms blisters, which crack when walked on. Splits form over deck joints, at insulation board edges and where the membrane is stressed by thermal shock or movement.

Surface ageing

Bitumen surfaces craze (alligatoring), lose their mineral surfacing and harden. PVC loses plasticiser, shrinks and becomes brittle. EPDM shrinks and pulls at its perimeter, bridging across the angle at upstands until it tears.

Wet insulation

In warm roofs, water entering through any defect spreads through the insulation below the membrane. Wet insulation loses its thermal value, adds load, rots timber decks, corrodes steel decks and fixings, and keeps feeding leaks long after the original defect is repaired. It is the most expensive hidden failure on a flat roof, because the roof cannot simply be re-covered.

Wind uplift

Mechanically fastened single-ply membranes rely on fasteners into the deck. Corroded or under-specified fasteners, and corners and perimeters with too few, let the membrane billow and tear in wind.

Liquid coating failure

Liquid membranes fail where they were applied too thin, over damp or dusty substrates, or across cracks that move. They crack, peel and blister, often over the whole area, and the cause is usually the original application.

From aerial imagery

• Ponding marks and dark rings where water has stood; vegetation in ballast and at outlets.
• Patches in different materials; areas of new membrane; coatings over old membranes.
• Plant added since the roof was laid, with its supports through the membrane.
• Lifted or billowing single-ply at corners and perimeters after wind.

On the roof

• Open seams and laps; blisters; splits over deck joints and board edges.
• Upstands lower than the water line; terminations pulled out; sealant at terminations failed.
• Outlets corroded, blocked, detached or set above the membrane level.
• Soft or spongy areas underfoot: wet insulation beneath.
• EPDM bridging at upstands; brittle PVC that cracks when folded.
• Wrinkles and ridges in the membrane over moving insulation boards.

Below the roof

• Staining, blistering paint and efflorescence on soffits; drips at deck joints.
• Corrosion of steel deck and fixings visible from below.
• Leaks that continue for days after rain stops: water held in insulation.
• In car parks under podiums, staining and corrosion in lines along joints.

In the records

• Membrane specification, installer and warranty, with date.
• Leak register and repair history; repeated repairs in different places.
• Roof surveys, core reports and moisture surveys.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A roofing consultant or building surveyor leads, with a specialist leak detection contractor where needed.

• Visual survey of the field, seams, upstands, terminations, outlets and penetrations, with a plan of defects.
• Core cuts to identify the build-up, membrane type and layers, and insulation condition; cores sealed after.
• Moisture survey by infrared thermography at night, capacitance or impedance meters on a grid, or nuclear moisture gauges.
• Electronic leak detection (low-voltage or high-voltage methods) to locate breaches, including under ballast or overburden; ASTM D7877 guides the methods.
• Flood testing of balconies and small decks, checking deck capacity first.
• Seam probing on single-ply roofs.
• Fastener pull-out tests on mechanically fastened roofs.

The report should state the build-up, membrane type and age, defects and their locations, the extent of wet insulation, and whether the roof can be repaired, recovered or must be stripped.

• Detail repairs: renew upstands, terminations, outlets and penetration flashings; often the best value on an otherwise sound field.
• Seam repair: reweld or re-tape seams, or overlay with cover strips.
• Recover: a new membrane over the old, only where the insulation is dry, the deck sound, and the code allows the extra layer.
• Restoration coatings over sound ageing membranes to extend life; not over wet insulation.
• Strip and replace: remove membrane and wet insulation, repair the deck, correct falls with tapered insulation, new membrane and outlets.
• Drainage: add outlets and overflows, correct falls, clear and maintain outlets.
• Protection: walkways to plant, raised plant supports, protection boards under pavers.

Interactions: replacing a membrane usually means lifting and reinstating whatever sits on it, which can cost more than the membrane; insulation upgrades may be required; roof safety systems and plant supports are renewed with it.

Moisture surveys and cores are lump sums. Detail repairs are itemised per outlet, penetration or linear metre of upstand. Replacement is priced by area with separate lines for strip-out, wet insulation removal, deck repair, new insulation and falls, upstands and outlets, and reinstatement of ballast, pavers, planters and plant. Reinstatement is often the largest component on terraces and podiums.

The usual shape where leaks exist and insulation status is unknown is two-part: moisture survey, then a conditional envelope that is a detail repair if the insulation is dry and a strip-and-replace if it is wet. Replacement is lifecycle; detail repairs and outlet clearing are repairs and maintenance.

• Australia. The National Construction Code calls up AS 4654.1 and AS 4654.2 for external above-ground membranes; roof drainage follows AS/NZS 3500.3. On strata buildings, roof, podium and balcony membranes are usually common property and a leading head of defect claims; New South Wales and Victoria have statutory defect and warranty periods for waterproofing.
• United Kingdom. BS 6229 sets the code of practice for flat roofs with continuously supported coverings, BS 8217 covers reinforced bitumen membranes and BS 8218 mastic asphalt; Approved Document L applies thermal upgrades on renovation, and Approved Document B and regulation 7(2) bear on roof build-ups on relevant buildings.
• United States. International Building Code Chapter 15 governs roof assemblies and re-covering; single-ply membranes follow ASTM D6878 (TPO), D4434 (PVC) and D4637 (EPDM); ASTM D7877 guides electronic leak detection. Insurers commonly require FM Approvals ratings for wind uplift and fire.

• Membrane specification, installer and warranty, with installation date.
• Leak register, repair history and any leak investigation or moisture survey.
• Roof drawings showing falls, outlets, overflows and the build-up.
• Records of plant added through the roof since installation.
• Outlet cleaning and roof maintenance schedule.
• Defect claims and expert reports inside statutory defect periods.