Single-Ply Roofing

Single-ply roofing is a family of low-slope and flat roof waterproofing systems in which the weatherproof layer is one sheet of flexible, factory-made polymer membrane. It is not built up on site from several plies of felt and bitumen. Sheets are overlapped at the edges and joined into a continuous skin. Thermoplastic membranes are joined by hot-air welding, and cured rubber membranes by adhesives and seam tapes.⁴⁶ The membrane is held in place by mechanical fasteners, by adhesive or by ballast.¹ In the United Kingdom the products are usually called "polymeric single ply". In North America they are often referred to by their polymer: TPO, EPDM or PVC.¹²

Single-ply systems now dominate new commercial low-slope roofing in North America and are widely used in Europe. In a 2023 survey of US roofing contractors, about 81% of commercial roofers installed single-ply, and it made up 36% of their commercial revenue, more than any other roof type.⁸

Membrane families

Single-ply membranes fall into two broad chemical groups.

Thermoplastics

Thermoplastic membranes soften when heated and harden again as they cool, so overlapping sheets can be fused into a homogeneous seam with hot air.⁶ The main types are:

• PVC (polyvinyl chloride). PVC resin, compounded with plasticisers, stabilisers, fillers and pigments, makes up more than half of the polymer content.³ The US product standard is ASTM D4434. It recognises fibre-reinforced (Type II) and fabric-reinforced (Types III and IV) sheets. Type I has been withdrawn because the product is no longer made.³ Types II and III have physical requirements based on a minimum 45 mil (1.14 mm) sheet. Fabric-reinforced sheets need at least 16 mil (0.40 mm) of material over the scrim.³²
• KEE (ketone ethylene ester) and PVC alloys. In KEE membranes the KEE polymer is at least 50% by weight of the polymer content. They are covered by ASTM D6754, with a basis thickness of 32 mil.³ Blends with less KEE are usually sold as PVC membranes to ASTM D4434.³
• TPO (thermoplastic polyolefin), also called FPO (flexible polyolefin). TPO is based on polyolefins and contains no chlorinated ingredients, which is often cited as an environmental advantage over PVC.⁹ It is somewhat stiffer than PVC.⁹ The US standard is ASTM D6878.²⁶

Thermosets

Thermoset membranes are cured (vulcanised) synthetic rubbers that cannot be re-melted. Their seams must be bonded with adhesives or tapes.⁴

• EPDM (ethylene propylene diene monomer). EPDM is the dominant thermoset. It resists weathering, ultraviolet light, ozone, thermal shock and cyclic fatigue well.⁹ The US standard is ASTM D4637.² Sheets are made non-reinforced, scrim-reinforced or fleece-backed, in thicknesses of 45, 60 and 90 mil (1.1, 1.5 and 2.3 mm).⁴

History

Origins in Europe and the United States

Single-ply roofing grew out of the post-war synthetic rubber and plastics industries. In Switzerland, the company Sarna was formed in 1958 to develop polymer products. In 1962 its chemists produced the first fabric-reinforced PVC roofing membrane, patented and sold as Sarnafil.⁵ Early PVC membranes suffered from brittleness, a problem since overcome by better formulations.⁹ In the United States, EPDM single-ply membranes were introduced in the same year, 1962.⁴

The 1970s boom

The oil embargo of the 1970s raised the price of asphalt and lowered its quality. That made multi-ply built-up roofs less attractive and gave single-ply membranes their opening.⁴ Most EPDM roofs of the period were loose-laid and held down by ballast stone. Their seams were cleaned with white gas and joined with a neoprene-based splice adhesive, which could lose strength under prolonged ponding.⁴ The UK industry dates its own single-ply experience from the late 1970s. The Single Ply Roofing Association's 2012 design guide drew on more than 35 years of British practice.¹

Better seams and reinforcement

The 1980s and 1990s brought steady improvements, especially to EPDM:⁴

• Mid-1980s. Butyl-based splice adhesive replaced the neoprene system, and scrim-reinforced EPDM sheets appeared.
• 1989. Perimeter securement strips, reinforced membrane strips fastened with plates, came into use for mechanically attached roofs.
• 1996. Fleece-backed EPDM was introduced. It raised the puncture resistance of a 60 mil sheet by about 300%.
• About 2005. Factory-applied splice primer and seam tape cut seam-related warranty claims by about 80% for one major manufacturer.

The industry's US trade association, SPRI, has represented single-ply manufacturers since 1981. It publishes ANSI standards, and its research on installation and performance informs building code development.⁷

The rise of TPO

TPO membranes were used in Europe from the 1980s and entered the US market in the early 1990s.⁶ Early generations had problems, including "false" welds at lap seams and difficulties traced to halogenated fire retardants. These were later reformulated.⁶ The first American standard for TPO, ASTM D6878, appeared in 2003 and was tightened several times:⁶

• 2006. The required ultraviolet (xenon arc) exposure was doubled, from 5,040 to 10,080 kJ/(m²·nm).
• 2011. Heat ageing before testing was extended from 670 hours (28 days) to 5,376 hours (224 days) at 116 °C (240 °F). The minimum coating over the scrim of a 45 mil sheet rose from 12 to 15 mil.
• 2013. The top coating had to be at least 30% of the sheet's thickness and no less than 15 mil.

Thickness preferences shifted in parallel. In the mid-1990s about two-thirds of TPO sold was 45 mil, and today about two-thirds is 60 mil.⁶ TPO became the leading single-ply membrane in the United States. Among contractors installing single-ply in 2023, 41% named TPO as driving their sales, against 30% for EPDM and 17% for PVC.⁸

Roof build-ups

A single-ply membrane is always part of a system, and its position relative to the insulation defines the roof type:¹

• Warm roof. The most common arrangement. Insulation boards lie directly under the membrane, above a vapour control layer, so the deck stays warm.
• Inverted (protected membrane) roof. The membrane lies on the deck. Moisture-resistant insulation, a filter layer and ballast go on top, protecting the membrane from sun, traffic and temperature swings.
• Duo or hybrid roof. The membrane sits between two layers of insulation.
• Cold roof. Insulation is at ceiling level with a ventilated void above. It is rarely feasible and is not recommended for single ply.

Single-ply membranes are also widely used as the waterproofing layer under green roofs. These may be intensive, simple-intensive, extensive or "brown" biodiverse roofs.¹⁹

Falls and drainage

Single-ply roofs are low-slope, not truly flat. The British code of practice BS 6229 requires a minimum finished fall of 1:80 (1.25%) at every point. To allow for deflection and building tolerances, designers are advised to use a design fall of about 1:40 (2.5%). Tapered insulation is commonly cut to 1:60 or 1:40.¹ Gutters and outlets are designed to BS EN 12056, typically for a 30 mm (1.2 in) head of water.¹

Attachment methods

The membrane must resist wind suction. The attachment method is chosen after the wind load has been calculated, in the UK to BS EN 1991-1-4 and its National Annex. The calculation takes account of building height, site altitude, topography and roof shape.¹ There are three methods:

1. Mechanically attached. Fasteners with plates or bars hold the sheet to the deck. They may sit in the seam overlap, run in lines across the sheet, or be hidden beneath it by induction welding or secret fixing.¹
2. Fully adhered. The membrane is bonded to the substrate across its whole area with solvent or water-based adhesive, or through a fleece backing.¹ On profiled metal decks, bonded systems need more than 49% contact area.¹
3. Ballasted. The membrane is laid loose and held down by rounded stone or concrete pavers, an approach common on early EPDM roofs.⁴¹ Ballast also protects the sheet and improves fire performance.¹

Whatever the field attachment, the membrane must always be mechanically restrained at the perimeter, at changes of slope and around details.¹ Where an insurer requires an FM Approved assembly, FM Global Data Sheet 1-28 applies as well.¹

Installation

On a typical warm roof the work proceeds in this order:

1. Deck preparation. The deck is checked for falls and deflection, and a vapour control layer is laid.¹
2. Insulation. Insulation boards are laid and fixed. In refurbishment work they may be tapered to create falls.¹
3. Membrane. Rolls are unrolled, relaxed and laid flat with the specified overlaps. Creases should be avoided, since they can cause micro-cracking in some thermoplastics.⁶⁹
4. Fixing. The sheet is fastened, bonded or ballasted to the pattern required by the wind calculation.¹
5. Seams. Thermoplastic seams are welded with automatic or hand-held hot-air welders at the manufacturer's temperatures and probed afterwards. EPDM seams are primed and taped.⁶⁴
6. Details. Perimeters, upstands and penetrations are flashed and mechanically restrained.¹

Workmanship at seams and details decides most single-ply failures. Specialist, trained installers and early testing of seams are recommended.⁹⁶

Performance

Thickness. Most membranes are about 1.2 to 2.0 mm (45 to 80 mil) thick, and thicker sheets are increasingly specified.⁹⁶

Fire. In Britain, single-ply warm roofs typically achieve AB or AC ratings in the BS 476-3 external fire test, and ballasted membranes can reach AA.¹ Under the European system the equivalent test is classified to BS EN 13501-5, for example as BROOF(t4).¹ The harmonised product standard for single-ply membranes, EN 13956, puts their reaction to fire at Euroclass E.¹

Heat and ultraviolet light. For TPO, heat rather than ultraviolet light caused many early failures. Laboratory tests found that TPO generally kept its properties under UV better than PVC, KEE and EV membranes on average.⁶ Some warranties exclude membranes whose service temperature exceeds 71 °C (160 °F). Membranes next to highly reflective walls or equipment can run hotter than the open roof.⁶

Puncture. Thicker sheets and fleece backings improve puncture resistance: a 90 mil EPDM sheet has about twice the puncture resistance of the original 45 mil sheet.⁴

Service life. In the UK, SPRA puts typical membrane durability at 25 to 40 years and requires members to give at least a ten-year product warranty.¹ Manufacturers must hold current British Board of Agrément (BBA) certificates.¹ Expected lives in UK specification literature are about 20 to 30 years for PVC and EPDM and about 20 years for TPO. Some PVC roofs have lasted more than 35 years, and some American EPDM roofs about 40.⁹ A ten-year study of 16 TPO membranes found 15 performing satisfactorily across different climates.⁶ Inspection frequency should increase as a roof ages, especially for TPO thinner than 60 mil after about six years.⁶

Common defects. Typical defects include:⁶⁴⁹

• Open or "cold" welded seams.
• Adhesive seam failure under ponding.
• Surface crazing, chalking or erosion of the top layer.
• Splits at creases.
• Punctures.
• Failures at poorly detailed penetrations.

Roofs should be inspected at least twice a year, typically in spring and autumn.¹⁹

Cost

In a 2005 UK whole-life study, single-ply membranes 1.2 to 1.5 mm thick cost roughly £35 to £45 per square metre to install on a simple warm roof. The cost varied with membrane type and design life. The net present cost over 60 years ranged from about £69 to £86 per square metre.⁹ Much of the cost lies in detailing, and the differences between membrane types were small.⁹

Environmental aspects

Light-coloured, reflective membranes and extra insulation are recommended in UK sustainability guidance for flat roofs.⁹ PVC contains chlorine and plasticisers, while TPO avoids chlorinated ingredients.³⁹

Organisations and standards

• SPRI (United States). The single-ply manufacturers' association, active since 1981. It produces ANSI-accredited standards and a wind design calculator.⁷
• Single Ply Roofing Association (UK). It publishes design guidance, member requirements and certification rules.¹
• Product standards. The main product standards are ASTM D4434 (PVC), ASTM D6754 (KEE), ASTM D6878 (TPO) and ASTM D4637 (EPDM) in North America, and EN 13956 in Europe.²³¹
• Design codes. Design codes include BS 6229 (flat roofs), BS EN 1991-1-4 (wind actions), BS 5250 (condensation) and BS EN 12056 (drainage) in the UK.¹