Single-ply roofing systems have become a popular choice of membrane over the past 30 years or so. The systems typically consist of one layer of waterproofing membrane which is applied on insulated low-slope (flat) roof decks, hence the term “single-ply”. The three most commonly used single-ply roofing systems are;
- TPO (Thermoplastic Polyolefin)
- PVC (Polyvinyl Chloride)
- EPDM (Ethylene Propylene Diene Monomer)
This article will inform you of the three most common single-ply membranes on today’s market, and help you understand potential defects that can arise from poor installation, workmanship and/or neglect. It will help guide you through what membrane defects to look for when inspecting a roof and help guide you through repair and replacement options.
As a matter of note, there are other single-ply membranes on the market such as Hypalon, TPE (Thermoplastic Polyolefin Elastomer), KEE and PIB (Polysio Butylene); however, these are not so common and therefore this article focuses on the “Big Three”.
Pros and Cons of Single-Ply Membranes on Low-Slope Roofs
Single-ply roofing materials are used on low-lope roof areas with a pitch equal to or less 3:12 (14 degrees). The primary function of the roof is to protect the building user and occupiers from the elements such as wind, rain, snow, and solar/ultraviolet (UV) rays. The roofs are usually protected by the waterproofing layer, in this case the single-ply membrane, and in most instances are insulated. We have listed the pros and cons associated with single-ply roofing systems below;
Table 1 – Pros and Cons of Single-Ply Roofing Systems
Thermosets Versus Thermoplastics Single-Ply Membranes: What’s the Difference?
The “Big Three” single-ply systems fall into two categories known as thermoplastic membranes – TPO (Thermoplastic Polyolefin) and Polyvinyl Chloride (PVC); and thermoset membranes – EPDM (Ethylene Propylene Diene Monomer). A thermoplastic is typically white with a fiberglass or polyester reinforcement layer welded between two plies, and a thermoset is usually a black rubber-like membrane.
The main difference between the roofing membranes is that a thermoplastic (TPO or PVC) can be re-melted and a thermoset (EPDM) plastic remains in a solid state once hardened. This means that the thermoplastics have a superior seam which can be welded (usually with hot air) while thermoset seams have to rely on an adhesive substance where plies overlap. In addition, the thermoplastics are considered to have better chemical properties and may offer more energy savings due to their solar heat reflective capabilities. Having said that, the thermoset (EPDM) win in some areas as coverings are considered to be a more durable field membrane, easier to install and are typically the most economic choice as the material and labor cost is generally cheaper.
Choosing between membranes depends on a variety of factors such as budget, climate and location. A qualified roofing consultant and product manufacturer should always be consulted with when selecting a membrane to ensure it is suitable for application.
Fully Adhered, Mechanically Fastened or Ballasted: Which Application Method Works Best?
What is the best attachment method for your roof? The answer varies depending on your building needs. Mechanically fastened roof systems tend to dominate the market, but fully adhered and ballasted roof systems are still popular for buildings in certain locations with specific needs. Below we discuss the three most common application methods for single-ply roofing systems and their associated pros and cons;
Fully Adhered Roofing System
A fully adhered roofing system is where the single-ply membrane is bonded to the substrate below (typically insulation boards) by a glue (adhesive). There is an industry perception that this method of roof application is more resistant to leaks than alternative attachment methods. In the event of a leak penetration of the roof membrane the water will not be able to travel under the roof membrane past the glue layer. Fully adhered roof systems can also provide higher windup-lift ratings when compared to mechanically attached systems. The drawback is that fully adhered systems are more expensive, slower to install and have additional challenges as the glue cannot be applied during poor weather or incorrect temperature conditions.
Mechanically Fastened Roofing System
A mechanically fastened roofing system is where the single-ply membrane sheets are rolled above the substrate and then mechanical fasteners (typically screws) are driven through the membrane and insulation boards to the roof deck. Typically, the fasteners are drilled at the edge of the membrane sheet rolls then covered with the edge of the next sheet roll. The edges are then heat welded together using hot air to form a watertight seal. Mechanically fastened roof systems can be installed in most weather conditions and are cheaper and faster then the alternative attachment methods. They do however, have the least wind uplift resistance and are most susceptible to leaks. If there is a hole in the field membrane, then water can track anywhere within the roofing system with no glue as a prevention barrier.
Ballasted Roofing System
A ballasted roofing system is where the single-ply membrane is not fastened or adhered in any way to the substrate below. Instead, loose-laid ballast (usually course gravel or stones) is spread over the surface of the roof which holds down the membrane and other roofing components. In some instances, pavers are used in lieu of the ballast material. As the ballasted system is loose-laid, with limited fasteners installed here and there to prevent excess movement, the installation is extremely simple and quick. Since there are no fasteners, and no need for heat or sticking materials, there is less clutter, smell and hot works during installation. It also has the best wind uplift resistance and pro-longs the useful life of the membrane as the ballast protects the filed from weather and ultra violet degradation. The downside is that ballasted roof systems are more expensive due to supply and delivery of ballast, and the structure needs to be designed to carry a heavier roof load. Another disadvantage is that the ballast tends to move and spread unevenly over time, which can become a healthy and safety issues if stone ballast falls off roof edges. The stone ballast also has the potential to damage the roof membrane when moving.
Common Failures and Deficiencies: The Checklist!
Like all roofing systems, single-ply membranes do not last forever. The anticipated useful life of single-ply roofing systems can vary from 15 to 20 years or more depending on quality of install and level or maintenance. A building purchaser, or owner, should undertake roofing evaluations to determine its type, condition and age so that they can undertake necessary repairs and budget for future roof replacement costs. There should be a strict audit to check the following;
1. General Maintenance Conditions
Check for debris or storage materials left on the roof that could block the flow or water or damage the membrane. During the due diligence period of a building acquisition, the purchaser should request information regarding warranties and maintenance schedule. If the owner has a well-documented maintenance and inspection record by an approved roofing contractor, then the chances are that the roof will last longer if the roof has not been cared for.
The field membrane should be inspected for signs of any punctures or penetrations. This is especially important on ballasted roofs. If there are only a couple of punctures, then these can easily be patch repaired. However, if there are multiple then it might be time to replace the roof.
The field membrane should be inspected for cracks. As single-ply membranes age and are exposed to solar ultraviolet (UV) degradation then the material begins to dry out, lose its elasticity and tighten up. Eventually cracks will form allowing water to enter the roofing system. Once cracks begin to appear then it is a sign that the roof has reached the end of its useful life and should be replaced.
Blisters, or bubble-like spots, are an indication that air or water is trapped between the roofing membrane and underlying insulation and/ or deck. When air or water is trapped in a blister, the weather can cause the blister to grow on a daily basis. Warm weather will cause the water or air to expand during the day then cool temperature will cause the trapped water or air to contract at night. Blisters that contain air can still be waterproof and are likely present due to poor installation. However, if they contain water then it is likely that the waterproofing layer of the roof, i.e. the single-ply membrane, has failed.
5. Fish Mouths/Open Seams
Fish mouths is a term used to describe “U” shape openings in the roof seams. This commonly occurs due to lose of adhesion on fully adhered systems or when mechanically fasteners are spaced too far apart on mechanically fastened roof systems.
Always check the condition of the flashings and sealants at walls and roof edges. Base flashings are often an extension of the single-ply system field membrane. As single-ply membranes age and are exposed to solar ultraviolet (UV) degradation then the material begins to dry out, lose its elasticity, tighten up an de-bond from the wall.
As single-ply membranes age and are exposed to solar ultraviolet (UV) degradation, then the material will change color. For instance, a black EPDM (Ethylene Propylene Diene Monomer) single-ply membrane will turn to a lighter grayish color with time in the sun, due to the composition of the material being altered by the UV exposure.
Drains, scuppers and gutter should be inspected to make sure thy are free of debris and ensure water has adequate discharge from the roof. On a typical low-slope roof system, it is recommended that the roof have no less than one quarter inch per foot slope to drain to allow adequate drainage. Any ponding water or staining to surface of the single-ply membrane is an indication that the roof drainage is inadequate or blocked.
Check that the ballast is spread evenly and is not falling from the roof edges. Any displaced ballast may result in the uplift of the roof membrane and insulation and therefore should be redistributed evenly.
10. Check for Leaks
Finally, the most obvious but often over looked is to check for signs of water leaks from inside the building. Leaks can often be hidden behind walls or within ceiling voids, so it is important that the inspection from within is thorough.
The replacement cost for single-ply systems can be anywhere from $15 per square foot for low-rise industrial park units to $40 for high rise city center location buildings. We therefore recommend you engage with a suitable qualified roofing consultant before deciding on the best course of action for your roofing needs. A recovery system should be considered at the time of replacement which has the potential to reduce overall project cost. However, the suitability of this method would need to be determined by core samples and/or inspection by the roofing contractor and manufacturer at the time of replacement to ensure warranties can be provided.
Two main warranties exist relating to roofs – a material warranty which covers the roofing product, and a contractor warranty which covers the installation by a roofing contractor/business. Usually, both warranties are provided on completion of a roofing project. Typically, roofing materials (the product which is installed) are guaranteed by a roofing manufacturer for a substantial period (say 20 to 25 years) against defects caused during the manufacturing process. A contractor warranty is typically much shorter (usually 1-2 years) and covers installation-related defects. The contractor warranty should detail what items are covered, and activities that will void them. It is important to ensure that copies of the warranties are obtained, and that building inspections and maintenance activities do not invalidate any warranties in place.
Other Related Websites
National Roofing Contractors Association – http://www.nrca.net/