Understanding Single-Ply Roofing Systems

Material options and installation methods for specifying single-ply roofing membranes in low-slope commercial applications
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Sponsored by Johns Manville
By Andrew A. Hunt
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Pre-molded Accessories

Manufacturers offer pre-molded accessories to help protect single-ply roof systems, reduce installation labor and increase flashing consistency. Flashing accessories are easy to install and offer protection from roof edges to parapets, from penetrations to walls and corners. Drain boots, exhaust boots, clad edges, wraps, curb and corner wraps, vents and many other accessories are available to protect corners that are fluted or inside. They may be heat molded to the membranes of PVC or TPO roofs. They can be installed by tape and primer or other specialty sealants per manufacturer recommendation.

Seam welders

Hot air welds are quick, durable and reliable. Seam welders make permanent and waterproof seams by fusing thermoplastic membrane sheets together without adhesive or glue. Seam welding can take place in many different conditions, even cold weather. A hand-held hot air welder is a convenient choice when welding membrane sections at corners or vertically. Hand-held hot air seam welders are also used to weld membrane sections together and to weld membrane to thermoplastic-coated metal.

New technologies are making seam welders capable of dual direction, and they are fast and simple to use. There are hot air hand tools for quicker warm-up times and improved adhesion at membranes. Consult your seam welder’s manufacturer for specific guidance on use and maintenance.

Mechanical fasteners

Today there are mechanical fasteners designed specifically for single-ply roofing systems, such as coated steel plates and fasteners. Mechanical fasteners come in different load strengths to suit a project, such as high load or extra high load or super heavy duty for wider membranes. Plate designs also have improved with the use of barbs and other fasteners.

More than 50 years ago, mechanical fasteners were designed to replace asphalt mopping. It took time for the fastening design of mechanical fasteners to evolve into deeper and finer threads with stronger holds and more resistance to pulling out, but today there are many such options available.

Thermoplastic Installation Options

This section will discuss the three main installation options for single-ply thermoplastic roofing systems: mechanically attached, induction welded and fully-adhered. There are advantages to each type of installation that must be weighed depending on locale and contractor experience. While it may be tempting to assume a certain type of system doesn’t hold up as well, the reality is that factors such as unpredictable weather, contractor error and poor maintenance will affect any type of installation. Each of the main options are discussed and evaluated for ease of installation, cost, structural advantages, and general warnings or concerns that must be addressed during installation for PVC and TPO roofing materials.

Mechanical attachment

Mechanically attached roofs are the most common of the thermoplastic roofing installation options. Mechanical attachment offers a fast, simple and low cost way of applying a single-ply system any time of year. Mechanically attached roofs can be installed more quickly than fully adhering the material, and the membrane comes in large roll sizes that save time and labor. Mechanically attached roofs are relatively easy to inspect and validate, as well.

The membrane is laid out and screws fasten through a layer of insulation into the nailable deck along the seams. The screws are covered with the next membrane sheet and sealed into the roof system. Finally, welding a watertight seal will close any gaps in seams to fuse the system into a single layer.

It’s important to prepare the roof deck before any type of installation by ensuring it is clean, dry and flat. Cut insulation for a close fit and taper to drains. Install wood nailers and preformed metal flashing at the perimeter as recommended. Roof substrates such as vapor retarders should be sealed off at edges and penetrations to prevent moisture from inside or out passing up into the roof system or down into the building. Incorporate air barriers onto structures with high internal pressure such as airport hangars warehouses.

Set the base and wall flashing and gravel stops as specified by the manufacturer. Immediately replace any materials that happen to get penetrated by water during installation. Occasionally, wind gusts can cause the installed membranes to flutter, possibly compromising the system and even causing noise and air leakage inside the building. For this reason it is crucial to know the wind uplift requirements in your area so you can adequately adjust spacing of the fasteners.

Induction welded

Induction welding enables a thermoplastic roof to be mechanical attached without penetrating the roof membrane. This can save labor costs by allowing for wider sheets to be installed and reducing the number of seams on the roof. Induction welding requires a hand-held machine welder. It is relatively straightforward to use and, when manufacturer guidelines are followed, limit the possibility of user error. The thermoplastic roof membrane is bonded to fastening plates that are factory coated with the either TPO or PVC compatible adhesives. Working from above with an induction welding tool, the welder bonds the membrane to each of the specially coated plates.

Induction welding involves no membrane penetration. Membranes can even be hot air welded in cold weather but may require preheating and a slower speed.

TPO and PVC weld differently, with PVC generally requiring a higher temperature than TPO membranes.

A hand-held hot air welder eases the work at corners or on vertical surfaces and can be adjusted for the appropriate temperature, but it is a skill perfected with experience. Machine hot air welding, on the other hand, allows for consistent high-speed welding at the seams. The speed is determined by the heat setting based on ambient air temperature. Overheating, for example, prevents a decent weld and requires patching.

With either method, it is important to test weld seams for integrity and continuity, and to complete a series of test welds at the start of each work period. As soon as seams cool, run a blunted tool along the seam while applying firm pressure but without scoring. Voids are indicated by any penetration and must be patched. If weather conditions change or the automatic hot air welder is shut off for any reason, follow up with a testing cycle.


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Originally published in January 2019