Home Contractor's CornerAn Inspector Calls Single-ply advice: Welding well

Single-ply advice: Welding well

by Matt Downs

This month, the Inspector tackles the hot topic of heat welding singly-ply membranes.

As discussed in last month’s column, single-ply membranes are increasing in popularity but so too is their ill-founded ‘notoriety’. This misplaced sentiment is mostly down to a lack of industry understanding and insufficient product awareness regarding installation.

One of the main reasons for this is the alleged vulnerability to water ingress. Now this is most commonly a result of poor application, with typically the most common source of ingress originating from a poorly sealed lap. A number of reasons may have contributed towards this: incorrect welding temperature, the presence of dirt or contaminants on the seam, or even moisture retention within the product – a by-product of poor material storage. All of these issues can be eliminated, however, with some basic product awareness.

The most common issue we see is the incorrect welding temperature being used. This is mainly due to the widespread belief that every PVC membrane will weld at one set temperature whilst all TPOs will weld at another. The truth is that different brands of PVC membrane will have their own required welding temperature and the same is true for TPO membranes.

Poor weld
Poor weld

Recommended temperature

With this in mind, the best way to establish what the recommended temperature should be is to check with the manufacturer and read the product literature and installation manuals. Each manufacturer will have a different recommendation and for good reason. Different compositions, blends and stabilisers all contribute towards varying recommended temperatures, and these small adjustments can make a big difference. Much like baking a cake, we follow a recipe for a reason – the ingredients and ratios used can all influence the recommended temperature that is set for the oven.

Local environment

You should also consider the temperature and conditions of the local environment. A membrane’s receptiveness to welding can vary depending on the temperature the material has been stored at. Furthermore, the first weld of the day will be different to the last, so ‘test welds’ are always recommended prior to installation. Test welds will get the equipment up to the optimal temperature and any small adjustments required can be made prior to full membrane application.

Hand held welder


Welding equipment can also vary as manufacturers may have different recommended temperatures for handheld welders and automatic welders. Manufacturers might also recommend different types of seam rollers, such as a silicone roller for PVCs and a PTFE / Teflon-based roller for TPOs. Each roller would apply a different level of pressure, which is better suited to each membrane type.

Clean laps

Another recipe for failure is contaminated and dirty laps. These prevent a secure weld so it is advised all the laps are cleaned prior to welding. This is particularly relevant when applying patch repairs to existing systems as the membrane already in-situ will have retained dust, dirt and possible contaminants during its life, which could compromise any future bond. It is therefore essential the areas due to be welded are cleaned with an appropriate agent (from that particular manufacturer) or cleaned with an approved alternative. Failure to do this usually means the contaminants will prevent a homogeneous weld and this could ‘pop open’ weeks later. Coming back to site to ‘repair the repair’ never looks good and would not instil the client or building owner with much confidence in your workmanship.

Correct storage

Lastly, returning to our theme of storage, if the single-ply membrane has been incorrectly stored i.e. left on the ground or exposed to the elements, it will retain moisture that is not visible to the naked eye. This moisture will compromise any weld in the product and again could cause the seam to ‘pop open’ weeks – sometimes months after installation. A lap that may have appeared secure following project completion could be the cause of a costly source of ingress, which is easily avoidable if materials are stored correctly in the first place.

Important points to remember:

  • Always store your single ply membranes in a suitable place.
  • Consult the manufacturer for advice and best practice.
  • Use the recommended welding temperatures.
  • Use the recommend rollers (silicone / Teflon).
  • Clean the product prior to welding.
  • Test weld, test weld, test weld.


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