Liquid applied roofing systems are increasingly being specified to refurbish ageing single ply roofs, and for good reason. But on mechanically fixed single ply roofs, the fixings beneath the membrane remain a silent but decisive factor, and successful refurbishment ultimately balances adhesion performance, material compatibility, structural integrity and an honest alignment of guarantees with existing assets.
Liquid applied roofing systems are increasingly being specified to refurbish ageing single ply roofs, and for good reason. They offer a practical way to extend service life, improve waterproofing and avoid the disruption, cost and waste associated with full roof replacement. Most industry discussion, quite rightly, focuses on adhesion: will the liquid system bond to the existing membrane? Will plasticiser migration be an issue? Has the correct primer been selected? These are valid and critical questions.
Richard Aldred, Head of Technical at Sika, highlights how there's another factor that receives far less attention, and it may ultimately determine whether a refurbishment genuinely performs for the long term.
What about the fixings beneath the membrane?
Overcoating single ply membranes is not the same as coating concrete, asphalt or reinforced bitumen. PVC and FPO membranes introduce additional variables, from surface degradation to changes in formulation over time, which is why adhesion testing is now widely accepted as best practice rather than an optional extra.
Done properly, with representative samples, realistic preparation methods, appropriate primers and correctly interpreted pull-off results, this testing provides confidence that the liquid system will bond and perform as intended. But good adhesion alone does not define durability. Single ply membranes are typically installed fully adhered, mechanically fixed, or through a hybrid of the two. Where membranes are fully adhered, a liquid overlay is largely self-supporting once bonded, and provided adhesion has been proven, the age of the original membrane is rarely the limiting factor. Mechanically fixed systems are different. In these systems, wind loads and other stresses are transferred through fixings to the deck, so when a liquid applied membrane is installed over the top, the new waterproofing layer may now be structurally dependent on fixings that were installed many years ago. This is where a potential contradiction starts to emerge.
The fixing longevity question
Most roofing fasteners are designed and tested to deliver a finite service life, which varies by manufacturer, fastener type, fastener coating and deck type. When a mechanically fixed single ply roof is refurbished using a liquid system, the new waterproofing layer becomes reliant on original fixings that may already be fifteen, twenty or more years old, and whose residual service life may be far shorter than the guarantee proposed for the new system.
In mechanically fixed systems, offering long guarantees without fully accounting for fixing age risks a mismatch between expectation and reality. This isn't a criticism of liquid technology, which has proven itself in many applications, but of how system performance is sometimes framed. A more responsible approach aligns guarantee duration with structural reality: younger mechanically fixed roofs may reasonably support meaningful refurbishment guarantees, older roofs may warrant shorter guarantees or further investigation, and where fixings are approaching or beyond their intended design life, overlays alone may not be the appropriate answer. In simple terms, a new waterproofing system cannot reasonably be expected to outperform the remaining life of the components holding it in place, and expecting decades of future performance from fixings already approaching the end of their design life may be optimistic, and potentially risky. This raises an uncomfortable but necessary question for the industry: should refurbishment guarantees always match those offered on new roofs?
This isn't about limiting solutions; it's about specifying them honestly. In some cases, the most robust refurbishment strategy involves resetting the load path altogether. Introducing a mechanically fixed recovery board with new fasteners removes dependence on aged fixings, provides a stable and known substrate, and allows longer guarantee periods to be justified, effectively transforming refurbishment into a genuine system upgrade. While this approach carries additional cost, it often provides far greater technical certainty on older roofs or those exposed to aggressive environments.
None of this suggests that liquid applied refurbishment is flawed. On the contrary, it remains one of the most valuable tools available for extending roof life sustainably. But as an industry, we should be wary of reducing refurbishment decisions to a single metric such as adhesion. A balanced specification should consider how the existing membrane is attached, the age and condition of the fixings, the environment in which they have performed, whether they will realistically outlast the proposed guarantee, and whether sufficient investigation has been undertaken where risks are identified.
If the answer to those questions is uncertain, it's the solution that should change, not the wording. Liquid applied membranes can deliver excellent long-term performance when specified appropriately. But on mechanically fixed single ply roofs, the fixings beneath the membrane remain a silent but decisive factor, and successful refurbishment ultimately balances adhesion performance, material compatibility, structural integrity and an honest alignment of guarantees with existing assets. Only when all four are considered together can refurbishment truly deliver the long-term performance, value and confidence it promises.
Author
Richard Aldred
Head of Technical - Building Envelope
Sika UK