Do membranes shrink?                                                                                              Article No: 100 03/09/2007

Membrane shrinkage has become topical recently, and we are often asked by specifiers how our systems perform in this regard.  Here we address the issue of dimensional stability as it relates to commonly used roofing membranes, and we hope that this answers some of your questions.

Dimensional stability is one of a number of factors that determine the durability of a membrane and it is important to keep this in mind when selecting a membrane system.  Shrinkage is evident when the membrane pulls away from up-stands, corners and penetrations.butyl stretch with text 2

Under normal situations most membranes, including butyl rubber, will shrink to a degree as they age. This is an unavoidable result of the effects of long term UV and ozone exposure.

The main problem caused by shrinkage is generally aesthetic. However, there can be an increased risk of mechanical damage from people walking, or working, on these areas.

A number of factors determine the dimensional stability of a membrane, including;

1. Manufacturing process

When manufacturing any preformed membrane including torch-on, rubber, PVC and others, the membrane passes through a number of complex processes, rollers and tension controllers, culminating in a roll of finished material.  The stresses associated with these manufacturing processes can be “locked into” the roll of finished material.  When the material is subsequently unrolled these stresses are gradually “unlocked” which is evidenced by some dimensional change to the membrane.

It is for this reason, that good trade practice requires the material to be “relaxed” prior to being installed.  This allows these dimensional changes to occur prior to installation rather than afterward.  The time taken to relax the material depends on the type of membrane – rubber for example, relaxes very quickly; torch-ons take longer, particularly in cooler temperatures.  It is also very important that membranes are not stretched during installation and that the correct adhesive application is applied as per Waterproofing Systems NZ Ltd's recommendations.  By dressing the membrane over angles, corners and vertical surfaces correctly as per our material installation specification, any membrane movement will be minimised.

2. Reinforcement type

Where the membrane is reinforced, then this reinforcement plays an important role.  Torch-on membranes, for example, have a layer of reinforcement running through the structure to provide tear resistance and tensile strength.  Reinforcement technology has developed significantly over recent years, which has resulted in a more stable product.  Historically, nonwoven polyester mats were used as the reinforcing layer, and these had relatively poor dimensional stability – over 0.5%.  Current technologies utilise stabilised spunbond polyester mats with less than 0.25% shrinkage. These mats combine the dimensional stability of glass fibres with the tensile strength of polyester to give the final membrane the best of both attributes.

Our bituFLAME® torch-on membrane system uses the latter reinforcement technology.

3. Installation stresses

butyl stretch with text 3 Another important factor that affects shrinkage is the stress built into the membrane at the time of laying.  Rubber membranes in particular, because of their high elastomeric properties, can be prone to stretching during installation.  Over time the membrane shrinks back to its original dimension.  It is important that applicators take care not to stretch these membranes into up-stands and corners, instead making sure that the rubber membrane is carefully dressed over the fillets.

It is important to understand that dimensional stability and shrinkage is only one factor that affects the membrane's performance.  High dimensional stability can have a downside, particularly when the membrane is to be used on timber substrates that are prone to movement.  Ideally in these situations, having a membrane that has high levels of flexibility and the ability to dimensionally accommodate movement can be beneficial.

So as you can see, a pre-occupation with one membrane attribute doesn’t always tell the full story.  Often selecting the right system for your application involves balancing a number of membrane attributes to give optimum overall performance.