This month, the Inspector puts bitumen primers in their place.
You can never put enough emphasis on the importance of primers. As the very foundation of a bitumen roof’s build-up, you might wonder why some choose to scrimp, substitute or simply ignore the role it has to play. Yet some do. The results can be damaging to the roof, the relationship with the property owner and the hit to the contractor’s pocket in remedial work.
The most frequent occurrences of this are probably in bituminous membrane applications. This might be down to the fact that any primer is quickly covered by the system layers, so the temptation might be to cut corners, use a primer of lower quality or simply apply less of it. Don’t forget the primer is there for a reason – it is designed to penetrate and seal dusty or porous substrates of masonry, concrete, cementitious screeds or renders, existing bituminous or asphalt waterproofing, structural steelwork, and other metals, prior to the application of bituminous membranes.
The correct primer will ensure adherence of the system and help secure a comprehensive guarantee, yet without it, the system may delaminate and fail to a catastrophic effect.
The right temperature and curing time
Typically, primers for a bituminous felt roof comprise a high-quality elastomeric bitumen and a hydrocarbon solvent, like BMI Siplast, and should be applied only when atmospheric and substrate temperatures are greater than 5ºC.
Attention must also be paid to the solvent content of a primer. Primers such as BMI Siplast contain a compound which aids in the curing of the liquid and allows the roofer to continue with the system installation, around one to three hours depending on temperature. However, these solvents or volatile organic compounds (VOCs) cannot always be used. The use of VOCs on or next to an occupied building or in an enclosed area should be discouraged, and indeed is in some cases prohibited.
Fortunately, there are solvent-free variants to call on such as BMI Icopal SF Bitumen Primer. These solvent-free elastomer bitumen primers are designed to be used where site constraints prohibit normal bitumen primers.
However, there is one caveat – the removal of the solvent compound lengthens the curing time significantly, which can make it an unattractive prospect for a contractor wishing to “get on” with the job. Despite the desire to crack on with the work, never ignore a low VOC specification as you are playing with the health of your employees as well as any members of the public within the vicinity.
Heat applied / flame free systems
Another area of confusion with bitumen primers is found when looking at heat applied or flame free systems. Typically, in these systems, the primer is quick acting and cures within 10 minutes. The system should then be applied immediately. These primers, like BMI Icopal S.A. Primer, are a high performance solvent-based low viscosity, black, quick drying polymeric priming solution that improves the bond adhesion of thermally-activated self-adhesive bituminous membranes to a variety of substrates. The reduced solvent content in the formulation quickly evaporates allowing for fast and permanent bonding to horizontal and vertical substrates. Use of a standard primer with a flame free system should be avoided.
Of course, any primer is only as good as the surface you put it on. So adequate cleaning of the substrate and the removal of surface contaminants will allow primers to have direct contact with the substrate. Roughing up the surface will provide increased anchorage of the applied material. A common misconception is that a new surface doesn’t need priming; this simply isn’t true, as a primer will offer the best opportunity for the product to bond to its surface and to ensure adhesion is at its maximum.
The correct specification of the primer, as mentioned above, is also just as critical as some waterproofing systems have substrate specific primers for the same application. It is therefore essential that the contractor is familiar with the system’s primer, the specific substrates they should be used on, and the minimum / maximum temperature ranges they should be applied in. Failures tend to occur when contractors try to find a more economical primer or adhesive from a third-party manufacturer. Slight variations in chemical compositions can have an adverse reaction. Whilst a few pounds may be saved on a less expensive primer or adhesive, it may cost a lot more to replace the covering should a failure arise in the future.