premier metal roof systems information

Roof systems and their component materials are often divided into two basic categories: low slope and steep slope. These are industry terms relating to the angle of incline, or pitch, which is calculated by the rise in inches (height) for every foot covered (length or “run”). That means a slope of 3:12 will be equal to a rise of 3 inches for every foot, so a run of 15 feet would total an increase in height of 45 inches. Any slope less than or equal to a pitch of 3:12 qualifies as a low slope roof.

Low slope roofs must be impermeable to water, or waterproof, since their lower angle will mean an increase in the amount of time it takes for water to be shed from the surface of the roof. That is not the case with steep slope roofs, as standing water is removed quickly by the effects of gravity.

Different materials are recommended for use on low slope roofs than for steep slope. Whereas concrete tiles and wood shingles are commonly installed on roofs with a pitch greater than 3:12 (or 14 degrees); they would not be appropriate for low slope applications because they are designed to be used as a water shedding system, which allows for the intrusion of water. That can cause serious damage to the roof deck - what the roofing material is normally attached to - and insulation. According to the NRCA (National Roofing Contractors Association), there are 5 generic classifications for low slope roofing materials.

1. Metal Panel Roof systems
2. BUR Built Up Roofs (“Tar & Gravel”)
3. Polymer-Modified bitumen sheet membranes (Two-ply)
4. Single-ply membranes (“Rubber Roofs”)
5. Spray polyurethane foam-based roof systems (“SPF”)

These systems all have inherent strengths and weaknesses, so the key is to find the one that best suits your needs.

As you would expect from the name, Built Up Roofs, or “Tar and Gravel” roofs as they are commonly referred to, are created by literally building up alternating layers of bitumen and reinforcing fabrics. Bitumen is composed of asphalt, coal tar, or cold-applied adhesive (solvent based asphalts that do not have to be conditioned in a kettle or tanker). This system is typically surfaced with an aggregate - like gravel, slag, or mineral granules - that acts as a barrier against the elements. Other common surfacing practices include mopping hot asphalt over the entire system or using various coatings. Note: care must be taken not to overheat the asphalt because surface blistering or fires may occur in the kettle. Splitting, surface erosion, and UV degradation in the event ponding (water building up in low areas) are other potential problems with BUR systems.

Polymer-Modified bitumen sheet membranes were invented in Europe during the early 60’s and were introduced into the U.S. market in the mid 1970’s. This system is similar to BURs in many ways, though there is one key difference: the bitumen is treated with a polymer so it can be manufactured into a roll, backed by the reinforcing fabrics. Because of this approach, the system can be limited to only two-ply.

There are two different forms of MB and depending on which type you use (SBS or APP), can be either installed with hot mopped asphalt or cold adhesive (SBS), or can be heat-welded or torch-applied (APP). Note: the NRCA warns against using the torch method if installing over a wooden roof deck, for obvious safety reasons. People have also complained of the fumes being emitted from cold adhesive, so proper steps should be taken to cover up any ventilation systems or other air passages that lead to the inside of the building during application. The modifiers in APP tend to render a “rubberized” quality to the asphalt, so it tends to be confused with EPDM, a thermoset membrane that is often termed “rubber roof” by professionals.

EPDM, or ethylene propylene diene monomer, is a form of single-ply roofing that is prized for its light weight and relative ease of installation. There are a multitude of different chemical compositions for this class of membrane, but most have two common characteristics. As mentioned earlier, thermoset membranes are manufactured sheets that utilize principle polymers that are chemically cross-linked or vulcanized (meaning “cured”). Once the material is heated and cooled, the only way to attach it to another sheet of like material is with adhesives. EPDM, CSPE, ECH, CR, and PIB all fall in this category.

The secondary form of single-ply membrane is known as a thermoplastic membrane, which has no molecular cross-linking, and as such, can be repeatedly softened and hardened by heating and cooling. PVC, or polyvinyl chloride, and a host of other chemical compounds - with TPO, thermoplastic olefin, being the most widely used - are included in this group.

The seams for single-ply are normally sealed by heat welding with hot air or the application of chemical solvents. Serious problems may arise if the seams are not completely sealed. High winds can cause sheets to peel back or billow, and while the membranes themselves can repel water, any gaps will not.

An option that does not include seams is Spray polyurethane foam, or “SPF”. This is an adhered roof system that consists of two components, isocyanate and polyol, being mixed together and then broadcast onto a substrate (roofing, structural deck, or insulation). It can then be surfaced with an elastomeric coating - that can stretch to at least twice its original length and still recover to its original dimensions - a single-ply system, or mineral granules or sand. This can increase the systems ability to withstand UV exposure and weathering.

Punctures and penetrations are an issue to consider with SPF systems. Foot traffic, wind driven debris, and hail can cause damage to the surface, in the form of pitting, cracks, or tears. If the damage is not repaired, UV rays can cause the situation to worsen over time, so periodic inspections of the roof are recommended. Though a single-ply covering, or using more material in anticipated walkways, can help mitigate the risks, unanticipated traffic or severe weather may still be problematic.

The final category of low slope system is the structural metal roof. This style of roofing has been around in one form or another for centuries and has proven to be extremely durable, even in the harshest environments. It comes in a single form for low slopes called structural metal panel, meaning the individual panels have the strength to span supporting members. The majority of these systems are installed over spaced structural supports, such as purlins, that actually act as a substrate, holding the panels up (for metal roofs, a purlin can also be called a z-bar or z-purlin, which is a steel beam shaped like the letter z, formed by folding galvanized steel). With this system, no actual roof deck need be constructed, so installation time is reduced and no underlayment (or asphalt saturated organic felt) is required. This system has several other unique advantages.