Low-slope Roof System Types

Most low-slope roof membranes have three (3) principal components:

1. Waterproofing layer or layers – The weatherproofing component is the most important element since it keeps water from entering the roof assembly.

2. Reinforcement – Reinforcement adds strength, puncture resistance, and dimensional stability.

3. Surfacing – Surfacing of membrane protects it from sunlight and weather. Some surfacings can also increase fire resistance, improve traffic and hail resistance, and increase solar reflectivity.

With some roof membranes, a single component may perform more than one (1) of these functions.

Built-up Roof (BUR)
Gravel surfaced built-up roof system.
Gravel surfaced built-up roof system.
Smooth surface built-up roof system.
Smooth surface built-up roof system.

Built-up roof systems (referred to as BUR) have been in use for over 100 years. Built-up roof systems are also commonly called “tar and gravel” roofs and are generally composed of alternate layers of bitumen and reinforcing fabric that together create a finished roof top membrane.

The reinforcement sheets are often referred to as plies or felts. Therefore a 4-ply system would consist of four (4) layers of bitumen on reinforced fabric. Sometimes a base sheet is utilized and mechanically fastened as the bottom-most ply.

The bitumen typically used in built-up roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or a mechanical spreader.

Surfacing for built-up roof systems include aggregate (gravel, slag or mineral granules); glass-fiber or mineral surfaced cap sheets; hot asphalt mopped over the entire surface; aluminum coatings; or elastomeric coatings.

Metal Panel Roof Systems for Low-slope
Low-slope metal panel roof system.

The only metal roof system used in low-slope roofing applications is structural metal panel. Structural metal panels are hydrostatic and therefore can be utilized in this application due to their waterproof characteristics. Structural metal panel roof systems can be used in steep-slope applications but this is the only metal roof system that can be used in low-slope applications.

Structural metal panels are designed to inhibit the ability of water to enter the system at laps and other joints by the use of sealant or anti-capillary designs used in the seams. Structural metal panel roof systems also possess the strength to span supporting members.

Roof Deck
Structural metal panel roof systems can be installed over a variety of decks. There are two (2) general categories of substrates: continuous or closely spaced decking that provides solid support or spaced structural supports (such as purlins) where the metal panels span between supports. Most structural metal panels are installed over spaced structural supports and not supported by a solid roof deck.

Underlayment (or felt paper as it is frequently called) is installed over the roof deck before installation of a metal panel roof system. This underlayment performs two (2) important functions: temporary water protection until the metal panel roof system is installed and it provides a secondary moisture barrier should moisture infiltrate the system.

Typical underlayments consist of asphalt saturated, non-perforated organic felts and are commonly designated as Type 15 and Type 30 or No. 15 and No. 30 which reflect a once used pound per square weight designation. The terms Type I and Type II are now used in lieu of 15 and 30 respectively.

For metal panel roof systems the NRCA recommends a minimum of one (1) layer of No. 30 asphalt-saturated felt on roof decks with a slope of 4:12 (18 degrees) or more and a minimum of two (2) layers of No. 30 for roof slopes of 3:12 (14 degrees) up to 4:12. Underlayment should be applied horizontally in shingle fashion.

Underlayment is not typically used with structural metal panel roof systems when intermittent supports are used. However if installed on a continuous or closely spaced deck, the NRCA recommends underlayment.

Condensation should be expected to develop on the underside of metal roof systems and careful consideration should be paid to vapor retarder, insulation and ventilation issues.

Modified Bitumen/Polymer-modified Bitumen Sheet Membranes
Granular surfaced modified roof system.
Granular surfaced modified roof system.
Smooth modified roof system.
Smooth surface built-up roof system.

Polymer-modified bitumen or modified bitumen (MB) is an asphalt-based sheet membrane composed of multiple layers of reinforcing fabrics that hold the hot polymer-modified bitumen as it is manufactured into a roll membrane material. Much like built-up (BUR) membranes, modified bitumen sheets are comprised of multiple layers and are typically installed as a two-ply system and are almost always fully adhered.

There are two (2) types of modified bitumen roof membranes:

SBS Modified Bitumen Membrane

SBS modified bitumen membranes are commonly attached in either a hot mopping of asphalt similar to BUR or with cold adhesive. Some SBS membranes are self-adhering.

APP Modified Bitumen Membrane

APP modified bitumen membranes are typically heat-welded or torch-applied. NOTE: The NRCA cautions consumers that APP modified bitumen should never be torch-applied directly to a wood deck which could cause a fire hazard.

Surfacing for modified bitumen membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing. A system that consists of a built-up membrane with 2 or 3 plies covered by a modified bitumen cap sheet are commonly referred to as a “hybrid” system.

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