Low-slope roofing: systems and questions to ask

Low-slope roofs are common on modern and mid-century homes, on additions, and over porches and carports — and they follow entirely different rules from the pitched roofs most people picture. Where a steep roof sheds water quickly down overlapping units, a low-slope roof sheds it slowly, so it is usually covered by a membrane or coating assembly rather than by lapped shingles or tiles. Understanding the families of membrane makes any conversation with a roofer far more productive.

Where the low-slope line falls

The Cool Roof Rating Council classifies a roof below 2:12 — less than a 2-unit rise over a 12-unit run, or under about 9.5 degrees from horizontal — as low-slope. Roofs like these are common on commercial buildings and multi-family homes, and on the flat-ish sections of many houses. Products used there commonly form a watertight membrane or coating; the permitted system and minimum slope depend on the product, the manufacturer's instructions, the deck and drainage design, and local code. The CRRC notes that some products used on low-slope roofs may also be used on steep-slope roofs, and vice versa.

Asphaltic membranes

Asphaltic membranes combine asphalt with one or more reinforcement layers, and come in two main varieties:

• Modified bitumen (“mod bit”). Asphalt modified with polymers — commonly APP or SBS — to give it flexible, roll-able qualities. Sheets are applied by torch, mechanical adhesion or adhesive.
• Built-up roofing (BUR). The traditional layered system: asphalt-coated felts built up in plies and usually topped with gravel or a cap sheet that protects the surface from water and sun.

The surfacing on these systems — smooth, granular or aggregate — affects how the finished roof reflects heat.

Single-ply membranes

A single-ply membrane is a flexible synthetic sheet applied in one layer, loosely laid, mechanically fastened or adhered. There are two broad types:

• Thermoset — for example EPDM, a flexible elastomeric membrane whose seams are joined with adhesives and tape rather than heat.
• Thermoplastic — for example TPO and PVC, whose seams can be hot-air welded together because the material re-softens with heat. PVC is naturally fire-retardant; TPO blends can include flame-retardants or UV absorbers.

Liquid- and fluid-applied systems

Some low-slope systems arrive as liquids. Roof coatings — elastomeric silicone, acrylic or polyurethane, among others — are sprayed, rolled or painted onto a substrate, and can renew an existing roof surface. Fluid-applied membranes go further, combining multiple liquid layers with a reinforcing fabric to build tensile strength and flexibility, sometimes over an existing roof. Aggregate (rock) surfacing is also used to shield a membrane from UV and add protective thermal mass.

Questions to ask

You do not need to pick the membrane yourself; you need to ask good questions:

• Which membrane family are you proposing for my roof, and why that one here?
• How will the seams, edges and penetrations be sealed and flashed?
• What is the condition of the deck and any existing roof beneath?
• Can this go over the existing roof, or is a tear-off needed?
• What surfacing or reflectivity are you specifying, and does local code require a rating?

You can look up rated products by type in the CRRC's Rated Roof Products Directory. Note what that rating is and is not: it measures a product's radiative properties — solar reflectance and thermal emittance — not its overall quality, installation or suitability for your roof. And whether a reflective or “cool” surface actually saves energy depends on your climate, as the EPA notes; it is not a universal gain.