Roof Systems We Evaluate
Large commercial and institutional buildings often carry several generations of roofing at once. A useful assessment starts with knowing how each system was built, how it ages and how it fails.
Built-up roofs: pitch, tar and gravel, and multi-ply
Built-up roofing (BUR) is made of several plies of felt or fabric laid in hot bitumen, usually finished with gravel or a cap sheet. Older roofs often use coal-tar pitch, which softens and can self-seal in warm weather and was traditionally used on dead-level or very low-slope decks. Asphalt built-up roofs are more common on slightly sloped decks. "Tar and gravel" usually means a built-up roof with an aggregate surface embedded in the top pour. On these roofs we look at ply condition, blistering, splits, exposed felts, gravel loss and wet insulation. Pitch and asphalt are generally not compatible with each other, which matters for repairs and recovers.
Modified bitumen
Modified bitumen is asphalt modified with polymers, sold as reinforced rolled sheets. SBS (rubber-modified) sheets are usually set in hot asphalt or cold adhesive, or self-adhered. APP (plastic-modified) sheets are typically torch-applied. Most installations use two or more plies with a granule-surfaced cap sheet. Typical issues include seam and lap failures, granule loss, and blisters. Hail can bruise or fracture the mat beneath the granules.
Single-ply: TPO, PVC and EPDM
- TPO (thermoplastic polyolefin): heat-welded seams, usually white and reflective, mechanically attached, adhered or ballasted.
- PVC (polyvinyl chloride): heat-welded seams, often specified where grease or chemical exposure is a concern.
- EPDM (synthetic rubber): seams joined with tape or adhesive; installed fully adhered, mechanically attached or ballasted.
On single-ply roofs we check seams, flashings, fastener and plate patterns, punctures, shrinkage, and membrane condition after hail or wind. We also check for wet insulation, often with infrared and core cuts.
Roof coating systems
Fluid-applied coatings are used to restore or protect a sound existing roof. They don't fix wet insulation or a failed deck, and choosing the right chemistry for the substrate matters.
- Silicone: very UV-stable and tolerant of ponding water. It tends to hold dirt, and generally only silicone adheres well when it's recoated later.
- Acrylic: water-based, reflective and economical, but generally not suited to ponding water. It needs suitable temperature and humidity to cure.
- Urethane: tough and impact- and abrasion-resistant. Aromatic urethanes are often used as base coats because they degrade in UV. Aliphatic urethanes are UV-stable and hold color, so they're usually used as topcoats.
- SEBS / rubber-based: solvent-borne, elastomeric coatings, commonly used over modified bitumen, built-up and metal roofs.
- Asphalt and aluminum: asphalt emulsions and cutbacks protect bituminous roofs. Aluminum-pigmented (aluminum roof coating) versions add reflectivity.
When a coated roof is damaged, the questions are whether the coating, the roof under it, or both were damaged, and what the manufacturer's requirements say about repair.
Slate
Slate is natural stone split into roofing units. Durability varies widely by source. In common usage, "hard" slates are dense and long-lived, and "soft" slates have shorter service lives and are more prone to delamination. The National Park Service's Preservation Brief 29 gives these general service lives by source:[1]
- Vermont and New York: about 125 years. This district produces green, red, purple and mottled slates, including weathering ("sea green") and unfading types.
- Buckingham, Virginia: 175 years or more. Blue-black, with a lustrous, high-mica appearance.
- Pennsylvania Soft-Vein (Lehigh and Northampton Counties): more than 60 years.
- Pennsylvania Hard-Vein: roughly 100 years. No longer quarried.
- Peach Bottom, Pennsylvania: at least 200 years. No longer quarried.
The brief says these figures are only a general guide to whether a slate roof is near the end of its service life. On slate we look at broken, slipped and delaminating units, fastener and flashing failure, and whether damage can be repaired with matching slate.
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