This page provides general information on low-slope roofing. For more detailed information refer to the reference pages on the individual roof systems. 

Roof system: The collection of roofing components installed above the roof deck, including base/separator sheets, felt plies, rigid board insulation, coverboards, membranes, fasteners, adhesives, coatings, flashings, and ballast.

Roof Assembly: The roof system plus the roof deck.

Ballasted Roofs: access the membrane at drains.


Manufacturer Links

The following list of manufacturers includes the products they produce with links to their installation manuals, and where to look for (manufacturer identification marks).








Roof Drainage, Design, Collapses, and the Codes     (9 pages) Codes don't adequately protect all low-slope roofs, learn to identify those at risk!

Roof Ponding NRCA     (5 pages) Excellent source on the basics!

Siphonic drains   (7 pages plus appendix) How they work, how to recognize them!


Pre-Inspection Research

Unlike with residential inspections in which you simply show up and proceed with the inspection, with commercial properties you want to know as much as possible about the installed systems before you arrive. Regarding roofs, you'll be asking for whatever documentation can be provided. Your objectives will be:

  1. Identification of the roof assembly, most importantly the roof system, including the general type (single-ply, mod-bit, EPDM, SPF, built-up or metal), and the manufacturer.
  2. Identification of proper application methods for the installed system, including field installation, flashing, seams. penetrations, terminations, etc.
  3. Identification of defects and deficiencies common to the particular roof system installed.

If you successfully complete all three objectives, you'll arrive on the roof knowing:

  1. What problems to look for;
  2. Where to look for them;
  3. How to recognize them;
  4. And with this Narrative Library, what to say about them.



Improper Repairs: The most common problem inspectors encounter with low-slope roofs is improper repairs; either the use of improper materials or improper methods.

Improper Installation: The second most common problem is improper installation of the various systems. Installation may be fairly generic or may require adherence to the manufacturer's installation instructions.

Deterioration Over Time: Materials and the connections between them all degrade over time. Moisture, wind, ultraviolet (UV) radiation from the sun, thermal cycling, oxidation, foot traffic, and incompatibility between materials are the most common- but not the only- forces working to deteriorate roof systems.


Single-ply Systems

Single-ply systems are those in which the weather membrane is only a single layer thick. Single-ply systems are divided into three general types.

  1. Thermoplastic Polymer Membranes

Membranes in this group come in rolls of various widths. Seams are heat-welded. It includes:

  • Polyvinyl chloride (PVC)
  • Thermoplastic polyolefin (TPO), increasingly common and less expensive than PVC;
  • Ketone ethylene ester (KEE), Less common than PVC or TPO.

Generally, application methods are the same for all three, but you should check anyway, in case something is different.

  1. Thermoset Polymer Membranes
  • Ethylene Propylene diene terpolymer (EPDM) is an extremely common synthetic rubber membrane.
  1. Thermoset Plastic Polymers

These are less common:

  • Chlorosulfonated polyethylene (CSPE) sold under the trade name HypalonTM
  • Polyisobutylene (PIB)