Commercial Roofing 101

*This is intended as a basic reference tool and not as a technical analysis.

A Commercial or Industrial Roof is Composed of Three Major Components:

1) Structural

2) Insulation

3) Membrane

Structural: What Holds You Up

Corrugated Metal Decking

Waterproofing Membrane: What Keeps Water Out of the Building

Bituminous Membrane

There are 3 Primary Waterproofing Systems:

  1. Single Ply – plastics or rubbers

    • PVC, TPO, EPDM

  2. Bituminous – petroleum based

    • Cold Processes, High Heat, Torch-Down, Hot Tar

  3. Metal – steel, aluminum, or copper

    • Standing Seam, Corrugated Panels

Insulation: What Insulates the Topside of the Building

Bundles of ISO

  • The most common product used is a rigid foam product called polyisocyanurate (ISO)

  • The insulating properties are measured by R-Value

    • The R-Value of 1 inch of ISO is 5.7

  • ISO can be made either flat or tapered if you want to create more slope on a flat roof

  • Boards are attached to the roof system either by screws and plates or foam adhesives

  • If water gets through the top waterproofing of the roof, the ISO can absorb the water and may not leak


  • Designed by an engineer or architect

  • Supports the roof system and rooftop equipment

  • Most frequently metal decking over steel beams or trusses

  • There are other decks such as concrete or wood


Steel I-Beam

How Different Membrane Systems are Secured to the Roof

  • Single Ply – TPO, PVC, KEE, EPDM:

    • Mechanically attached with screws and plates into the decking

    • Or, adhered to insulation with adhesive (Fully Adhered)

  • Cold Process Modified Bitumen (Cold MB)

    • No heating involved

    • Solvent based adhesive is used between plies of roofing sheets

    • A multi-ply roof system generally with a surface of granules

  • Standard Built Up Roofing

    • Hot asphalt is mopped in place before  a sheet is set.

    • The asphalt cools and is the waterproofing part of the system

    • Multiple plies of fiberglass sheets hold the asphalt in place

  • Metal

    • Always mechanically attached