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Rubber Membrane (EPDM) Roofing Basics:

Rubber Membrane (EPDM) Roofing Basics:

Rubber membrane roofing, or EPDM (Ethylene Propylene Diene Polymer), has been the go-to roofing material for both flat roof and low slope roof applications for the last 45 years. Even with the introduction of more modern materials such as PVC and TPO membrane, EPDM continues to maintain advantages over other flat roofing materials.

Listed below are basic terms and definitions used in the EPDM roofing industry:

Low Slope - Refers to roof pitch; roofs that are flat or have very little pitch are low slope roofs and require specially engineered roofing systems.

Single Ply Membrane - Roofing material used in low slope situations such as EPDM, PVC, or TPO consisting of one layer (or ply) adhered together to create a water impermeable membrane.

Roof Decking - The horizontal, solid material of which the structure of the roof was built. 


Flat roof decking can be plywood, metal, concrete, wood planks, or gypsum/fibrous cement. The single ply membrane is installed over the roof decking.

Ballasted - In situations where insulation board cannot be mechanically attached to the roof decking, such as with concrete decking, the insulation board is loosely set on top of the roof decking. The EPDM membrane is then loosely set on top of the insulation board (not fully adhered) with only the seams glued together. Weight must then be added on top of the insulation board and membrane to hold everything in place. The added weight is called ballast. A flat roof can be ballasted with gravel (most common), concrete pavers, or other materials. Manufacturers recommend a minimum of 10 lbs of ballast per square foot of roofing material.

Mechanically Attached - Refers to the fastening of solid insulation board (either ISO Insulation Board or Fiberboard Insulation) to the roof decking using metal plates and crews. Insulation board can be mechanically attached to any type of roof decking except concrete. Manufacturers recommend a minimum of 1 fastener and plate for every 8 square feet of insulation board.

Fully Adhered - The use of adhesive or glue to attach EPDM to insulation board. In situations where insulation board is mechanically attached, the EPDM can be fully adhered to the insulation board making ballast unnecessary.

ISO Insulation Board - Polyisocyanurate (polyiso) is closed-cell rigid foam board made with recycled content. This is considered a premium insulation choice since it is long lasting, provides the highest R-value, and cannot be damaged by water in the event of a roof leak.

Fiberboard Insulation - High-density insulation board made from perlite (volcanic glass) and asphalt. This is considered a low-budget insulation option since it is less expensive to install versus ISO and is easily damaged by moisture.

Expanded Polystyrene Insulation (EPS) – Rigid, closed cell foam insulation board usually white in color. Sometimes referred to as “Styrofoam”. This type of insulation board is most often used as a filler or part of a layered insulation system since it is not strong enough in most circumstances to be a base layer for EPDM, and EPDM cannot be directly glued to EPS.

Tapered Insulation System - The sheets of insulation board can be cut horizontally to create a gradual taper in order to direct rainwater toward a drain or gutter and reduce ponding (standing water) on a flat roof. A tapered insulation system can be custom engineered and custom cut at the factory to produce a water drainage system for any situation.

Parapet Wall - A portion of a wall extending above the roof. A parapet wall could be built as a requirement for an interior firewall or as part of a water drainage system.

Wall Coping – Finishing material installed at the top of a masonry wall structure or parapet wall; sometimes referred to as wall caps. The material can be made from metal, stone, or concrete and help keep water out of the wall.

Roof Drain - Rainwater must be directed off of a flat roof using roof drains. Roof drains can be located on the roof deck and connected to an interior drainpipe. Rainwater could also be directed into a gutter system or scupper system around the perimeter of the roof.

Scupper - A type of roof drain. An opening or drain on the sidewalls of a building through which water can pass from the roof deck to outside the exterior wall.

Mil - Unit of measure (a thousandth of an inch) used to describe the thickness of roof membranes meaning. EPDM comes in standard thicknesses of 45 mil, 60 mil, 70 mil, and 90 mil. Although the thicker material is heavier thus more labor intensive, it is more durable and has a longer service life.

The terms listed above are commonly used in the flat roof industry. Understanding the terms will help consumers communicate with contractors and suppliers and improve the decision making process.

About the Author:

Pro Roofing, LLC is a Licensed and Insured Roofing Contractor Located in Madison, WI. Pro Roofing, LLC has been Wisconsin’s Premier Residential and Commercial Roofer Since 1999.

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