SpecTopics: PVC Type III and Type IV ASTM D4434 Standards

The ASTM D4434 PVC Standard Specification for PVC (Poly Vinyl Chloride) divides PVC roofing membranes into four types.

• Type I is no longer used, and no manufacturer currently produces a Type I membrane product.
• Type II usually includes fiberglass reinforcement; however, that reinforcement is not used for improving its performance in elongation or tensile strength, it is used for dimensional stability of the membrane. Membrane minimum thickness is 45 mils.
• Type III products are internally reinforced and may include a fabric backing. Membrane minimum thickness is 45 mils.
• Type IV products are internally reinforced and may include a fabric backing. Membrane minimum thickness is 36 mils.

It is important, when specifying, to look at membrane attributes and minimum membrane requirements per the ASTM standard to find the performance you are looking for in a PVC membrane. As an example, comparing Type III and IV membranes by their minimum performance attributes for breaking strength, elongation, and tensile strength (see below table).

Minimum Membrane Properties	Type III	Type IV	% Increase
Breaking Strength (lbf/in)	200	275	38%
Elongation (min %)	15	25	66%
Tensile Strength (lbf)	45	90	100%

Even though the minimum membrane thickness of the Type IV category is only 36 mils, its performance requirements are higher than those of Type III membranes, which are 4 mils thicker. Choosing a Type IV membrane means superior PVC membrane performance. As the thickness of the membrane increases, the qualities of breaking and tearing strength similarly rise, resulting in a product with enhanced performance.

For further information on PVC and KEE HP membranes, consult the Carlisle SynTec website.

For more information on Type III versus Type IV, consult the Sure-Flex PVC Type III vs. Type IV Sell Sheet.