1. About TPO Roofing
  2. Inspection Considerations

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1. ABOUT TPO ROOFING

TPO was first introduced into the North American market in the early 1990s. It is a single-ply, thermoplastic roofing material supplied in sheets and compounded from polypropylene and ethylene- propylene rubber polymers. Seams are hot-air welded according to manufacturers recommendations. Unlike PVC, it varies in feel, from soft and flexible to rigid and board-like.
TPO is manufactured with various quantities and formulations of fire retardants, UV resistance and resistance to heat-aging.

They have the following attributes:
Resistant to bacterial growth;
Resistant to degradation from animal fats, some hydrocarbon oils, and vegetable oils;
Typically white or tan but available in other colors; and
Typically not warrantied against ponding.
Their resistance to animal fats means that inspectors may see them used on restaurant roofs.

Fire Resistance:
TPO is not fire resistant and requires the addition of retarders to obtain a fire rating.

Thickness:
TPO roofing is available in 45, 60, 72, 80, and 90 mil thicknesses. Thicker membranes allow for more stabilizers and greater breaking strength  and thickness over scrim, which improves puncture resistance. For these reasons, the national Roofing Contractors Association (NRCA) recommends a minimum thickness of 60 mils for single-ply TPO roofing applications.

Reinforcement:
TPO membranes are typically internally reinforced with polyester scrim or fabric that is positioned during manufacturing near the middle of the finished thickness.

Attachment:
TPO membranes may be mechanically fastened, adhered or ballasted, according to the manufacturers recommendations, typically depending on the roof deck material.

Flashing:
Membrane flashing typically used with TPO is of the same material as field sheets, although unreinforced TPO is typically used for field fabrication.

Accessories:
Accessories include pre-formed flashing, adhesives, sealants, and pourable sealants.

 

2. INSPECTION CONSIDERATIONS

Blisters:
Blisters can develop in TPO for several reasons:
Inadequate flash-off time: Some sealants need time for solvents to off-gas. If the membrane is mated to the substrate without allowing inadequate time of off-gassing, blisters may appear as a result.
Entrapped air/moisture: If the roof deck is damp or has gaps, when the membrane gets hot, moisture vapor/air will expand, causing the overlying membrane to lift, forming a blister.

Don’t recommend that blisters be repaired unless they are damaged. Blisters do not affect the water-tightness of the roof, but a repair might. Also, warranties often cover water tightness but not blisters.

Air Movement Across the Membrane:
A characteristic of mechanically attached membranes is that they are subject to lifting between fasteners. This condition is commonly called flutter, bellowing, or ballooning, and may be active, static, or a combination of the two.

Whatever form is evident, it is an indication of air exchange between the building interior and exterior, typically caused by pressure differentials created by wind. Wind directed over the roof creates low pressure above the membrane due to loss of laminar flow. This condition causes the membrane to lift, which in turn creates low pressure beneath the membrane that pulls air into this area from the building interior. This condition creates two problems:
1.  It contributes to mechanical fatigue of various roof assembly components, including the field membrane, flashings, mechanical attachment components, and the roof deck.
2.  It has the potential to raise moisture levels in the roof assembly.

This condition may be caused by the lack of an air retarder installed above the substrate. An inspector might identify this condition as a problem, but would not recommend correction because of excessive expense.

Heat-source Degradation:
Some TPO membranes, especially those manufactured before about 2009, have a reputation for degradation/accelerated aging from heat sources including direct and reflected solar radiation and thermal loading from heat-emitting, roof-mounted mechanical equipment, heat exhausting vents or materials laid on the roof that may retain heat.

Reflected sunlight may be from glass, metal, or highly reflective curtain walls or flashing.
PV systems are a concern, not only because they are large and dark and retain heat, but also because anchoring the arrays may create penetration-related problems, especially with ballasted systems.

Not all TPO is the same in its vulnerability to heat accelerated degradation. Different formulas have been used over the years and variation in TPO performance characteristics varies by manufacture. Inspectors working in hotter climates should be especially aware of the potential for heat-related problems and vigilant in looking for heat sources and areas of deterioration.