- About PVC Roofing
- Installation Considerations
- Inspection Considerations
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1. ABOUT PVC ROOFING
Polyvinyl chloride (PVC) is a single-ply, thermoplastic roofing material supplied in sheets. Seams are hot-air welded according to manufacturers recommendations. Hot-air welded seams are stronger than the sheet material.
PVC membranes have the following attributes:
Resistant to bacterial growth;
Resistant to industrial chemical atmospheres;
Resistant to plant root penetration;
Good fire resistance;
Typically white but available in other colors
Typically warranted against ponding
PVC is not compatible with bituminous materials or with polystyrene insulations and require the use of a separator sheet when installed with these materials.
PVC roofing is available in 36, 45, 60, 72, and 90 mil thicknesses. The national Roofing Contractors Association (NRCA) recommends a minimum thickness of 45 mils for single-ply roofing applications.
PVC membranes are reinforced with fiberglass or polyester scrim, mat, or fabric. Some PVC membranes are manufactured with non-woven polyester fabric adhered to the back of the sheet. Fabric-backed membranes may provide for adhesion to substrates and/or serve as a separator from the substrate. PVC membranes may also be internally reinforced with fibers or fabric.
PVC membranes owe their flexibility to the addition of plasticizers. Plasticisers may be of different forms, some more vulnerable to loss to evaporation and/or leaching, and some such as polymeric materials (coplolymer blends/alloys), ethylene interpolymers (EIP), and ketone ethylene esters (KEE) more stable over the long term.
PVC membranes may be mechanically fastened, adhered or ballasted, according to the manufacturers recommendations, typically depending on the roof deck material.
Membrane flashings typically used with PVC membranes are of the same PVC material as field sheets, although unreinforced PVC is typically used for field fabrication.
Accessories include pre-formed flashing, PVC-coated metal, adhesives, sealants, and pourable sealants.
PVC may be combined with a number of different polymers to enhance certain characteristics. These include:
Resistance to certain oils and greases; and
Improved flexibility and weathering characteristics.
Inspectors will not be able to identify specific PVC chemical profile without documentation.
Before installation, sheets should be unrolled and allowed to relax for at least 30 minutes. Inspectors may see problems related to failure to allow sufficient time.
Wood nailers, curbs, penetrations, etc. and the substrate prepared according to the manufacturers guidelines before the membrane is applied.
Sheets should be overlapped so that the flow of water will not be against the laps.
Areas where 3 sheets intersect should have T-joint covers installed. T-joint covers are sections of membrane installed directly over the intersection of 3 seams.
2. INSTALLATION CONSIDERATIONS
Mechanically Attached Membranes
Membranes are mechanically attached to substrates using a variety of fasteners and fastening schedules:
Although fasteners will typically not be visible during inspections, knowledge of acceptable fasteners will contribute to overall understanding of single-ply roofing systems. Some manufacturers allow the use of commonly available fasteners, others require specialized, pre-assembled fastening components.
Fastener choices may be influenced by roof deck types, warranty considerations, and proprietary manufacturers concerns.
Nailing disks placed within seams and fastened through the membrane to the substrate.
Metal or plastic bars placed within seams and fastened through the membrane to the substrate.
Metal or plastic bars over disks placed over the membrane, fastened through the membrane to the substrate, and covered with a strip of PVC membrane.
Polymer-coated metal disks used with fasteners to attach rigid board insulation to roof decks and heat-welded to the underside of thermoplastic (PVC/TPO) membranes using electromagnetic induction welding equipment.
Other proprietary methods.
Fastening Schedules, Sheet Installation, and Uplift:
Fastening schedules and sheet installation will vary according to wind exposures, which in turn vary by area, roof shape, slope, height, and orientation to the prevailing winds. Designers often specify half-width sheets with an increased number of fasteners be installed near roof perimeters and corners where uplift is expected to be higher, so inspectors may see this condition in certain areas. If these precautions are lacking, look for evidence of problems relating to uplift in these areas.
Adhered membranes are typically applied using a liquid-applied contact adhesive, although some have fleece backing that allows other types of adhesives to be used, and some are self-adhesive.
With adhered membranes, the resistance to wind-source uplift previously described is dependent upon the method of attachment of the base sheet or rigid board insulation to which the membrane is adhered.
Smooth-backed membranes use cold-applied bonding adhesives to bond PVC membranes to substrates. Adhesives are typically applied with a roller, but may be also applied by spray, brush, or squeegee. Application may be to the membrane, or to both the membrane and substrate. Seams should not receive any bonding adhesive.
Before the membrane is mated to the substrate, the adhesive is given time to flash-off (partially cure). Once the membrane is mated to the substrate, it is brushed or rolled to promote good bonding.
Fleece-backed membranes may be adhered with cold-applied adhesive, hot asphalt, or low-rise foam. A large variety of adhesives are available that inspectors will have no way of identifying, and each having manufacturers requirements with which inspectors will not be able to confirm compliance. Home inspectors are not required to confirm proper installation, and application information supplied here is meant only as general information.
PVC may be applied as a loose-laid (not attached to the substrate) membrane, secured to the roof only around its perimeter and held in place in the field by the weight of gravel, typically to 1 in size and applied at a rate of 1,0001,200 lbs. per 100 sq. ft. Pavers may also be used for ballast.
Perimeter securement may be made with or without fasteners, and either penetrating or non-penetrating types may be used. Most methods are proprietary and fasteners will typically not be visible.
Concerns are wind and the ability of the structure to support the additional weight. Inspectors should look for signs of problems caused by these issues. Because most of the membrane will be hidden, inspections of these roofs will be very limited and they should be disclaimed for those without specialist training exceeding that provided by home inspection courses.
3. INSPECTION CONSIDERATIONS
By Kenton Shepard
Air Movement Across the Membrane:
A characteristic of mechanically attached membranes is that they are subject to vertical displacement 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.
Blisters can develop in PVC 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.
Embrittlement is a major failure mode of older, thinner, unreinforced PVC membranes. It is caused by the gradual loss of plasticizers and result in increasing fragility over time. In the past it was thought that embrittlement was due to UV exposure, but with time it became evident that ballasted-shielded PVC membranes were affect at a more rapid rate than membranes directly exposed to sunlight. Look carefully at older ballasted PVC membranes for signs of failure.
Loss of plasticizer results in membrane shrinkage that can in turn result in the membrane pulling away from the substrate. This is often first visible at roof/wall intersections or at the bases expansion joints, skylights, and platforms supporting mechanical equipment.