Fully Adhered vs Mechanically Attached

A low-slope roof membrane must be held against wind uplift. The three main attachment methods for single-ply membranes such as EPDM, TPO and PVC are fully adhered, mechanically attached and ballasted. Some systems also use induction-welded plates, and bituminous membranes can be torched, mopped or self-adhered. The choice affects wind performance, cost, appearance, noise and how the roof behaves if punctured.

Fully adhered

The membrane is bonded across its whole area to the insulation, cover board or deck. Canadian guidance describes folding back half the sheet, applying adhesive at the manufacturer's rate, rolling it back in and brooming or rolling out trapped air, while keeping adhesive out of the lap area. Most manufacturers limit adhered sheets to a maximum width of about 3 m (10 ft). Fleece-backed membranes can be set in hot asphalt or low-rise foam adhesive.

Advantages: smooth appearance, no membrane flutter, good performance on irregular roofs. Limitations: higher labour and adhesive cost and temperature limits on adhesives.

Mechanically attached

The membrane is fastened through the laps with plates or bars screwed into the deck; the next sheet covers the fasteners and is welded or taped over them. Canadian guidance notes that sheets usually run perpendicular to the deck flutes, and that fastener spacing follows the manufacturer and local wind pressures. A recommended minimum slope of 1:50 (2 percent) applies to both adhered and mechanically fastened thermoplastics.

Advantages: fast, fewer temperature limits, lighter than ballast. Limitations: the membrane can billow in wind and fastener pull-out must be designed for each zone of the roof.

Ballasted

Loose-laid membranes are held by stone or pavers. Canadian guidance gives about 60 kg/m² (about 1200 lb per square), more in high-wind areas, and limits loose-laid ballasted roofs to slopes of no more than 1:6. The structure must carry the extra load.

Induction welding

Some thermoplastic systems fasten coated plates to the deck under the membrane and bond the membrane to them with an induction tool, avoiding fasteners through the sheet.

Design basis

In all methods, perimeter and corner zones receive higher attachment rates than the field, following wind-load calculations and approval listings.