Mechanically Attached Roof System

In a mechanically attached roof system, the membrane is held down by fasteners driven through it into the structural deck. Most often the fasteners and plates (or continuous metal bars) are placed in the overlap of each seam, so the next sheet covers them and the seam is welded or taped over the top.

Variations

• In-seam (lap) fastening: fasteners along one edge of each sheet, covered by the next sheet's lap. This is the most common method for TPO.
• Strip or bar attachment: continuous bars in or over the seams, or across the field with cover strips.
• Point attachment: individual plates through the sheet with cover patches.
• Induction welding: coated plates under the membrane, bonded by induction heat, with no penetrations.

Insulation is usually fastened separately with its own screws and plates, or held by the same fasteners.

Wind design

Fastener spacing and sheet width are set by wind-uplift calculations. Because suction is highest at roof corners and perimeters, these zones use narrower half-sheets or closer fastener spacing. In North America assemblies are tested and listed by FM Approvals and UL, and fastener pull-out strength in existing decks can be checked with the ANSI/SPRI FX-1 field test.

Advantages and limitations

Mechanical attachment is fast, lightweight, works in cold weather when adhesives struggle, and suits steel decks. Its drawbacks include "billowing", where wind lifts the sheet between fastener rows and draws interior air into the roof, possible fastener back-out, and the need for a deck with adequate pull-out strength. Steel deck must be thick enough to hold screws, and some concrete or gypsum decks need special fasteners.