Ballasted Roof System

In a ballasted roof system, the waterproofing membrane is not fastened or glued across its surface. Instead, the membrane and usually the insulation beneath it are laid loose, attached only at perimeters, penetrations and flashings, and held in place against wind by a layer of heavy material on top.

Ballast materials

• rounded river stone or crushed stone of specified sizes
• concrete pavers or linked paver systems
• in green and inverted roofs, growing medium, paving or gravel serving the same function

Ballast also shields the membrane from UV, hail and foot traffic.

Where it is used

Ballasted systems are common with EPDM and other single-ply membranes, and are the usual method for protected membrane (inverted) roofs, where the ballast also keeps the insulation boards from floating. They suit large, simple roofs on structures able to carry the extra dead load.

Design

Wind design is critical. In North America the wind design of ballasted single-ply systems is covered by the ANSI/SPRI RP-4 standard, and corners and perimeters, where suction is highest, typically need heavier ballast or pavers. In the UK, zero-fall inverted roof guidance requires ballast heavy enough to prevent insulation flotation so water can reach the outlets.

Advantages

• fast installation with few fasteners or adhesives
• membrane protected from weathering
• membrane and stone can be reused at reroofing

Limitations

• heavy, so the structure must be designed for it
• stone can be scoured or blown off at corners in high winds
• leaks are harder to locate because the membrane is covered
• dark stone cancels the benefit of a white membrane; light ballast is needed for cool-roof performance