Earthquakes and Roofs

Earthquakes shake buildings sideways, and the forces a structure must resist are roughly proportional to its mass. The roof is at the top of the building, so a heavy roof increases the lateral forces on the walls below and the risk of collapse if those walls are weak. Roof coverings that are not well fastened can also fall from buildings during shaking.

Heavy roofs on light frames

The January 17, 1995 Great Hanshin (Kobe) earthquake in Japan, which killed more than 5,000 people, is a widely cited example. Many traditional wooden houses had heavy tiled roofs, weighing up to about two tonnes, that were intended to stay in place during typhoons. When the light timber frames beneath them failed, the roofs crushed the walls and floors below. Post-earthquake reconstruction in Japan moved toward reinforced walls and lighter roofs, although lighter roofs are more vulnerable to typhoon winds, illustrating a trade-off between hazards.

Roof weight and design

Clay and concrete tiles and natural slate are among the heaviest roof coverings; cement-based tiles have long been among the heaviest products because their porosity requires thickness. Metal panels, asphalt shingles and lightweight synthetic products weigh much less. In seismic regions, engineers account for roof weight when designing walls, bracing and connections. Replacing a light roof with a much heavier one, or adding heavy rooftop equipment or solar arrays, may require a structural review.

Diaphragms and connections

A roof deck acts as a horizontal diaphragm that collects seismic forces and distributes them to the walls. Good performance depends on:

• adequate nailing of sheathing to framing;
• secure connections between roof framing and walls;
• bracing of gable ends, parapets and chimneys, which often fail in earthquakes;
• continuous load paths to the foundation.

Tile attachment

Loose or mortar-only tiles can slide off during shaking and become falling hazards. Mechanical fastening of tiles (nails, screws, clips or wires), particularly at ridges, hips and eaves, reduces the chance of tiles dislodging during shaking.

Masonry chimneys and parapets

Unreinforced masonry chimneys frequently break at the roofline and can fall through roofs. Bracing, reinforcement or replacement with lightweight flues are common retrofit measures in earthquake-prone areas.