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Damage to buildings and building systems from volcanic ash can range from complete or partial roof collapse to less catastrophic damage of exterior materials and interior rooms, including appliances and computers, floor coverings, and electrical and mechanical systems. These effects depend on several factors, including the thickness of ash, whether it is wet or dry, the roof and building design, air-handling systems, and how much ash gets inside a building. Damage to the interior of a building can be significantly reduced by taking several key steps before an ash fall begins. For example, shutting down a building's mechanical systems and air conditioners, protecting air intakes, and closing other openings (doors and windows).
After an ash fall, removing ash from the roofs of buildings is usually a top priority in order to (1) prevent roof collapse; (2) reactivate the ventilating and air-handling systems; and (3) coordinate community-wide clean-up efforts—clean from roof to ground (roofs need to be cleared first so that ground-level areas are not covered again by windblown ash from roofs above). Rapid cleanup and restoring normal operation of public buildings can significantly improve public morale and confidence after an ash fall.

CAUTION: Be extremely careful when working to (1) remove ash from roofs or gutters; and (2) seal a home or building before an ash fall—ash can make surfaces extremely slippery. Falling from a roof or ladder can result in injury or death.

Roofs & Gutters  || Loading & collapse || Corrosion || Gutters || Ash removal ||

Ash loading on roofs

A primary concern during ash fall is the potential collapse of buildings from the accumulation of ash on roofs, which can lead to widespread injuries and deaths. For example, the collapse of roofs from falling ash during the explosive eruption of Mount Pinatubo on June 15, 1991, killed about 300. Historical examples of the effects of ash accumulating on roofs are provided below.

If ash fall is expected, a survey should be made of the strength of roofs in the area and of the maximum thickness of ash that they will bear without danger of collapse, especially for critical facilities and buildings which are expected to provide refuge for people during ash fall. Such surveys must take into account the density of both dry and wet ash.

The effects of volcanic ash on roofs depend primarily on:

Ash density and thickness
The specific weight of dry ash can vary from 400 to 700 kg/m3, and rainwater can increase this by 50-100 percent or more if the ash becomes saturated by rain, sometimes reaching more than 2,000 kg/m3. The problems of loading by ash are similar to those from loading by snow, but the effects of ash accumulation are much more severe—the load due to ash is typically much greater (see table below), ash doesn't melt, and the ash can clog gutters and cause them to collapse, especially after rainfall.
In areas that have snow-loading codes, some protection against ash may result but his is highly dependent on the location of structures because snow load levels vary with altitude and location.


Density & load comparison, 10 cm of snow and 10 cm volcanic ash
(Johnston, 1997; p. 75)

Description Density kg/m3 Load kPa
New snow 50-70 0.05-0.07
Damp new snow 100-200 0.1-0.2
Settled snow 200-300 0.2-0.3
Dry uncompacted ash 500-1,300 0.5-1.3
Wet compacted ash 1,000-2,000 1.0-2.0

The load on a bulding is given by the equation:

Equation for calculating load on a building


Graph: Load (kPa) vs. Ash Thickness

Loading of volcanic ash on a roof for wet ash and dry ash. The ash is assumed in this example to have a dry (compacted) density of 1,000 kg/m3 and a saturation of 50% water by volume and therefore a wet density of 1,500 kg/m3.

From Johnston, 1997; p. 74

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Building design & construction || Historical case studies ||

The effects of ash loads on buildings vary greatly depending of their design and construction, including roof slope, construction materials, roof span and support system, and age and maintenance of the building. In general, flat roofs are more susceptible to damage and collapse than steeply pitched roofs, and roofs made of smooth materials like sheet metal and glass are more likely to shed volcanic ash than roofs made of rough materials like thatch and asphalt or wood shingles.

Buildings designed to withstand a heavy load of winter snow will clearly support thicker accumulations of ash than buildings not engineered for any type of load or shear stress. Surveys of buildings damaged from the accumulation of ash during the eruptions of Mount Pinatubo in the Philippines and Rabaul Caldera in Papua New Guinea indicate that roofs with wide spans (for example, warehouses) are more vulnerable to collapse than buildings with short spans typical of small homes.

Contrasting effects of ash loading on buildings, 1991 eruption of Mount Pinatubo, Philippines

Collapsed building on former U.S. Clark Air Base in the Philippines
Collapsed building with long roof span
House on former U.S. Clark Air Base in the Phillipines
Nearby home with short roof span
Ashfall from the 15 June 1991 eruption of Mount Pinatubo resulted in the accumulation of  5-10 cm of wet ash in the area of former U.S. Clark Air Base, located 20 km northeast of the volcano. Densities of ash samples collected here ranged from 1,200 to 1,600 kg/m3 (dry) and 1,500 to 2,000 kg/m3 (wet).

A survey of damaged buildings in Castillejos, 27 km southwest of Mount Pinatubo identified the roof structure as the most significant indicator of damage in a comparison of residential and nonresidential buildings. Sixteen percent of surveyed buildings with short-span roofs suffered major damage, with 43 percent having no significant damage. Seventy-five percent of buildings with long-span roofs (greater than 5 m clear span) were severely damaged, and only 17 percent were without significant damage (Spence and others, 1996, p. 1059). See online report, Building damage caused by the Mount Pinatubo Eruption of June 15, 1991.

Complex roof profiles or geometry and obstructions on roofs such as chimneys, parapets, roof tanks or solar panels may lead to a greater accumulation of ash next to these features if the ash is drifting with the wind. Such uneven accumulation of ash on roofs can lead to an unbalanced load on the roof, increasing the potential of roof failure (Blong, 1984, p. 206-208). Also, ash loads against these obstructions may lead to their failure and, indirectly, to failure of the roof.

Historical eruptions: effects of ash loads on buildings

1994 Eruption of Rabaul, Papua New Guinea

The effects of various ash loads on buildings in Rabaul, 1994

Ash thickness1 in mm

Estimated load2 in kPa

Observed damage to roofs
<100 1.5 - 2.0 Roofs and guttering generally remained intact.
<200 3.0 - 4.0 80-90% of roofs remained intact with little apparent damage. Sagging or partial collapse occurred in some buildings.
<300 4.5 - 6.0 More than 50% of roofs did not collapse.
500-600 7.5 - 12.0 More than 50% of roofs collapsed.
>600 9.0 - 12.0 It is doubtful that buildings survived without significant damage even when the roof remained relatively intact.
1Ash fell wet
2Using this equation and assuming ash density of 1,500 to 2,000 kg/m3
Data from Blong and McKee, 1995.

Mount Pinatubo, Philippines, 1991 (Spence and others, 1996)

Collapse roof and upper story of a building due to load of wet ash 15-20 cm thick in Castilleejos, PhilppinesA survey was conducted of 51 buildings damaged in Castillejos, a town with a population of less than 50,000 located 27 km southwest of Mount Pinatubo. The thickness of ash in the town was about 20 cm. For the survey, the building analysis included identification of (1) principal constructional materials used; (2) number of stories; (3) roof structure, shape, and pitch; and (4) building's usage (residential or nonresidential). The principle cause of damage to the sample of buildings was that the load of ash on the roof exceeded the strength either of the roof sheets or of the roof supporting structure, or both. The load of 15-20 cm of water-saturated ash on roofs would have exerted a force of 2kN/m2 (1 kiloNewton per square meter is about 200 kg/m2.

In a summary of the damage, the authors identified the following:

  1. Although many roofs had been cleared by the time of the survey, there was evidence from the uncollapsed roofs that the wet ash was able to accumulate to depths of at least 15 cm on metal sheet roofs of pitch up to 25 degrees, without slipping from the roof.
  2. Out of the total sample of 51 buildings in Castillejos, 17 suffered partial or complete roof damage, while 18 suffered no damage or only light damage.
  3. Buildings tended to suffer worse damage if they were (a) constructed with long-span roofs (greater than 5 m clear span), rather than with short-span domestic scale construction; (b) of timber frame rather than reinforced concrete frame construction; (c) of higher rather than lower roof pitch; or (d) non-residential rather than residential.
  4. Some other factors that seem to have contributed to damage, though statistical evidence is inadequate to demonstrate their significance are (a) unbraced supporting walls or columns; and (b) large unsupported roof overhangs.

According to the authors "to protect lives, roofs of buildings exposed to possible ash fall should be designed for a superimposed load related to the probable level of ash fall, in a manner analogous to design for now loading in cold climates."

Ash corrosion on roofs and exterior materials

Ash can cause corrosion and may be electrically conductive. To minimize effects, tape plastics (garbage bags, plastic wrap, tarps) over external building electronics and metal surfaces, for example, security system displays, swipe card door locks, alarms, and electrical panels.

Metallic roof surfaces, particularly older galvanized roofs which are pitted, and lower gauge galvanized roofs are most susceptible to increased deterioration from the properties of ash. To prevent or reduce the accelerated deterioration of roof coatings by mildly acidic property of ash, clean and/or protect the roof surfaces accordingly.

Ash may clog & collapse gutters and drains

Because gutters and drains are designed to collect water from roofs, they are perfect "ash traps" and one of the most susceptible parts of a building to damage from ash fall. A gutter that fills with ash, especially if the ash is wet, can easily pull apart or collapse from a roof.

The many downspouts and drains attached to gutters may also become clogged with ash, especially when it rains or if water is used to remove ash from a roof. If the drain pipes deliver water to a dry well, the ash can seal the well, making it inoperative.

For these reasons, it is important to keep roof drains and gutters clear of ash as much as possible.

If a building's down-spouts and drains are designed to deliver water to a community's wastewater delivery system and water-treatment facilities, effort should be made to disconnect or block a building's roof drains before or immediately after an ash fall to prevent ash from entering the drains and waste-water systems.

Removing ash from roofs

The most obvious action to take for ensuring the safety of a building is the removal of ash from the roof. Before beginning a cleanup operation, it may be advisable to check insurance policies to see if any actions undertaken, or inaction in some circumstances, may void the policy with respect to damage due to the ash or cleanup process.

When should ash be removed from roofs?

The range of ash densities, roof design, and construction techniques make it difficult to determine when during an ash fall that ash should be removed from a particular building's roof. Clearing ash from a roof may prevent collapse but such a decision must be weighed against the risk of personal injury working in a dark, ash-rich environment—people easily slip from roofs, fall from ladders, and fall through weak roofs while clearing and removing ash. Also, if an ash fall is accompanied by rain, the roof may become slippery and the wet ash could be difficult if not impossible to shovel or sweep.

It may be advisable to remove ash before it exceeds a thickness of 10-15 cm, but only if the roof is easily accessible and the ash can be removed safely. Because it is often dark or "pitch black" during an ash fall, it may not be possible to safely remove ash until a later time. After an ash fall, buildings which have received more than 10-30 cm of ash and that have not collapsed still run a high risk of load damage (for example, with the addition of more weight during cleanup operations). The ash should be removed, however, as soon as it can be done safely.

Things to consider before removing ash from roofs:
  • Use extreme caution when working on a roof.
  • Know what plan your community has developed for disposing ash that you collect.
  • Coordinate your clean up efforts with those of your neighbors and community to prevent the need for more than one ash-removal project, especially from public streets.
  • Communities—(1) promply notify building owners to remove ash from roofs in a timely manner (or according to a specific schedule) to prevent streets from having to be cleaned many times; and (2) inform the public of effective methods for removing ash from roofs and property and preparing it for pick up by clean-up crews and organizing neighborhood cleanup activities (see Cleaning up volcanic ash).
  • Prevent ash from entering the waste-water delivery system (or sewers) by (1) disconnecting drains and downspouts from roofs; and (2) not placing ash where it can be swept by water into the waste-water system UNLESS told to do so by your community and neighborhood cleanup plans.
  • Remove ash before the first rain if possible; to reduce the amount of billowing during clean up, first dampen the ash with a light spray of water (do not use large amounts of water because the ash may form a glue-like "cake" material, which is difficult to remove and adds considerable weight to the roof).
  • Do not flush ash into drains and downspouts, because ash may clog them. Ash flushed into dry wells can seal them, rendering them inoperable.
  • Small vacuum equipment is usually not very practical for removing ash from roofs because of the abrasiveness of ash and the enormous volumes of ash that typically needs to be removed.
  • Take action to prevent ash from entering the building during ash removal (for example, plastic coverings over windows and doors).
  • Ash shovelled or pushed off a roof may accumulate at the sides of a building and exert pressure against the walls; it may be necessary to remove this material at the same time the roof is cleared of ash in order to reduce or eliminate the pressure.
Safety steps to take into account when removing ash from a roof:
  • Be extremely careful when working on a roof, especially roofs with even a low to moderate pitch and slippery material.
  • Use personal protective measures when removing ash from roofs; for example, use a strong ladder, safety harness, filter face mask, gloves, and eye goggles.
  • Prevent unnecessary damage to roof material and surfaces by using protective measures during cleanup; for example, use planking, mats, plywood sheets, and pliable footwear to prevent damage from impact and abrasion; when using shovels, rakes, or other tools, be careful of the underlying roof surface; the full force of water from fire hoses can easily break lap shingles or tear lap roofing material.

Recommendations for removing ash from roofs:

  • Remove all traces of ash near intakes of ventilation systems.
  • Cover and seal intakes of ventilation systems around the building.
  • Prevent ash from entering building through windows and doorways seal doors and windows and control access into buildings.
  • On flat roofs, hand sweep the ash into rows and transport it by wheelbarrow to the edge of the roof; use planking, mats, plywood sheets, and pliable footwear to prevent damage from impact and abrasion; hoppers with a funnel pipe suspended above a loading truck may be helpful; to remove the final residue or thin layer of ash, an air compressor may be useful but only if the pressure can be regulated.
  • On steep shingle roofs, place dams in the troughs to prevent ash from reaching the drains and downspouts, then hose down the ash and clear it from the eave troughs and gutters. This operation must be performed with care to avoid deforming the gutters and tearing them loose.
  • On low-slope bitumastic mopped roofs, where there is only a thin ash layer or small residue, flush the ash with water; use extreme caution from high-pressure water hoses, which can easily damage such roofing materials.

Modified from, FEMA, 1984

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Air-handling Systems

The abrasive and mildly corrosive nature of ash can damage mechanical and electrical systems. Air-handling systems and air conditioners are vulnerable to ash damage and air-filter blockage, especially if air intakes are horizontal surfaces. Damage can be prevented by turning off such systems before an ash fall begins or immediately at first signs of ash fall. In many cases damage can be avoided by taking steps to avoid use and contamination during ashy conditions and thorough cleaning of equipment.

In buildings where air quality is critical, for examples hospitals, any action should be taken under the advice of qualified personnel and engineers.

Recommended steps for air-handling systems (FEMA, 1984):
  • Shut down air-handling and air-conditioning systems prior to or duing the initial onslaught of ash fall.
  • Close and seal air intakes; use internal circulation only to create positive pressure inside building.
  • Close and seal windows, doors, and other openings of buildings.
  • Before re-starting air-handling system, clean and remove ash from near external air intakes and roof area adjacent to the intakes; clean or replace filters; inspect, clean or lubricate moving portions of the system following prescribed routine maintenance procedures.
  • Restrict vehicle and foot traffic near air intakes.
  • Consider installing intake hoods that extend higher above the ground.
  • Install pre-filters.

To restart air-handling systems:

  • Clean the air intakes and the roof area adjacent to the intakes.
  • Clean or replace filters.
  • Inspect, clean or lubricate moving portions of the system following prescribed routine maintenance.

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Electronics  || Electric motors || Computers || Appliances ||

The abrasive and mildly corrosive nature of ash can damage computer and electronic systems. When possible, the best damage-preventive strategy is to shut down all computer, electric motors, and electronic systems until the ash has been completely removed from the equipment and the surrounding area, including air-supply and ventilation systems. Electrical panels can short out because of the high level of electical conductivity of wet ash. For electronical systems and components, the goals are to keep ash out of electronics, controlling what gets in, and cleaning and disposing of the ash.

Electric motors

Ash can increase the wear on brushings, brushes, thrust bearings and commutators on electric pump motors and other drive motors. Small motors blanketed with ash can generate heat and could become fire hazards (FEMA, 1984).

Recommendations for cleaning motors (FEMA, 1984):
  • Turn off electric motors and machinery before cleaning them. Throw the motor switch as well as the main circuit breakers.
  • Clean the electrical equipment using air pressure of 30 psi or less to avoid getting a sandblast effect on delicate parts. Vacuum, where possible, and change filter bags often. Avoid damaging surfaces by rubbing or brushing them. Do not blow ash into places that should be kept clean, and always follow manufacturer's recommendations for cleaning equipment operated under dusty conditions. Increase maintenance servicing as recommended.
  • Watch for electric shocks when operating ashy and dusty equipment. Be alert to the possibility of overheating and fires.

Computer systems

Computer heads and discs, and any high-voltage circuits, are especially vulnerable to ash upset and damage. Ash on digital circuits will not cause much of a problem because of the low voltages involved. High-voltage or high-impedance circuits are very vulnerable to leakage caused by semiconductive ash. Ash that is acidic is conductive as well as corrosive. Continual cleaning and aggressive protection of computer systems should allow for continued operation in all but the heaviest ash fallout (Labadie, J.R., 1994).

Techniques for cleaning include (from Labadie, J.R., 1994):
  • Clean and condition surrounding air to keep ash out of equipment.
  • Cotton mat filters used in separate clean rooms were found to be best for filtering particles, but they reduce the air flow. A solution is to use larger fans to maintain required air flow. Rack-mounted equipment can be modified to add a larger fan, but smaller instruments or components with a built-in fan would require design change to increase fan capacity. Use fluted filters as a compromise; increases surface area but reduces air flow by only about 20%.
  • Digital integrated circuits can vary 5-10% in performance (depending on type of circuit) and still be acceptable. It is difficult to generalize about other equipment (e.g. high-voltage power supplies).
  • Humidifying ambient air (for example, wetting carpets) will help to control ash reentrainment.
  • Ash on equipment can be blown out with compressed air. If the air is too dry, static discharge could damage sensitive components (for example, integrated circuits). If the air is too damp, the ash will stick. Relative humidity of 25-30% is best for compressed air.
  • Cleaning with a pressurized water-detergent mix and a hot water rinse is quite effective. However, this process requires at least partial disassembly.
  • Ash should be blown or brushed away from power supplies.
  • Ash may have high static charge and be hard to dislodge, thus equiring brushing to dislodge particles.
  • Accelerate filter change; use pre-filters.
  • Change to absolute filters; these will keep out particles down to 1 micron and smaller.
  • Keep computer power on to operate filtration, but don't run (especially disk drives).
  • Maintain "room-within-a-room" configuration; restrict access; re-circulate air; accelerate cleaning of the critical area.


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Removing Ash || Inside buildings & households || Disposal || Roofs ||

Inside buildings and households

In general surfaces should be vacuumed to remove as much ash as possible from carpets, furniture, office equipment, appliances, and other items. Portable vacuum systems equipped with high-efficiency particulate filtering systems are recommended whenever possible. The severity of ash intrusion depends on the integrity of windows and entrances, the air intake features, and the care exercised to control the transport of ash into a building or home via shoes and clothing (see recommendations for keeping ash out). Care should also be taken to avoid further contamination during the emptying, cleaning, and maintenance of vacuum equipment.

Suggested ash removal from buildings and households (FEMA, 1980):
  • Cleaning by blowing with compressed air or dry sweeping should be minimized.
  • A dustless method of cleaning such as washing with water and an effective detergent/wetting agent is recommended. Damp rag techniques should be used whenever possible to remove the substance from small surface areas or flooring. On those areas where damp rag techniques cannot be implemented (for example, carpets) vacuum cleaning methods should be applied.
  • After vacuuming carpets and upholstery may be cleaned with a detergent shampoo. Avoid excess rubbing action because the sharp ash particles may cut textile fibers.
  • Glass, porcelain enamel and acrylic surfaces may be scratched if wiped too vigorously. Use a detergent soaked cloth or sponge and dab rather than wipe.
  • High-shine wood finishes will be dulled by the fine grit. Vacuum surfaces and then blot with a cloth treated to pick up ash. A tack cloth used by furniture refinishers should work well.
  • Floor sweepers with side brushes should not be used to clear aisles and floors beacuse they may re-entrain dust particles into the air.
  • Ash-coated fabrics should be rinsed under running water and then washed carefully.
  • Soiled clothing will require extra detergent. Wash small loads of clothing, using plenty of water so the clothes will have room to move freely in the water. Do not mix heavily soiled clothes with garments that are lightly soiled.
  • Be sure clothes are free of ash before putting them in an automatic dryer Ash may scratch the inner surface of the dryer.
  • For several months after an ash fall, filters may need replacing often. Air conditioner and furnace filters need careful attention. Clean refrigerator air intakes. Clean any surface that may blow air and recirculate the ash. Stove fans and vents should be cleaned thoroughly.
  • Each employee should be responsible for clean-up of his/her own work area to minimize exposure potential during a work shift. This should be accomplsihed at the beginning of each workshift. Damp rag or vacuum techniques should be used during this operation.

Ash Disposal

Ash removed from inside buildings and homes should be disposed in accordance to community plans and directions (for example, preparing it for pick up by clean-up crews as part of neighborhood cleanup activities). It may be advisable to request that people separate volcanic ash from normal garbage for collection or disposal at a designated location—mixing ash with normal garbage can result in damage to collection vehicles and take up space in landfills. Small amounts of ash from vacuum cleaners have been disposed successively in household gardens and lawns.

See full discussion of ash disposal issues.

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Keeping Ash Out  || Air-handling systems ||

During and after ash fall, keeping ash out of buildings and homes will significantly reduce cleanup costs and prevent damage to surfaces, electronics, appliances, floors, and other equipment.

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Blong, R.J., 1984, Volcanic hazards: a sourcebook on the effects of eruptions: Academic Press, Australia, 424 p.

Blong, R., and McKee, C., 1995, The Rabaul eruption 1994: destruction of a town: Natural Hazards Research Centre, Macqauarie University, Australia, 52 p.

Labadie, J.R., 1994, Mitigation of volcanic ash effects on aircraft operating and support systems, in Casadevall, T.J., ed., 1994, Volcanic ash and aviation safety: Proceedings of the first international symposium on volcanic ash and aviation safety, Seattle, Washington, July, 1991: U.S. Geological Survey Bulletin 2047, p. 125-128.

Federal Emergency Management Agency (FEMA), Region X, 1984, The mitigation of ashfall damage to public facilities: lessons learned from the 1980 eruption of Mount St. Helens, Washington: [Seattle, Wash.], FEMA, 70 p.

Spence, Robin J.S., Pomonis, Antonios, Baxter, Peter J., Coburn, Andrew W., White, Mark, Dayrit, Manuel, Field Epidemiology Training Program Team, Building damage caused by the Mount Pinatubo eruption of June 15, 1991, in Newhall, C.G., Punongbayan, R.S. (eds.), 1997, Fire and mud: Eruptions and lahars of Mt. Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology, Quezon City and University of Washington Press, Seattle, 1126 p.

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