Reroofing Dilemma


Karen Warseck,  AIA

Too often, roof replacement comes as a surprise because there are no regular maintenance programs in place; and it always seems to happen when money is least available. The problem then becomes, how can one design a quality roof at a low cost? One option is recovering an existing roof with a new membrane. This may not be the preferred way to reroof a building but, when cost becomes the significant factor, knowing that the expense of a tear off can be significantly reduced and sometimes even eliminated in a re-cover makes it very attractive. It is an option, but one that must be thoroughly investigated. With a recover, you are relying on the condition of the original roof assembly to provide a firm foundation for the new installation. If the existing membrane is in very poor condition, tearing it off may be the only option. It is illogical and foolish to put an expensive new roof on a deteriorated substrate.

In a tear off, everything is removed right down to the deck, which not only allows the new roof to start with a clean slate but also enables the facility executive to see the condition of the structure and make appropriate repairs. This is especially critical for decks that are prone to water damage and structural deterioration from long-term moisture exposure, I such as gypsum-based poured-in-place, plank and cementitious-wood-fiber decks. Other decks that need close examination are steel, wood and lightweight concrete fill. All of these will deteriorate with long-term exposure to water. One caveat here. Do not assume that the deck is sound if structural deterioration is not visible when looking up at the deck from the floor below. Examining the deck from the underside may not reveal structural damage. Water usually enters the roof assembly from the top, so what may look acceptable from the ground may be almost rotted through from the top. The only way to know for sure is to remove the roof membrane and look. If there is any doubt whatsoever about the condition of the structural deck, a recover is a very bad idea.

Although unscrupulous roofing contractors may let facility executives think so, keep in mind that a recover does not mean simply plopping a new membrane down over the old one. The old roof must be repaired so that all blisters and delaminations are cut and patched before the crew roof is installed. Some membrane manufacturers also require the original roof to be scored to allow any moisture vapor trapped in the old roofing system to vent. This will add to the cost of the recover but is essential if the new roof is to last. What's more, moisture may be retained in the existing roof system from the leakage that made a reroof necessary in the first place. Trapped moisture will cause the new roof to deteriorate. Moisture under an asphalt built-up or modified bitumen roof system will leach plasticizing oils out of the membrane, making it prematurely brittle. Pressure exerted by moisture vaporizing during a hot day will blister asphalt and coal tar pitch built ups, modified bitumen membranes and coated polyurethane foam systems. Trapped moisture is also a problem in single ply roofs. It can rust fasteners in a mechanically attached EPDM or thermoplastic single ply membrane system. Even if the fasteners don't rust, the holes around them in a steel deck will. The steel is not protected at that point by galvanizing or paint. Rusting will enlarge the hole and increase the chance of blow-offs. Dampness also reduces the insulating value of most types of insulation.

Question of Cost

All roofing materials manufacturers require that damp materials be removed from the roof before installing a recover. Most recovers should therefore involve a partial tear-off. The question then becomes how much can be removed before it no longer makes sense to recover? Conventional wisdom and many building codes say 25 percent is the maximum limit; some sources say 15 percent. From a purely economic viewpoint, after about 25 percent removal is needed, the cost of recovering begins to approach that of a complete tear off. Thus, the best candidate for a recover roof is one that has been maintained over the years, so that the spread of moisture through the system has been kept to a minimum.

Testing Options

The best way of determining the amount and location of wet materials to be removed is through a moisture survey. Moisture surveys generally use one of four methods - nuclear isotopic, infrared thermographic, electrical capacitance or electrical resistance testing - to locate the moisture retained in the roof system. None of these testing methods actually measures moisture, however. Instead they measure properties of materials affected by water.

For example, infrared testing relies on the fact that insulation generally works less effectively when wet. As a result, at night, when the outside temperature drops, there will be more heat loss through the wet areas of insulation than through dry areas. An infrared camera picks up this heat loss and marks it as wet. This method of determining what is wet and what is dry can be confused by anomalies in the roof. Areas of thinner insulation may read as wet because they will insulate less. Areas in shadow during the day may not pick up as much heat as surrounding areas and also show as wet. Damp areas will retain heat longer than dry areas. In locations where heat gain is a problem, the infrared camera will note hot areas as wet. Thus, areas of piled gravel may read as wet because they will retain heat longer than the surrounding areas.

Nuclear isotopic testing works by shooting hydrogen ions into the roofing materials and counting how many bounce back. The theory is that water contains two hydrogen ions in its molecule, so a high number of hydrogen ions will indicate the presence of moisture. However, thick layers of asphalt in a built-up roof will confuse the readings of the nuclear gage, since asphalt is a hydrocarbon-based material. The gage may also be confused by wet roof surfaces. Electrical capacitance testing relies on the ability of materials to conduct electricity. Damp materials will conduct electricity more easily than dry ones will. In electrical capacitance testing, an electrical signal is sent through the materials; the strength of the signal indicates the extent of moisture. This gauge too has its anomalies. EPDM roofs conduct electricity easily so the gauge cannot be used. Surface moisture and embedded metal items in the roof will also confuse the gauge. Electrical resistance meters work in a way that is similar to electrical capacitance testing - although instead of measuring a wet material's ability to conduct electricity, they measure a dry material's ability to resist the flow of electrons - and have the same shortcoming as a capacitance meter. All types of metering systems should be correlated by taking core cuts to verify the readings and to eliminate false ones. The roof map generated by the results of the testing can be used to remove the damp materials from the building. One caveat here, too. Different materials tolerate different amounts of moisture, so what may be considered wet in one material may be dry in another. There are no set standards of what is considered wet.

Below the Membrane

Although some roofing contractors will still install additional plies of roofing directly onto the existing roof, good roofing practice requires a layer of insulation (a recover board) between the old and new roof. The recover board is used to provide a suitable, smooth surface for the re-roofing membrane to adhere to, provide protection from gravel or other detritus on the roof, or to isolate chemically incompatible materials. Lack of a recover board usually results in debonding of the new roof from the existing one, allowing water to penetrate between the layers. Additional insulation is also sometimes installed to increase energy efficiency. Tapered insulation may be added to promote drainage.

In any case, when designing the recover roof, the finished elevation of the roof should be determined ahead of time to ensure that the recover will not raise the height of the roof so much that it does not allow for good roofing practice or code compliance. For example, raising the height of the insulation may raise the roof elevation above the heights of through wall flashings at masonry penthouse walls. This will block water entering the wall system from draining back to the exterior and cause leaks. Another potential problem is flashing heights at perimeters and mechanical equipment that may be diminished below the eight inches normally required by warranties and many codes. Worse, overflow scuppers designed to keep the building from collapsing in the event of blockage of the primary drainage system may be covered or reduced in size. Anything that interferes with the overflow scuppers is foolhardy at best and dangerous at worst.

But while there are many reasons for tearing off the roof, there are situations where a recover makes much more sense. The downside of a tear-off is that the interior of the building is exposed to the weather for as long as it takes to replace the torn off area. In some locations and climates, this is not a major concern. However, if the area is prone to sudden showers, the interior of the building and its contents can be severely damaged. This can be especially serious over mission-critical areas like computer facilities, clean rooms, operating rooms and switching stations, where moisture intrusion can be devastating. The existing roof, even in its marginal state, will provide some protection while the recover is being performed, not only from the weather, but also from dust and debris generated by the reroofing process. And of course, there is the money.

One final caution about recovers. Although recovers can be an excellent tool for saving money in a reroofing project, they are only as good as the consultant who designs them and the contractor who installs them. Be especially careful when the designer and the contractor are the same entity. Experience shows that the better contractors avoid recovers because of their higher failure rate. Too often what happens is that the facility executive or building owner calls three contractors, gets bids and takes the lowest without fully understanding the ramifications of the proposal. The lowest price may be a recover in disguise. Be very careful when the proposal says that the contractor will remove materials down to a "smooth, workable surface." This may mean that the contractor intends only to spud off the gravel and recover. Find out the exact meaning of these words: Do they mean the contractor will be removing the roof or recovering it? If the contractor is proposing a recover, is a recover board included in the price?

Facility executives should also ask the contractor if there has been a moisture survey done to find out ahead of time how much wet materials will be removed. Also, find out if the contractor is proposing a unit price for removals with the final cost to be determined at a later time. If this is acceptable, determine up front who is to measure the square footage of removed materials to avoid an unpleasant surprise when the change order comes through.

If a facility executive needs to conserve costs on a reroofing project, it's a mistake simply to get three bids and go with the lowest. The result may be a need to reroof again in three years. Thoroughly investigate a roof's particular situation to determine if a recover is right for the roof and then proceed with caution. An architect or engineer that specializes in roof consulting can help assure that the proper steps are taken and that a recover is designed to work on the building at the best price. Caveat emptor.

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Karen Warseck, AIA. CSI, is president of Building Diagnostics® Associates, a Hollywood. Fla., firm that specializes in the analysis of roofing and waterproofing problems.

This article was reprinted from the July 2000 issue of Building Operating Management magazine. For other great articles, log on to Facilities Net at

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