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Replacing a Roof: Hot or Cold System?

By Stephen Varone, AIA and Peter Varsalona, PE

The board of our six-story cooperative recently called in two contractors to look at our roof, which has needed replacing for some time. The first contractor recommend what he called a hot-mopped built-up roof while the second one told us that a cold-applied roofing system was more suitable for our building because we have a wood-frame roof deck. What exactly are hot-mopped and cold-applied roofs, and what are the relative advantages and disadvantages of each? Is there a standard type of warranty that we should expect with a new roof?

hot mopped roof In a built-up roof, successive layers of hot asphalt are applied using a mop.

The first thing to keep in mind when undertaking a roofing replacement program is that a new roof of any type is only as good as the installation. Furthermore, even when a new roofing system is properly installed using the right materials, it must be regularly maintained and protected to ensure its longevity.

Built-up and hot- or cold-applied systems are commonly used methods for replacing roofs. Which type is best depends on the particulars of your building, especially given that it has a wood roof deck.

Depending on the condition of the existing roof, replacement usually begins with removing the old roofing membranes, any underlying insulation, and associated roofing accessories. This exposes the underlying roof deck, typically constructed of wood, concrete, or steel. The roof deck, after any necessary repairs are made on its structural members, is cleaned and primed (if concrete) before the new roofing membranes are laid down.

In a typical roofing configuration, an approximately two- to three-inch thick layer of insulation is secured to the roof deck, underneath the roofing membranes. (One notable exception: In a configuration known as an inverted roof membrane assembly, or IRMA, the insulation is installed on top of the roofing membranes.)

Built-up Roofs

The most traditional and time-tested roofing system, the built-up version gets its name from the successive layers of asphalt and roofing membranes laid on top of each other. The most common type of built-up roof is installed using a hot-mopped method. In this application, a thin layer of hot asphalt is spread over the roof with a mop, followed by rolling down a felt—a bituminous membrane reinforced by glass fiber or polyester—on top of the asphalt.

A typical built-up roof configuration consists of four layers of asphalt and three layers of felt. The asphalt provides the waterproofing capabilities while the felt reinforces the asphalt.

To protect the roof from damaging ultraviolet rays, which cause it to dry, shrink, and eventually crack, the roof surfacing is usually treated in several ways. A common application is to spread gravel onto a layer of asphalt. This aggregate surfacing, as it is known, helps protect the roof not only from the elements but also from punctures and other penetrations.

Another way to protect the top layer is with a mineral-surfaced cap sheet, a membrane with granular bits embedded on the top side that gives the roof a slightly textured surface. A mineral cap sheet's relatively smoother surface, compared to gravel, makes it easier to inspect for things like nails or broken glass, but at the same time it also makes it more vulnerable to punctures from such debris.

A third method of treating the top layer of a built-up roof is to apply an aluminum-based reflective coating. The coating reflects sunlight, thereby keeping the roof from getting too hot and lessening damage from ultraviolet rays. One drawback with this surface is that it needs to be painted every five to seven years to maintain its reflective qualities.

Modified Bitumen Systems

In a second type of roofing system, called modified bitumen, typically two layers of roofing membrane are laid down: the bottom one has a smooth surface while the top one is embedded with a mineral surface, creating an effect similar to the multiple membranes of a built-up roof.

The layers in a modified bitumen system can be applied two ways: In the first method, known as APP (atactic propylene), a torch is used to heat the asphalt pre-applied to the bottom of the felt, adhering the membrane to the layer below. In the SBS (styrene butadiene styrene) method, the mineral cap sheet is usually hot mopped over the smooth-surfaced first layer, or in some cases, torched down as in an APP system.

Cold-applied Systems

The third major type of roofing system also uses fiber-reinforced bituminous membranes. Instead of asphalt, however, a liquid adhesive and a catalyst are mixed in a cold solution and applied to the surface to promote adhesion.

An increasingly popular type of cold-applied system uses self-adhesive roofing membranes. As with the modified bitumen systems, a bottom layer with a smooth surface is used with a mineral cap sheet on top. Instead of hot mopping asphalt or torching the layers down, however, the self-adhesive membranes come with their own pre-applied adhesive elements and are directly applied to the substrate using primer.
In a second kind of cold-applied system, a waterproofing liquid is distributed onto the roof deck and a reinforcing fleece membrane is rolled over it. The fleece absorbs the underlying liquid, adhering to the deck and forming a seamless layer. Additional layers are applied in the same way. Sand can be spread into the top layer to achieve a gritty finish if desired.

Single-ply roofing systems—i.e., those with only one membrane instead of multiple layers—are also sometimes used, especially in new commercial construction. These types of roofs do not offer the durability or consistency of either the built-up, modified bitumen, or cold-applied systems, so they are not recommended for multi-family buildings.

Wood Deck Considerations

One limitation with built-up roof systems comes into play with wood roof decks, such as in your building. Applying hot-mopped asphalt requires heating it in a kettle at approximately 400° F. The New York Fire Department regulations, however, prohibit open flames on any roof with a wood deck. These regulations similarly rule out using the torch-down method for applying membranes.

As an alternative, the asphalt is heated in a kettle on the ground level of the building and pumped up to the roof. This method, however, can be used only for buildings approximately six stories or lower because of the logistics of pumping a piping-hot material alongside a building. The temperature of the asphalt, for example, cannot fluctuate too much from the time it is heated in the kettle to when it reaches the roof. Another problem with placing the kettle on the ground is that asphalt gives off potentially hazardous fumes, inviting fines from the city's Department of Environmental Protection. In such cases, a low-fuming type of asphalt should be used along with additional fume-lowering methods.

For buildings with wood roof decks, which require pumping the asphalt up, or buildings with concrete terrace decks where hoisting a kettle of hot asphalt is impractical (such as on a tall building with a series of set-back roofs or terraces), a cold-applied roofing system may be more feasible.

Whether the adhesive being used in a roof replacement is hot asphalt or one of the cold solutions, the material is sensitive to heat, humidity, and how it is applied. Asphalt that is too hot or cold, or spread too quickly or slowly, will not adhere properly, resulting in gaps and blistering in the membranes. Hiring a contractor with experienced, well-trained workers who carefully follow specifications from the manufacturer and engineer is critical.

Warranties

Whatever type of roof your cooperative decides to install, you should request what is called a "No Dollar Limit" warranty. Standard warranties cover only defects in the roofing materials on a pro-rated basis, diminishing in value as the years pass, and they won't pay for shoddy installation. NDL warranties, on the other hand, will reimburse for the full cost of replacing the roof for both defective materials and substandard workmanship, even if the system fails in the last year of the warranty. NDL warranties typically come in 15- or 20-year terms. Expect to pay approximately $17 to $20 a square foot to replace a standard roof, which includes demolition and disposal of existing roofing materials and asbestos containing debris, installation of new materials, labor, and an NDL warranty. (Independent engineering consulting fees would be separate.)

During the design phase, tests will need to be conducted to determine if any of the roofing materials contain asbestos. The presence of asbestos will require licensed asbestos workers to dispose of the material in an approved landfill facility.

Maintenance and Care

A newly replaced roof will eventually be compromised if not maintained and cared for. The building's staff should regularly inspect the roof for damaging items such as debris and sharp objects (nails, glass, pieces of metal), clogged drains, faulty pitch pans, and defective flashing and counterflashing (metal or other types of membranes used to cover the intersections between the roof and chimneys and parapets). Pay special attention to ponding on the roof, which could indicate that the roof is not pitched at a sufficient enough angle to promote proper drainage. Using a roof for recreational purposes not designed for such activities will also shorten the roof's longevity and weaken its waterproofing (and possibly structural) capabilities.

A roof replacement program is a major capital investment, so a great deal of care and consideration should go into selecting a qualified contractor to perform the work. You would be well advised to engage an independent engineering or architectural firm to prepare construction documents and solicit at least four to six sealed bids from reputable roofing contractors. The specification and bidding process will assure that the board receives competitive pricing for the project and that the proper materials and methods will be used.

Stephen Varone, AIA is president and Peter Varsalona, PE is principal of RAND Engineering & Architecture, DPC. This column was originally published in the July/August 2003 issue of Habitat Magazine.

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