The built-up asphalt roof waterproof layer is really multiple layers of asphalt and felts (usually 3-5 layers). Built-up roofs are usually covered with small rocks (gravel) if the slope is not steep or the wind is not strong. The main advantage of these roofs is the redundant layers of waterproofing. The disadvantage to the owner is the cost and the use of torches and kettles. The cost is high due to the cost of the asphalt (competing with roads and road repairs). Other factors that drive up the price is that there are fewer roofers that want to install these types of roofs, which are tough on the installers, and fewer roofers have the knowledge to install these kinds of roofs.

A modified bitumen waterproofing layer consists of asphalt derivatives that come in 3′ wide rolls. There is usually a base sheet and a cap (top) sheet that make up the system. The cap sheet comes in different thicknesses. Both the base and cap sheets can be attached to the substrate in different fashions (torched, mechanically fastened or adhered). There are two types of modified bitumen – APP and SBS. The APP type is primarily asphalt. The SBS type is a mixture of rubber and asphalt. The most important difference is that APP is not flexible in normal New England winter temperatures so we do not recommend using this product. The SBS type remains flexible in sub-zero temperatures so is more appropriate for New England. Both the base sheet and the cap sheet are typically twice as thick as single-ply membrane. If a roof is to receive heavy foot traffic or heavy wear then this may be a good choice due to the extra thickness. The downside of the modified roof is that it costs more than single ply, since roll widths are only 3′ wide there are more seams to seal correctly (usually with a torch), and it is more difficult to seal around penetrations with the thicker membrane.

A single-ply membrane performs the waterproofing with a single sheet of material. The material is either rubber-based (EPDM) or plastic-based (TPO or PVC). The material comes in varying thicknesses – .040″ thick to .090″ thick. EPDM is usually black but can come in white (much more expensive). EPDM can come with a fabric mesh embedded in the rubber (called reinforced) or not. EPDM rolls are joined together with seam tape (double-sided tape) within the last 15 years and prior to that adhesive. TPO and PVC are plastic based but chemically different from each other. Both TPO and PVC always have the mesh fabric embedded to provide strength. TPO and PVC usually come in white, gray and tan (sometimes other colors). TPO and PVC seams are heat welded with hot air. The advantage of a single-ply waterproofing layer is cost and flexibility. It also comes in wide sheets which require less seams and is easier to flash around penetrations (less chance of applicator error). The disadvantage is that it can be punctured more easily than modified bitumen or built-up roofing waterproofing layers. If you want a white roof then choose TPO or PVC. If you are comparing prices of EPDM to TPO/PVC, then to compare it properly the EPDM should be reinforced EPDM.

PVC is generally the most expensive, followed by TPO, reinforced EPDM, then non-reinforced EPDM. The thicker the membrane the higher the cost

Between the waterproofing layer and roof deck there are layers of materials that can be installed. Some layers are mandated by the building code, some are mandated by the waterproofing layer manufacturer. Vapor barriers try to keep moisture from inside of the building diffusing and condensating into the roofing layers above the deck. Air barriers attempt to stop any gusts of air from pushing on the roofing system from the bottom and could be tied into the air pressure systems of the building. Insulation prevents hot and cold energy leaving or entering the building. Coverboards are dense materials that are used just under the waterproofing layer to protect the roofing system from traffic or next to the roof deck to protect the roofing system from fire.

The vapor barrier is intended to stop moisture from condensing in the roofing layers. Vapor generally flows from an area of high to low vapor pressure. The vapor pressure depends on the temperature and the amount of relative humidity. Certain recreational and industrial activities result in a high interior relative humidity (pools, fruit storage, processes with lots of open liquids, etc.). If your building generates a high relative humidity then a vapor barrier should be considered upon re-roofing. The best way to prevent vapor transmission through the roofing system (and other building components) is through adequate ventilation. Many times the practicality and cost of doing this ventilation is prohibitive and a roofing vapor barrier is the next choice. If a vapor barrier is installed its effectiveness is lessened as screws that hold down roofing material over the vapor barrier puncture it. Roof penetrations also must be sealed to prevent vapor from escaping around them. If you have high humidity, the roof deck itself should have some protection from underneath. We have seen steel deck rust from underneath and eventually require replacement. For a more technical discourse on vapor control please refer to “The Manual of Low-Slope Roof Systems” by C.W. Griffin and Richard Fricklas.

The air barrier is intended to stop air from moving through the roofing system. It is generally to stop air pressure from inside the building from pushing up on the roofing material. This is particularly important where there are alot of openings in the walls. On single-ply roofs that have the membrane mechanically fastened an air barrier can also help stop air from being pulled from one area and pushed into another. If you are re-roofing a building there should be some experience as to the air flow and what roofing systems are/were on the building. If the building is strictly climate controlled or could result in negative pressure on the roof then an air barrier should be installed. The air barrier should be sealed around all penetrations.

If a fire occurs below the roof deck the most important thing the roofing assembly can do is to not spread the fire. The first line of defense is the material the roof deck is made of. When re-roofing the building it is highly unlikely to consider replacing the entire structural deck component just in case a fire might happen. The roof assembly is a different story. The most fire resistant material should be next to the roof deck. The most common material we see is a gypsum like board from Georgia-Pacific called Dens-Deck. There are other competitors that also perform well in a fire. With the more stringent enforcement of fire prevention systems and practices, and insulation materials tested and fire rated, we see less coverboards immediately above the roof deck specified by architects, consultants or roofing companies.

The basic questions are how much, how many layers, what kind and does it need to be tapered or sloped.

How much – if you are completely tearing off the old roof you have to install insulation whose R value meets or exceeds the state building code. For most buildings this is generally around R=20. The codes vary depending on your location and how much glazing (windows/skylights) are in the walls and roof. If you have special refrigeration requirements then you will need to go much higher. The National Roofing Contractors Association has a program that estimates energy savings as more insulation is added. There are other professionals that will also attempt to quantify savings of additional insulation. If your roof is completely insulated but your windows leak then the cost estimates may be inaccurate. The cost estimates can be used, with a level of skepticism, to generate payback for an increased amount of insulation. Obtain the cost for an additional inch or two of insulation and compare it to the anticipated savings – then decide. If you are going over an existing roof then no insulation may be required. A go over may be used as an opportunity to install more if your energy costs are high. Many times a thin layer of insulation is used to separate and/or provide a clean/known substrate from the existing roofing system to the new waterproofing layer

How many layers – two layers of insulation are better than one. The reason for this is the gaps between the insulation boards leak energy. If two layers are used the layers are staggered so the gaps are not over each other, resulting in better coverage. Two layers of insulation cost more than one layer. One 4″ thick insulation board costs less than 2 – 2″ thick boards. One 4″ thick board costs less in labor to install. It is difficult to determine the cost of any potential energy leaking from any gaps between the boards and the extra installation cost.

What type of insulation – the general answer to this is isocyanurate or polyisocyanurate. This insulation has the lowest cost and is one of the thinnest boards per R value, is closed cell so does not soak up moisture well and performs well in fires. It is the standard in the industry. There are a few other types of insulation that can be less expensive but come with drawbacks. The most common in re-roofing is fiberboard. Fiberboard is less expensive than isocyanurate but, and this is a big but, it soaks up water like a sponge. What typically happens is that a leak occurs in the waterproofing layer and the water dripping in spreads sideways through the fiberboard. This generates a number of bad consequences. First the leak isn’t noticed quickly and then a much larger area of roofing material gets saturated. Once the moisture is in the roofing system it constantly attacks downward to rot the bottom roofing system or deck and in hot weather it turns into a gas and attacks the weakest spots in the waterproofing layer. This will shorten the life of the roof and make it more costly to replace. Fiberboard is typically used when going over an existing roof and we do not recommend it. There are two common types of plastic based insulation that we see once in a while – extruded polystyrene (XEPS) and expanded polystyrene (EPS – also known as beadboard). The extruded polystyrene is more costly per R value than isocyanurate. The expanded polystyrene (same material as the cheap white coolers at the local market) is less expensive than isocyanurate but is only 2/3rds the R value per inch as isocyanurate and performs extremely poorly during a fire. Neither XEPS nor EPS can be placed next to a steel deck due to fire concerns.

Does the insulation need to be sloped – if the roof deck is flat the insulation needs to be sloped or tapered to force the water to drain off the roof. To determine if the roof deck is flat take test cuts at different points where the roof drains (a high and low point) and if the measurements result in a different thickness then the slope is in the insulation, not in the roof deck. If the slope is generated by the insulation and it is being removed then tapered insulation must go back on. Tapered insulation costs much more than flat insulation. A tapered insulation system is designed by the roofing manufacturer and then put together like a 3-dimensional puzzle by the roofing installer. Our experience is that 90% of buildings have the slope in the roof deck, making the roofing much easier (and cheaper). Even if the roof deck is sloped ponding will sometimes occur. Prior to any new roofing work careful attention should be paid to any existing ponding on the roof. Sometimes this ponding will be alleviated by new flat insulation. Most of the time in order to alleviate the ponding tapered insulation or crickets should be installed. If the ponding is in the middle of an area it may be more cost efficient to install an interior roof drain at the low point of the ponding. Ponding typically occurs between roof drains, at the high side of roof top units, skylights or other penetrations, and between scuppers/leader-heads and downspouts. All of these places should have crickets installed between them to facilitate drainage.

If the re-roofing involves removing the existing roofing New England states require a certain amount of insulation to be installed. Generally the insulation can be directly covered with a waterproofing layer. If people and machines regularly utilize the roof it is advisable to further protect the insulation with a coverboard that is a more dense material than the insulation. Material like DensDeck or Firestone’s High Density ISOGARD or even plywood may be installed. This is also advisable if solar panels are being installed. If the existing roofing or some layers of existing roofing are being roofed over and receive high traffic this material may also be used. There is some thought that dense material directly underneath the waterproofing layer will help prevent punctures.

If a new waterproofing layer is being installed on existing roofing it is typical for a manufacturer to require a separation layer between the old roofing and new waterproofing layer. This requirement can depend on the type of existing roofing being covered. Single ply roofing cannot be laid directly on an asphalt and gravel roof. There are a few choices for this separator layer – 1″ isocyanurate, 1/2″ fan board (extruded polystyrene) and fiberboard are the most common. We do not recommend fiberboard (see insulation section). Certain types of single ply membrane (called fleece-back or felt-back) come with a mesh layer already attached to the bottom of the membrane which can serve as the separator layer as well as strengthen the membrane.

Extra traffic on a roof may require extra protection for the waterproofing layer. This usually occurs with either walkpads, walkway rolls or pavers. Many manufacturers require a walkpad outside a hatch and at the service side of a roof top unit. If the waterproofing layer is built-up roofing or modified bitumen roofing this may be all that is necessary. If the waterproofing layer is single ply membrane the extra protection becomes more important. If the roof is to be used as a deck then interlocking pavers should be installed. Concrete pavers on a single ply roof must have an underlayment pad. Carlisle, Versico and other manufacturers make interlocking rubber pavers that may be of interest. There are more paver options entering the market as roof decks become more popular.