If you are at this point you have hopefully read through our recommendations on how to check out your existing roof and prepare it to receive a new roof. Choosing what type of roofing system involves a lot of factors – existing substrate, functional requirements, warranty requirements, cost sensitivity and building codes. A 10+ year system would involve installing a new comprehensive waterproofing layer, possibly with additional insulation or substrate separation layer at a low cost. The difference between a 10-15 year warrantied system and a 20-30 year warrantied system is the 20-30 year usually has a thicker waterproofing layer, may have added redundancy at key future failure points like flashings or the roof edge, may require it be the only roofing system installed and the warranty costs more per square foot. 90% of our customers are happy with the 10-15 year warrantied system.
- Is it important to reduce heating and cooling costs? It may not be if you have a warehouse or shop type environment. If it is important then how much will more insulation reduce the heating and cooling bills? There are different ways to figure this out – use EnergyWise from the National Roofing Contractors Organization or hire a professional to check the roof and/or the whole building. The best advice you can buy will be just an estimate. There are too many variables for someone or some computer program to say exactly what the savings for increased rooftop insulation are. The roof isn’t the only place that can lose hot or cold air. If you are serious about saving the energy bring in an infra-red camera and check the entire structure as well as any machinery in place. The building codes for New England for new roofs generally require an R value between 25 and 30, depending on how much window glazing there is in the walls and the location of the building. Isocyanurate insulation is the most cost effective insulation and it has an R value of 5.7 per inch. If the R value in your roof is close to the building code requirements it could be years before the return on investment from increased insulation is seen. Thicker insulation costs more in insulation material, thicker wood blocking at the perimeter of the roof to match the thicker insulation height and longer fasteners to secure the insulation to the roof deck. Check what you have existing on the roof and ask your roofer for a separate cost for thicker insulation.
- Does the roof or area receive lots of foot traffic – choose strong insulation/underlayment materials and a tough waterproofing layer with redundant layers. Install walkpads, walkway rolls or pavers as well.
- Are strong winds possible? This could occur from being near an ocean, lake or large empty field. The building height and the presence of a parapet wall (and how high the parapet is) around the perimeter of the building also can generate more uplift on the roofing system. If there is more wind make sure the manufacturers warranty specifies a greater wind speed so the design of the roofing system is appropriate.
- Will the roof deck structure be able to support the new roofing system plus any snow load – if there is any question a structural engineer must be brought in.
- Is there a plan to cover the roof with solar panels – again the structural load must be checked and the roofing system should be one that lasts a longer time. The solar panels may require a white roof to either cool down the panels for efficiency or actually utilize the reflected sunlight to convert to electricity.
- Have you considered adding skylights – doing it during re-roofing saves on installation costs. Skylights can allow most lights to be turned off in the daytime. There are new types worth checking into (prismatic lenses, solar tubes).
- Is the existing drainage adequate – if not add more downspouts, scuppers, interior roof drains, crickets (raised areas) to force water away from ponding areas. Tapered insulation may be needed on the entire area to generate correct drainage.
- Are there areas in the walls that open up to create negative pressure on the roof (truck bay doors) – install an air barrier and extra insulation/waterproofing attachments in those areas. Make sure all roofing material will be secured to the roof deck appropriately.
- Is a vapor barrier required – is the area a pool, sauna, hothouse, uses lots of moisture, etc. – check with a professional.
- Are grease or oil products exhausted onto the roof – use sandboxes around the fans and special protection for single-ply membranes (PVC is much better than EPDM or TPO).
- Is your building insured by FM Global – they will need to be involved in the re-roofing process.
- Is the building a freezer or cooler – in addition to extra insulation certain roofing details need to happen at the roof edges and around all penetrations – make sure the roofer knows what they are doing or money will be wasted through continual energy loss.
- Are there special color requirements for the perimeter metal that shows from the ground – Toyota dealers require all metal to come from one place – special orders – long lead times.
- Are there areas where snow/ice can slide off the roof and cause injuries – discuss snow guards/rails. This is especially important on roofs with a pitch greater than 1″ in 12″ like metal roofs covered with single-ply membrane or coatings that offers less resistance to sliding snow.
Waterproofing layer - What are the choices?
The 3 main choices for a waterproofing layer in New England are asphalt built-up, single-ply membrane and modified bitumen. Will a coating last 10+ years – maybe, but since we haven’t seen one do that we are not speaking to that option. We also haven’t seen a spray foam roof last 10+ years in New England – so we aren’t speaking to that option either.
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.
Modified Bitumen roofing
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.
Single-Ply Roofing at a High Level:
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.
In our experience the pricing hierarchy starts with white EPDM or PVC at the top. Reinforced EPDM, non-reinforced EPDM, then TPO. This can vary. Some depending on the size of the area and number of projections to flash. EPDM requires seam tape which is an extra cost PVC and TPO do not have. It is more labor intensive to flash projections with PVC and TPO. The thicker the membrane the higher the cost but the more resistant to damage.
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.
Air barriers are advisable in white roofs as the air flow within a roof assembly can help generate condensation. On black roofs that get hotter any condensation is more likely to evaporate.
Extra layers of 1/4″ to 5/8″ boards are generally used for either protecting a roof from fire spread below or above the roof system, or to enhance the toughness of the roofing system.
Generally some type of gypsum board like DensDeck from Georgia-Pacific is used on the bottom of a roofing assembly immediately above the deck to prevent the spread of fire from below and the more combustible the deck (wood) the better it is. This type of coverboard is also sometimes used immediately above the deck to adhere a vapor barrier to (typically on a steel deck).
Coverboards can also be used immediately below the waterproofing membrane. The boards are typically more dense than any insulation. There is some evidence that the denser the coverboard the fewer holes in the membrane will occur. A coverboard can also provide a uniform substrate for the membrane to lay on and act as a separator between the membrane and an incompatible substrate (like EPDM and asphalt).
Cover boards also help prevent crushed insulation from heavy foot traffic or machinery.
The most popular coverboards are fiberboard (which we do not advise using as they absorb water), gypsum based (very dense and fire resistant), and high density isocyanurate (medium density, light weight).
The proper use of coverboards can enhance your roof assembly.
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=25 in RI and 30 in MA. 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. An overlay 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. If the “insulation is exposed” in a re-roof in New England then typically new insulation to meet the building codes R value must be installed.
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.
Coverboards/Separators Directly Under Waterproofing Layer:
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 high density isocyanurate 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″ high density 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.
Coverage over the Waterproof Layer:
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 roofng 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.
There are a number of issues to consider.
- Will the paver stay on the roof? Even 2″ x 2″ x 2″ concrete pavers may require interlocking in a high wind zone. Other enhancements may be required at the edge if there is no wall or parapet to prevent the wind getting under the perimeter.
- Is there space for the water to drain?
- How much maintenance is required to keep the paver looking good?
- Will the structure support the weight of the deck?
- Are slip sheets included to provide extra protection to the water proofing layers?
- Is there insulation or a cover board that is 80 psi or higher to prevent the weight of the pavers crushing the insulation?