In July 2003, Professional Roofing asked various NRCA technical committee members to choose a roof system for a fictional building and explain their choices. The article generated significant interest, and Professional Roofing once again has asked NRCA technical committee members to approach a fictional building.
This year, the characteristics of the building are as follows:
Contractors were asked to explain their choices in the context of roof system performance attributes and the contractors' local geographic conditions, not necessarily lowest cost.
The following contractors contributed to this article: Alex Hernandez, vice president of Clark Roofing Co., Broadview, Ill.; Allen Lancaster, president of MetalCrafts Inc., Savannah, Ga.; Jack Moore, project engineer for West Roofing Systems, La Grange, Ohio; and Lisa Sprick, vice president of Sprick Roofing Co. Inc., Corvallis, Ore.
Alex Hernandez
Generally speaking, I recommend keeping the type of roof system as consistent as possible across all sections of a building. A system that is abuse-resistant and easy to repair and modify would be advantageous to the school. Most roofing companies will be capable of making appropriate repairs or modifications to the roof systems I describe using readily available materials. The fact that ballast or pavers will not cover the waterproofing membrane means any problems can be identified and repaired quickly and easily. Finally, foot traffic and rooftop equipment maintenance is common on school roofs in our area, and for these reasons, a durable, redundant roof system is needed.
I assume slope will be incorporated into the roof decks and appropriate placement of expansion joints will be executed. NRCA, FM Global and building code requirements would be considered with regard to attachment, fire resistance and good roofing practice.
Over the low-slope steel deck of the classrooms and natatorium, I would mechanically fasten 1/2-inch- (13-mm-) thick Dens Deck on which I would mop with hot asphalt a two-ply Type IV vapor retarder. This would allow other trades to penetrate the roof with mechanical equipment before installation of the final roof system. Over the vapor retarder, I would mop two layers of 1 1/2-inch- (38-mm-) thick polyisocyanurate insulation with the seams between the layers staggered. I then would mop in a 3/4-inch- (19-mm-) thick perlite board on which three plies of Type IV fiberglass felt, mopped in hot asphalt, would be installed. The system would be capped with a dual-reinforced, granular-surfaced, fire-rated modified bitumen cap sheet. The flashing system would consist of two base plies of Type IV fiberglass felt and one ply of the same modified bitumen cap sheet.
The field house would receive a fiberglass-reinforced base ply mechanically fastened to the low-slope cementitious wood fiber deck. I then would mop two plies of Type IV fiberglass felt as a vapor retarder. If I assume a 2-inch- (51-mm-) thick cementitious wood fiber panel, the R-value for that panel would be about 3.5; the addition of 2 1/5-inch- (56-mm-) thick polyisocyanurate insulation over the vapor retarder would help avoid condensation within the cementitious wood fiber deck. Over the insulation, I would install much the same roof system as the one installed over the classrooms and natatorium.
With regard to the auditorium and shop classrooms, I assume required R-value will be incorporated into the low-slope lightweight insulating concrete. I would mechanically fasten a vented base ply to the deck. The vented base ply would run up the walls and projections and continue over any edges. Then, I would mop three plies of Type IV fiberglass felt followed by a dual-reinforced, granular-surfaced, fire-rated modified bitumen cap sheet. Again, I would follow the same construction as previously stated.
The steep-slope wood deck over the office and administration section would receive a waterproofing underlayment from the gutter edge up the slope. To determine how far up the slope to run the underlayment, I would consider several factors, including building code requirements, slope and rafter length. Generally, I like to see 6 feet (1.8 m) of waterproofing over the heated space of the roof. The remainder of the roof would have two layers of No. 30 roofing felt and synthetic slate.
Over the cafeteria roof's steel deck, I would mechanically fasten a composite insulation with an oriented strand board nailing surface followed by a self-adhering, high-temperature waterproofing underlayment. I then would install a copper standing-seam metal roof system.
It virtually is guaranteed, in the current value-engineered world, that the roof systems I describe would not be "the low bid" systems. As a contractor interested in long-term performance and low life-cycle cost, I suggest the described roof systems as an extraordinary value.
Allen Lancaster
The first area noted is a low-slope steel roof deck over the classroom areas and natatorium. I would recommend a layer of polyisocyanurate insulation be mechanically attached to the metal deck. Thickness would be determined by the thermal value required for the roof construction. Next, I would recommend a layer of 1/2-inch- (13-mm-) thick perlite be installed in hot asphalt over the polyisocyanurate insulation. Because no ballast or gravel is allowed, I would recommend a hybrid modified bitumen roof system consisting of two plies of fiberglass felt (Type IV or Type VI) and a layer of 250-g, polyester-reinforced, granular-surfaced, fire-rated modified bitumen cap sheet installed in hot asphalt. If there was not the stipulation of no ballast, I would recommend the use of four plies of fiberglass felt (Type IV or Type VI) with a gravel surface installed in hot asphalt. In my experience, this is the most durable, long-lasting roof system on the market when conditions allow a gravel-surfaced built-up roof (BUR) system to be installed.
The second area of roof over the field house has a cementitious wood fiber roof deck. This roof deck has some sound-absorption properties and insulating value. Therefore, the insulation thickness may be less than the insulation used over the metal deck. Again, this would have to be determined based on the total thermal value required for the roof system construction. I would recommend a slip sheet or rosin paper be installed over the roof deck and layer of fiberglass base sheet mechanically attached to the cementitious wood fiber roof deck over the slip sheet. A layer of polyisocyanurate insulation and layer of 1/2-inch- (13-mm-) thick perlite would be installed in hot asphalt over the mechanically attached fiberglass base sheet. The slip sheet or rosin paper would help prevent any asphalt migration through the base sheet and into the roof deck or building. The roof system I would recommend is the same hybrid modified bitumen roof system noted over the steel roof deck.
The third area of roof over the auditorium and shop classrooms has a lightweight insulating concrete roof deck; this type of roof deck frequently is used in my area of the country. It can provide insulating value to a system, provide slope and remain in place through several reroofing generations because the lightweight concrete deck does not have to be replaced each time the roof is replaced. I would recommend a venting-type fiberglass base sheet be installed over the roof deck and secured with mechanical fasteners. No roof insulation should be required because the thermal insulation would be installed in the lightweight concrete deck. Therefore, I would recommend the same hybrid modified bitumen roof system be installed, consisting of two plies of fiberglass felt and a layer of 250-g, polyester-reinforced, granular-surfaced, fire-rated modified bitumen cap sheet installed in hot asphalt.
The next area of roof over the office and administration areas has a wood deck and is a steep-slope construction. The building construction needs to be reviewed for thermal insulation requirements and fire-protection needs because this is a combustible roof deck. I am assuming no thermal roof insulation is required at the roof level and this building will meet fire code requirements based on interior fireproofing, such as a sprinkler system or other fireproofing methods. Therefore, I would recommend a 24-gauge Galvalume™ standing-seam metal roof system with a Kynar® 500 paint finish be installed over a slip sheet or rosin paper over a layer of self-adhering membrane underlayment installed directly to the wood roof deck. I would recommend a standing-seam metal roof system that does not require exposed fasteners to secure the roof system. Standing-seam panels should not be wider than 18 inches (457 mm) to help minimize oil canning.
The last area of roof is located over the cafeteria. I would recommend a layer of polyisocyanurate insulation be mechanically attached to the metal roof deck. A layer of self-adhering membrane underlayment would be adhered to the insulation with a slip sheet or rosin paper placed over the underlayment and under the metal roof panel. The roof system would comprise a metal roof system formed from 24-gauge Galvalume standing-seam metal with a Kynar 500 paint finish installed over the slip sheet. The metal roof panels would be secured by use of concealed clips set 12 inches by 12 inches (305 mm by 305 mm) by 20-gauge galvanized bearing plates and secured with fasteners long enough to attach to the steel roof deck. Flashing details will have no exposed fasteners and panel seams at a maximum of 18 inches (457 mm) on center.
Jack Moore
I would recommend the following with regard to the fictional high school's roofing needs:
For the field house and auditorium, I would apply a primer approved by a spray polyurethane foam (SPF) manufacturer to the cementitious wood fiber deck. I would install test areas over the lightweight insulating concrete to ensure proper adhesion. If the results are unsatisfactory, I would install a hot-mopped base sheet before installing the SPF. Next, I would robotically install a tapered SPF layer with a minimum of 3 inches (75 mm) of insulation or as specified by local building codes. The SPF should be coated with a minimum of 22 mils (0.56 mm) of silicone elastomer in two coats with ceramic granules embedded into the top coat at a rate of 40 pounds per square.
For the main classrooms and natatorium, I would secure 5/8-of-an-inch- (16-mm-) thick fire-rated siliconized gypsum board to the steel deck using polyurethane insulating adhesive. Next, I would install a minimum of 3 inches (75 mm) of SPF coated with a minimum of 22 mils (0.56 mm) of silicone applied in two coats with ceramic granules embedded into the top coat at a rate of 40 pounds per square.
For the roof system above the offices and administration areas, I would ensure adequate insulation has been installed below the roof deck. Then, I would nail a No. 30 felt base sheet, ice-dam protection membrane on the perimeter and valleys, and high-quality dimensional shingles that complement the balance of the structure.
For the cafeteria's roof system, I would mechanically fasten 5/8-of-an-inch- (16-mm-) thick fire-rated siliconized gypsum board and install a minimum of 1 1/2 inches (38 mm) or more of SPF to achieve the specified R-value. Then, I would install a base coat of 10 mils to 12 mils (0.25 mm to 0.3 mm) of silicone elastomer and a custom color top coat of 10 mils to 12 mils (0.25 mm to 0.3 mm) of silicone elastomer with matching granules installed at a rate of 40 pounds per square. The top coat color will complement the balance of the structure. Using SPF will provide seamless and monolithic flashings at the vaulted skylights and ensure long-term, renewable flashing performance. Crickets will be sprayed upslope of the skylights to prevent any ponding water.
Lisa Sprick
Considering we are a 52-year-old company that completes most of its work in the cool, moist climate of the Pacific Northwest, following are my thoughts given the building parameters and what has worked best for us. My comments also are based on the assumption that the building meets or surpasses all building code requirements and is designed to withstand necessary roof loads. Cover boards are assumed to be tapered as needed to achieve a minimum drainage of 1/4-in-12 slope (1.2 degrees). The insulation thickness is based on a 2-inch (51-mm) minimum with the understanding that it be increased as needed to meet required R-values.
On the low-slope steel deck sections, I would mechanically fasten 2-inch- (51-mm-) thick polyisocyanurate insulation, then mop on a 1/2-inch- (13-mm-) cover board, offsetting the cover board and insulation seams accordingly.
Over the lightweight insulating concrete deck, I would mechanically fasten a vented base sheet, mop on the insulation and proceed with the cover board. Depending on any interior roof deck ventilation, additional perimeter and penetration venting would be installed according to the deck manufacturer.
For the cementitious wood fiber deck, I would mechanically fasten a base sheet and mop on the insulation with the cover board. Over this and the lightweight insulating concrete deck, evenly distributing the load weight during installation becomes imperative as structural damage to these deck types can be caused if they are not properly supported.
Over all low-slope sections, I would install a BUR system consisting of a base sheet, three plies of Type VI fiberglass felt (or four plies, if budgeted) with embedded gravel overlay. I'd choose gravel (rather than a cap sheet) only if the roof is visible from the ground for a "softer" look. Otherwise, a cap sheet would be preferred as a lower-maintenance option. Thermoplastic or thermoset single-ply systems also are viable options; however, because asphalt fumes during installation would not be an issue and many of our BUR systems last more than 30 years, I tend to lean first toward the tried and true.
For the steep-slope wood deck sections, we'd nail a No. 30 fiberglass underlayment and laminated asphalt composition shingles with a minimum 40-year warranty. Or, preferably, (for longevity's sake) install a metal roof of standing seams or horizontal panels. Insulation over these sections is not being addressed, so I assume it to be elsewhere other than the roof deck. Over the steel steep-slope sections, I would mechanically fasten a nail-base insulation (thickness to coincide with required R-value), nail on No. 30 fiberglass underlayment, and proceed with the installation as per manufacturer specifications.
What do you think?
The roofing options offered in this article are only some ways roofing contractors may approach the fictional roof presented. If you have another approach you would like to share, e-mail it to professionalroofing@professionalroofing.net or mail it to Professional Roofing, 10255 W. Higgins Road, Suite 600, Rosemont, IL 60018. Readers' responses will be posted on www.professionalroofing.net.
Ambika Puniani is editor of Professional Roofing and NRCA's director of communications.
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