8 Rules for Designing Roof Systems

Our trusty building envelope and moisture control experts provide eight must-follow rules for designing effective roof systems.

February 01, 2008 |

1. Refer to the NRCA Roofing Manual and FM Global Property Loss Prevention Data Sheets.

“A common mistake made when these documents are not followed is that roof-edge fascias end up being inadequately secured,” says Judd Peterson, AIA, president of Judd/Allen Group, Edina, Minn.

2. Install wood cants that are not pressure-treated against decay around parapet perimeters.

“Pressure-treated cants tend to warp after installation, particularly in heat, and pull coping flashings apart,” says Peterson. He recommends that subflashing membranes be lapped over parapet blocking. Also, use neoprene-gasketed, stainless-steel screws to attach sheet metal flashings and counter-flashings.

“For single-ply roofing, cants may be omitted, but at the very least use wood blocking to provide a place to secure the edge of the roofing,” adds Richard Keleher, AIA, CSI, LEED, senior architect at Richard Keleher Architect, Concord, Mass. “A common mistake is to fail to connect the roofing system to the air barrier of the exterior wall.”

3. Elevate perimeters of roofing deck surface with canted and tapered insulations.

“This will help keep water away from curb flashings,” says Peterson.

4. Provide continuous vapor barriers on the decks, under the roofing.

For roofs installed over metal decks on low-humidity structures, such as office buildings, the vapor barrier may be omitted, says Peterson. “This makes it easier to find leaks.”

5. Beware of the following:

• Coal-tar pitch roofs will flow even in the coldest temperatures and must be designed with the correct slope, perimeter felt layer enveloping, and sump scuppers with grates to avoid sloughing down drains.

• Asphalt roofing will release its solvent oils and dry out, particularly where the roof is not sloped enough and ponding occurs.

• Heat-welded or solvent-glued membrane seams can be discontinuous or fail, even if installed properly, particularly in below-grade applications.

• Leaks through loose-laid membranes are nearly impossible to find and repair. Always fully adhere the roofing membranes.

6. Double-check the rain leader piping that runs down through the building for chases and conflicts with the structure and building spaces.

Keleher also suggests insulating the underside of drain pans with one inch of spray polyurethane foam to prevent condensation on the pan, particularly in a climate like New England's. “I also insulate the piping for 10 feet with a vapor barrier jacket on the insulation,” says Keleher.

7. Verify the thickness of insulation and the elevation of the deck height and perimeter through-wall flashing.

Verify tapered insulation requirements and determine whether an additional drain would minimize tapering work. “A common mistake here is to randomly locate roof drains without consideration of the constraints of tapered insulation,” says Peterson.

8. Show detailed drawings requiring base flashings and curb flashings, but only if you know which manufactured system is being used.

“Avoid providing details that call out elements of the roofing system that disagree with manufacturers' details,” says Keleher. “Oftentimes, we don't know which manufacturer is being used.”

However, if the manufacturer is known, providing the base and curb flashing details can help to anticipate the drainage slopes against adjacent perimeter wall materials and to coordinate flashing levels, weeps, and lapping of materials.

“Too often, these are not detailed and not coordinated,” says Peterson. “What happens is, the masonry work goes in first, and then the roofing comes in higher than the flashing level. The correction is ugly, difficult, and time-consuming, and just when the owner expected the roofing to be completed and the building to be kept dry.”

Author Information
Richard Keleher can be reached at 978-369-4550;kel@rkeleher.com. Judd Peterson can be reached at 952-224-5050;jpeterson@juddallen.com.

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