Guide to Low-Slope Roofing
Learn the alphabet soup of materials, but first get the slope and underlying details right.
A good low-slope roof (3-in-12 pitch or lower according to the National Roofing Contractor’s Association, or NRCA)—rarely gets any attention. A bad one brings the kind of attention nobody wants. I see a lot of bad low-slope roofs and don’t know which is more astonishing: how little additional effort would have been required to build it right, or the extraordinary cost of correcting the roof and repairing the associated damage.
Karen L. Warseck, AIA, president of Building Diagnostics Associates, a Florida-based firm that specializes in identifying and fixing building-envelope problems, identifies the causes of roof failures in an Architect magazine article titled, “When It Leaks, It Pours”: “Normally we find that it’s about 60% to 70% construction, 20% to 25% design, and 10% materials.” In short, the roof is the wrong place to try to save money.
Though Warseck’s experience proves that the skill of the workers and the quality of the roof design are more important than the choice of materials, the first question I usually hear is, “What’s the best material?”
Well, the answer is, “It depends.” If there were one best roof for every application, we would have figured it out by now. Even standard asphalt roofing shingles can be used on roofs as low-sloped as 2.5-in-12, as long as the underlayment is installed in a double-coverage configuration. That simply means that there must be two layers of underlayment everywhere. So, instead of lapping a 36-in. underlayment the typical 2 in., the overlaps are 18 in. each.
For lower roof pitches, there are two broad categories of roofing: asphalt based and polymer based. But the best material choice depends on a variety of factors: location, weather, type of construction, intended use of the building, configuration of HVAC and rooftop equipment, anticipated foot traffic, foreseeable potential for damage, and ability to maintain the roof.
Good roofs start with good design
Drainage is the single most important consideration in roof design. According to the article in Architect, “while roofs only make up about 2% of construction costs, water intrusion accounts for more than 70% of construction litigation.” Most of the problems I see relate to inadequate drainage, which can mean inadequate slope, a lack of crickets where needed, and drains that are not properly constructed or maintained.
We often hear low-slope roofs referred to as “flat roofs,” but no roof should be dead level. The closer a roof is to level, the more likely sagging rafters will make a pond. An adequately pitched roof with a sagging middle will still slope enough to drain. Codes prescribe a minimum slope of 1⁄4 in. per ft., while the NRCA calls for “no ponding water on the roof 48 hours after a rain during conditions conducive to drying.” Roofing manufacturers have their own requirements. It’s a good idea to become familiar with all three perspectives and build to the most restrictive.
It’s unusual for the field of a roof with adequate slope to leak. Penetrations, obstructions, terminations, drains, improper flashings—here is where trouble lies. Anywhere the designer has to lift the pencil is a potential leak. To avoid putting the roofer in an impossible situation, the designer and the builder need to provide structural support for all anticipated loads, adequate slope, a suitable surface for attachment (which varies depending on wind conditions), sound parapet configuration, smooth surface conditions at transitions and changes in elevation, adequate ventilation, and proper installation of the sheathing. All openings in the roof should be completed before the roofer shows up. In high-wind areas, I have increased the thickness of the roof sheathing to provide better fastener anchoring. It costs a little more, but I sleep better.
Talk before work starts and as needed during the job. Make sure the framer, plumber, electrician, HVAC contractor, and site superintendent are involved. Clear detail drawings are crucial—you don’t want the lowest-paid roofer on the job reaching for a tube of caulk because he doesn’t know what else to do.
Have the roofer review the drawings prior to construction and comment on curb heights, nailing surface for flashing, distance from walls for penetrations, and configuration and location of pipe penetrations. One of the ways I have been able to get a better product at no additional cost is to ask the roofer in advance what he needs to see when he shows up—blocking, nailers, curbs, crickets, even access details and material storage areas—anything that makes his life easier will make my life easier, too.
Good workmanship needs guidance
I don’t suppose many people show up for work planning to do an inferior job, but there is a lot we can do to help everyone do even better work. “Built to code,” or “That’s how we always do it,” simply isn’t good enough. Clear standards of quality and workmanship should be agreed on before the contract is signed, reviewed before the crew gets on the roof, enforced throughout the job, and proven in inspections upon completion. The manufacturer’s specifications and the NRCA Roofing and Waterproofing Manual should be referenced in your contract. This is how all parties can agree to standards that are impartial, relevant, achievable, and verifiable. The NRCA Roofing Manual provides excellent design details, but at $650 for a non-member, it’s not cheap. Fortunately, the manual is broken into four books, and for low-slope roof details you can buy the Membrane Roofing Systems book for $195—far less money than even the smallest leak will cost to fix.
Editor’s note: If you want to learn how to build a deck on top of a flat roof, read the article Decking Over a Roof
From FineHomebuilding #269
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