Keeping low-slope roofs dry in northern climates

Figure 10: Blown-in insulation. Photos courtesy Inspec.   Photos © Dwight Benoy
Figure 10: Blown-in insulation.
Photos courtesy Inspec. Photos © Dwight Benoy


The contractor elected to work on the perimeter prior to conducting replacement work in the field of the roof (Figure 6). The former proved to be time-consuming and would have reduced the size of the area available for reroofing on a daily basis if the perimeter work was done in conjunction with the field of the roof. The contractor could also schedule the perimeter work on the days when unfavorable weather was forecasted, as the area could be enclosed rapidly should precipitation be imminent.

The contractor fabricated a Z-shaped transition metal instead of a U-shaped one. This served the same purpose as a vapor/air barrier transition material (Figure 7). However, there were areas of the previous mold remediation where additional framing done as part of that work interfered with the installation of the Z-shaped metal. Therefore, a two-piece, U-shaped metal was installed with the connection between the pieces accomplished with aluminum tape (Figure 8).

A short width of vapor/air barrier was then installed, followed by a parapet reinforcing assembly of plywood and sheet metal angle, and, lastly, the field of the roof vapor/air barrier (Figure 9).

Figure 11: Mold remediation paint.
Figure 11: Mold remediation paint.

The contractor had local insulation and plumbing subcontractors on call to complete varying amounts of work, depending on what was uncovered and anticipated each day. Perimeter work required the presence of the insulation subcontractor to vacuum insulation and install spray foam (in the rim area) and new blown-in insulation (Figure 10).

Mold remediation was handled by the contractor, alleviating the need for a specialty contractor. This eliminated coordination and delay issues. The contractor cleaned any discolored areas that were within the limits for moisture content, and then painted them with an anti-microbial paint (Figure 11). Most of the parapet that was left in place was remediated when the perimeter work was being constructed, which proved to be the most efficient.

The estimated amount of existing roof deck sheeting removal, based on the 60 invasive inspections openings, was 743 m2 (8000 sf). The actual amount of existing roof deck sheeting removal was 557 m2 (6000 sf).

While conducting the invasive inspection openings, and subsequently during the reroofing work, it was observed the TPO roof membrane plates were severely corroded in much of the roof area. This reduced the wind-uplift resistance of the roof membrane. The contractor was conscious of the need to respond quickly should a high wind event occur. Fortunately, the reroofing work was completed without incident.

Figure 12: Perimeter safety rails.
Figure 12: Perimeter safety rails.

The contractor removed tear-off debris from the site daily. The debris was lowered by crane into dump trucks. New materials were hoisted daily with only a one- to two-day stockpile on the roof. The crane and roofing materials were staged on the streets running adjacent to the building, but only at certain locations, thereby resulted in long travel distances across the existing roof in some areas. The City of St. Cloud, Minnesota, allowed the streets to be temporarily closed. Access to the retail establishments and egress from nearby buildings was continuously maintained, but was an ongoing public safety challenge.

Perimeter safety was accomplished by attaching rails to the parapet (Figure 12). A safety monitor was also assigned to work with the crew applying the low-rise foam adhesive for the insulation attachment.

The fully adhered EPDM membrane over the tapered insulation system provided a fully draining roof with a finished appearance. Even with all of the construction challenges, the roof was completed in a timely manner.


The owner, contractor, and A/E worked together to achieve the goal of taking a sick building and making it well. All parties understood from the start shortcuts could not be taken. As with most projects, some surprises were encountered, but these were quickly resolved with input from all parties. Cost efficiencies were considered and implemented only if they did not compromise the design intent. The project was completed with minimal disruption to the operation of the building and its occupants.

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