October 28, 2015


by Jason Allen

Reroofing w Steep Slope

You’ve probably seen many articles and photos of buildings with low-slope roofs converted to steep-slope metal panel roofs.  Aesthetics and leakage are driving these conversions, but mostly leakage of the low-slope roof.  Owners are frustrated with leaks and believe a steep-slope metal roof is their answer, and it certainly can be!  There are a number of issues to be resolved and decisions to be made to ensure long-term performance of the new metal roof.

Retrofitting a roofing system

A new structure, such as MBCI’s retrofit systems, should be attached directly to the existing structural members.  Removal of small sections of the existing roof is required for direct attachment.  Fastening through the existing roof means attaching through a soft substrate, allowing for compression over time and movement of the fasteners, and their eventual loosening.  If possible, installing new flashing at the attachment points (with spray foam?) provides a second water barrier.

Steps to take after retrofit installation

Conversion to a steep-slope metal roof creates an attic space that now needs to be ventilated to remove heat and moisture, but also to provide adequate airflow for HVAC intake requirements.  Determine intake air requirements—how many air changes per hour are required in the attic to satisfy fresh-air intake requirements?  Significantly more than code-required ventilation amounts may be needed if many HVAC units are enclosed.  Inadequate ventilation could result in an extremely hot and humid attic space and overworked mechanical intakes.  Poor ventilation could increase the need to cool a building, especially single-story buildings (e.g., schools) converted from low-slope to steep-slope roofs.  Conversely, the vent stacks are also enclosed.  Perhaps vent stacks need to be vented through the roof?

To prevent condensation issues in the newly created attic space, it’s prudent to install mechanical ventilation units that are controlled by a humidistat, not a thermostat.  Maximum humidity levels can be determined for summer and winter seasons, and the humidistat can be set to remove humidity and air when the maximum relative humidity levels are reached.

Drainage at roof edges must be well thought out.  If the new roof includes overhangs, gutters and downspouts and eave vents can be installed.  If there are parapets, creating an “internal gutter” where the new metal roof meets the parapet can be difficult to detail properly for a long-term, low-maintenance solution.  Overhangs and eaves, even where parapets exist, are the best solution.

This is blog one of two.  More on this topic next time!

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