Posted by McElroy Metal ● Jul 14, 2020 10:06:13 AM

How to Retrofit a Flat Roof with Metal

Metal is known for its durability, resistance to weather, low maintenance costs, improved energy efficiency, longevity, and ease of installation. Because of its great benefits, metal is commonly used to when retrofitting a low slope or flat roof.

Before beginning the design of the new metal retrofit system, you must consider the original roofing material that you want to recover. 

Low slope or flat roofs typically fall into one of several categories: built-up style (alternating layers of reinforcing fabric, and asphalt with gravel on top), modified bitumen, or single-ply rubber membrane.  The most common low slope roofing materials being used currently are modified bitumen and single-ply.

Retrofit a Low Slope Roof With Metal

There are two ways to approach using metal to retrofit an existing low slope or flat roof: a compact retrofit system or a frame-up retrofit system. A compact retrofit system is created by attaching a new metal roof directly over the top of the existing roofing system; the existing building slope remains the same and only a minimal amount of additional height or framing is added to the building. 

A frame-up retrofit system is created by installing a new frame on top of the old roof and attaching the new structural members to the structure below to increase the roof slope. In either case, a structural roof system designed for installation over open framing is required, and standing seam roof systems are best for keeping the system watertight.

Standing Seam System for Flat Roof Retrofit

Of the structural panels available, the symmetrical, T shaped, vertical ribbed standing seam like McElroy Metal’s 238T provide the highest wind uplift capacity, the most watertight seam design, and most importantly the ability to remove and replace individual panels anywhere in the field of the roof after installation.

The ability to repair individual panels after installation is very important given that the galvalume coated steel is projected to last 50 to 60 years. Although metal is an extremely durable roofing system, there is a lot that can happen to a roof over that time frame.

When choosing which type of retrofit will be a good fit for your building, the roof slope and gutter system of the existing building should be considered. A compact retrofit system can be used if there is a consistent minimum of ¼” per foot slope built into the structure and external gutters.

In the photo below the new roof is attached directly over the existing roof and attached to the deck or structure below. Like any other re-roof project, wet insulation or rotting, damaged wood would have to be replaced. One of the advantages of doing a compact retrofit system is that you do not have to bring the building up to the current energy code.

The current energy code requires a certain amount of insulation on all new construction and ‘remove and replace’ roofing projects; often, this insulation requirement is more than the existing building has installed and additional insulation is required. This can be quite onerous to install and can become expensive. Recovering a roof is one of the exceptions to the code and therefore is not needed unless the building owner requests it.

How to Retrofit a Flat Roof with Metal

Frame-Up Flat Roof Retrofit

The second type of retrofit, a frame-up retrofit, is often used to increase the slope of the existing roof and is also often used to change the rainwater flow of the original building. It is not uncommon for flat roof buildings to use gutter systems that are internal to the building.

These systems can cause costly damage to the entire building (and its contents) if they fail. A frame-up retrofit can reroute the rainwater flow to get water to the outside of the building, where it is much less likely to do damage if overloaded. 

Frame-up retrofits also offer a variety of aesthetic choices for your building. If you want the benefits of a metal roof without changing the overall appearance of the building, the new roof can be pitched as low as ¼” on 12”.

How to Retrofit a Flat Roof with Metal

You can also increase the pitch or slope of the new roof to eliminate roof leak problems and create a new look for the building or hide unappealing rooftop mechanical equipment.

How to Retrofit a Flat Roof with Metal

Regardless of the retrofit system type you choose, the new roof will require rigid attachment into the building structure. McElroy Metal recommends that a local structural engineer be brought in to evaluate the existing structure for suitability and to determine the best method of attachment of the new roof to the existing structure.

The existing structure can be bar joists, purlins, bulb tees, or a structural concrete deck. The direction, spacing, gauge, and depth of the structure will all determine the method of attachment. Once this has been done, the design process can commence.

With both compact and frame-up systems, when required, new roof panels can be manufactured on-site, allowing a single panel to run from the eave to the ridge or in the factory and shipped to the job site.

For roofing that consists of long panel runs, panel laps will be required to stitch multiple panels together to create one long panel. Panel laps are not only expensive to install but are also typically the weakest link in any roof system; therefore, it is often advantageous to avoid them with on-site roll forming, which creates a single piece panel from eave to ridge and produces the best available retrofit roof system for your building.

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About McElroy Metal

Since 1963, McElroy Metal has served the construction industry with quality products and excellent customer service. The family-owned components manufacturer is headquartered in Bossier City, La., and has 13 manufacturing facilities across the United States. Quality, service and performance have been the cornerstone of McElroy Metal’s business philosophy and have contributed to the success of the company through the years. As a preferred service provider, these values will continue to be at the forefront of McElroy Metal’s model along with a strong focus on the customer.

Topics: Recover and Retrofit, 238T Metal Over Metal, Low Slope to Steep Slope

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