Cases When Metal Roof Coating Is Not The Best Choice

Millions of square feet of existing metal roofs are coated each year, but before assuming all metal roofs are suitable for coating you should look a little deeper. From time-to-time, roofing contractors will encounter an existing metal roof coating project only to be challenged as to whether the roof is suitable for a coating or not. Several conditions may exist that would challenge the existing metal roof’s projected service life, weathertightness integrity and required minimum design loading. These conditions could be that the roof has been coated before, the roof has too many penetrations or the dynamics of the roof and its structural integrity may be jeopardized.

Metal roofs are unlike most other roofing types. Here are some cases where a coating may not be the best choice:

Metal roofing is a STRUCTURAL COMPONENT of the building and its design. It is common to see some corrosion on older metal roofs but when it is severe we need to look more closely. The panel end laps, curbs, equipment penetrations, ridge and eave areas should be looked at carefully inside the building as well as outside on the roof.

Leaky fasteners can result in large corroded weak areas around the fasteners. The fasteners not only hold the roof down, but the roof also provides critical diaphragm strength to the entire structure. A fastener cap will not keep the roof on the building or transfer loads to the structure.

Leaky panel laps at the ridge, eave or improperly installed penetrations can cause the structural purlins and eave struts to be reduced to something resembling Swiss cheese. If you see this, the building is structurally compromised, coating it may hide the problem but it may also put you in the liability chain should a structural failure occur. Legally, we are roofing professionals and should recognize when to seek additional expert advice.

Thermal Movement:
Metal roofs are subjected to significant thermal expansion and contraction during everyday temperature changes (+/- 1” per 100’). Especially vulnerable are those roofs that have exposed structural fasteners (thru-fastened roofs). Roofs where the panel runs (from eave to ridge) exceed 80 lineal feet are especially at risk. Standing seam roofs that have thermally active clips are designed to allow movement and typically do not suffer from this same problem. If you observe bending or cracking of long run panels, slotting of fastener holes, torn apart end laps or other indicators excessive movement; coating will only hide the problem and it will reoccur.

Roof mounted equipment and pipe penetrations are a constant source of problems in metal roofs, not only because they may have been installed incorrectly but because of differential movement between the roof and the curb mounted equipment. Coating may help for a while, but the forces are huge and frequently need further attention.

Eliminate roof mounted items whenever possible and never block the flow of water upslope from any curbs.

Additional Weight:
Metal Roofs are sensitive to additional weight. Look for indications of sagging of the purlins between frames or significant dimpling around fastener heads. It is common to find existing buildings have had suspended or mounted service equipment added since its initial construction. This equipment could include fire sprinkler systems, HVAC equipment and plumbing, lighting, ceilings and insulation. Add up the weight of the suspended materials and equipment and that 20 PSF live load could easily be 10 PSF. The result is simply, the building is no longer compliant with the building code governing the life safety of its inhabitants.