Commercial metal
On a metal building, the roof is a system of movement
Metal roofs on commercial buildings fail in two places: at the fasteners, and wherever the designer forgot that a long steel panel changes length between a January night and a July afternoon. Everything else is detail work.
The two systems
| Consideration | Standing seam | Through-fastened |
|---|---|---|
| Fastening | Concealed clips; no penetrations through the water plane in the field | Screws through the panel face into purlins, with a sealing washer |
| Movement | Clips allow panels to move as temperature changes | Movement works against the fastener, which elongates the hole over time |
| Typical use | Roofs where long service life matters, and anything visible | Agricultural and industrial buildings, and re-roofs where budget is the constraint |
| Maintenance | Attention goes to flashings, penetrations and end details | Periodic fastener inspection and replacement across the whole roof |
| Failure mode | Detail failures at ends, transitions and penetrations | Leaking fasteners, often hundreds of them at once as washers age together |
Retrofit over an existing metal roof
A leaking through-fastened roof on an occupied building is the classic candidate for a retrofit: a sub-framing system over the existing panels, insulation in the new cavity, and a new standing seam roof above. The building keeps operating, there is no tear-off exposure, and the result is a better roof than the original.
- The existing structure has to be assessed for the added load and for the condition of the purlins the new framing attaches to.
- It is an opportunity to add insulation that the original building probably did not have.
- Existing rooftop equipment, curbs and penetrations have to be raised and re-detailed, and that is where the real design work is.
- Drainage, gutters and edge conditions change with the new roof plane and need to be part of the design rather than the last thing considered.
What decides performance
Allow for movement everywhere
Clip type, panel length, expansion at the eave and the ridge, and at every transition. A roof that cannot move tears its own fasteners out and it does it slowly enough that nobody connects the cause.
Fabricate the flashings
Ends, transitions, valleys, curbs and wall junctions are fabricated to suit the profile. Off-the-shelf trim forced to fit is the most common source of leaks on otherwise good metal roofs.
Detail every penetration deliberately
Every pipe, conduit and unit support wants a proper boot or a curb, positioned so panels are not cut at their seams.
Get the perimeter right
Edge and eave conditions are where uplift is concentrated, and this market records damaging wind far more often than hail.
On any existing through-fastened roof, look at the fasteners before deciding it needs replacing. A roof full of aging washers is a legitimate maintenance program with a predictable scope, and it can buy years while a capital plan is built.
Metal building roof leaking at the fasteners?
Talk to us about the buildingCan a metal roof be coated instead of replaced?
Coating systems are a legitimate option on a structurally sound metal roof with fastener and seam issues, provided the preparation is done properly and the details are addressed rather than painted over. They are a maintenance strategy with a service life, not a permanent roof.
How disruptive is a retrofit?
Less than a tear-off, which is usually the reason it is chosen. The building stays dry throughout because the existing roof stays in place until the new one is over it.
Is condensation a problem on metal roofs?
It can be, particularly on uninsulated buildings where warm moist air meets a cold panel underside. It is frequently mistaken for a leak. A retrofit that adds insulation and a vapor strategy often solves a "leak" that was condensation all along.
Will you work on someone else's metal building?
Yes. Most commercial metal work is on buildings that were put up by a builder who is long gone, and the useful starting point is a survey of the fastener and flashing condition.
All work is performed by licensed and insured local roofing professionals.