Rooftop Solar on an Older Metal Building
Strengthen the purlins and replace the roof in one pass, from the roof
Older metal buildings are good candidates for solar: big, unshaded roofs over owners who use the power. The problem is the structure. Purlins designed for light roof loads often can't take the added weight of an array. The TopHat Subframe strengthens the existing purlins from the roof and carries a new metal roof for the array to mount on. TopHat has supplied multiple solar projects, including a 200,000 sq. ft. Port of Cleveland warehouse in 2025.
Why solar stalls on metal buildings
A solar array adds dead load to the roof, and it can change how snow drifts and how wind acts on the roof. Building codes generally require existing structural members to be checked when an alteration adds significant load, so the engineer of record has to confirm the purlins, frames and connections can carry it. On many older metal buildings the purlins are the weak link: Z purlins under a through-fastened roof are only credited with part of their full strength, because their flanges are poorly braced against twisting.
The usual fixes all happen inside the building:
- adding purlins between the existing ones,
- bolting reinforcing members to each purlin (sistering), or
- adding bracing along the purlins.
That work goes on overhead, around lights, sprinklers, ducts and racking, often in a building that has to keep operating.
Strengthening the purlins from the roof instead
The TopHat Subframe is a hat-section member fastened along the top of each existing purlin, through the old roof. Its closed shape is stiff in torsion, so it braces the purlin along its whole length and lets the purlin reach more of its strength. The new metal roof is then fastened to the TopHat, and the array mounts on the new roof. How TopHat strengthens purlins explains the mechanics.
| 16 ga purlin | Without TopHat | With TopHat | Increase |
|---|---|---|---|
| Wind uplift capacity factor | 0.382 | 0.920 | +141% |
| Gravity capacity factor | 0.811 | 1.672 | +106% |
TopHat strengthens the purlins; it does not bypass them. Loads still travel through the purlins to the frames, so the engineer of record checks the frames, the purlin connections and the fasteners for the new loads. TopHat Framing Systems can provide project engineering and shop drawings on request.
Example: Port of Cleveland warehouse, 2025
In 2025 the Port of Cleveland renovated a 200,000 sq. ft. warehouse. The solar array the port wanted to install led the engineering team to find that the existing purlins could not carry the added weight. They chose 3-1/2", 14 ga TopHat Subframe to strengthen the existing purlins and meet the design loads, saving both time and money compared with a conventional strengthening approach from inside the building.
Reroof first, then install the array
A solar array is expected to last 25 years or more. If the existing roof is near the end of its life, the array will probably have to come off and go back on when the roof is replaced, which is expensive. Doing the reroof and the purlin strengthening first gives the array a new roof that will last as long as it does, in one project, without tearing off the old roof. TopHat works with standing seam and exposed-fastener panels for the new roof.
What to send us
- Building location, size and roof slope
- Existing purlin size, gauge and spacing, and the frame spacing (bay length)
- Existing roof panel profile (for the punch-out pattern)
- Array layout and weight, and the racking or attachment method
- Any structural report already done on the building
Popular TopHat sections ship from inventory, others typically in about three weeks, with delivered pricing anywhere in the US. TopHat is made in the USA, manufactured in Pennsylvania from domestically produced steel.
Common questions
Can an older metal building roof carry rooftop solar?
Often not without help. Many metal buildings were designed for light roof loads, and their purlins have little spare capacity. A solar array adds dead load, and it can change snow drifting and wind loads on the roof. An engineer has to check the purlins, the frames and the connections for the new loads before the array goes on.
How do you strengthen purlins for solar without working inside the building?
Retrofit sub-framing that braces the purlins can be installed from the roof. The TopHat Subframe is fastened along the top of each existing purlin through the old roof. Its closed hat shape restrains the purlin against twisting, so the purlin carries more load. In AISI base tests on a 16 ga purlin, the capacity factor rose 141% for wind uplift and 106% for gravity. The new roof goes on the TopHat, and the array mounts on the new roof.
Should I reroof before installing solar on a metal building?
If the existing roof is near the end of its life, usually yes. A solar array is expected to last 25 years or more, and removing and reinstalling it later to replace the roof is expensive. Reroofing first with TopHat gives the array a new roof and adds purlin capacity in the same project.
Does the TopHat Subframe carry solar loads straight to the main frames?
No. TopHat strengthens the existing purlins by bracing them; the loads still travel through the purlins to the frames. The engineer of record checks the frames, the purlin connections and the fasteners for the new loads.
Has TopHat been used on solar projects?
Yes. TopHat has supplied multiple solar projects. Send the building size, purlin size and spacing, existing roof panel and the array loads, and we can review whether the TopHat Subframe fits.
Planning solar on a metal building?
Send the building and array details and we will tell you whether the TopHat Subframe fits.
Ask for a reviewAlways have the design checked by a professional engineer licensed in the project's state. See also: testing and approvals.