Stronger Purlins, Not Just a New Roof
How the TopHat Subframe raises the capacity of the existing structure
Most retrofit sub-framing only carries the new roof. The TopHat Subframe also braces the existing purlins it is fastened to, and independent AISI base tests measured the result: higher uplift and gravity capacity from the same purlins. That matters when the reroof also has to carry something new, such as solar panels, fire sprinklers, rooftop equipment, or higher code loads.
What limits an existing purlin
The Z-shaped purlins in most metal buildings rarely reach their full material strength. Under wind uplift the bottom flange is in compression and has little bracing, so the purlin tends to twist and buckle sideways before the steel yields. Design codes account for this with reduction factors: under the AISI specification, a purlin supporting a through-fastened roof is credited with only part of its full bending strength.
What the TopHat Subframe changes
A TopHat member runs along the top of each purlin and is screwed to it through the old roof at close spacing. The closed hat shape is stiff in torsion, so it restrains the purlin against twisting and lateral movement along its whole length. A better-braced purlin reaches more of its strength before it buckles.
Engineers sometimes call this "composite purlin" action. Strictly speaking, the gain comes from bracing, and it is measured directly by testing the purlin and subframe together, as described below.
What the tests measured
Force Engineering & Testing ran AISI base tests (uplift and gravity) in 2008 and 2009 on 8" × 2.5" Z purlins spanning a 25-foot bay, with and without a 3-1/2" 16 ga TopHat Subframe under a new 26 ga panel. The result is a capacity factor for the purlin; a higher factor means the purlin carries more load.
| 16 ga purlin | Without TopHat | With TopHat | Increase |
|---|---|---|---|
| Wind uplift | 0.382 | 0.920 | +141% |
| Gravity | 0.811 | 1.672 | +106% |
Full reports, including the 12 ga purlin gravity result and the 14 ga TopHat uplift test, are on the test results page and in the document library.
How an engineer uses the results
- The tested factors replace the default reduction factors in the purlin check for configurations the tests cover.
- For other purlin sizes, spans or panel types, the engineer applies the results with judgment or calls for project-specific analysis. TopHat Framing Systems can provide project engineering on request.
- Connections still need checking: fastener pull-out from the existing purlins and the frames' capacity for any new load.
When this matters most
On a plain reroof with no added load, a simpler notched Zee Strip is usually the economical choice. When the reroof also has to carry solar arrays, sprinkler piping, rooftop units, or higher wind and snow design loads, the TopHat Subframe can often provide the new roof and the extra purlin capacity in one pass from the roof, instead of strengthening the purlins from inside the building. TopHat has supplied multiple solar projects. See solar on older metal buildings.
It also suits buildings with limited interior access or strict weight limits: occupied plants, warehouses full of racking, high-bay spaces. All the work happens on the roof, the old roof stays on as a weather barrier, and the added weight is mainly the new panels and the light-gauge sub-framing.
Have a building with overstressed purlins?
Send the building size, purlin size and spacing, existing panel profile, and the new loads.
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