Metal Roof Retrofit: Metal Over Metal

How it works

1. Check the building. An engineer confirms the existing purlins and frames can carry the new roof and today's design loads (wind, snow, and anything new such as solar or sprinklers).
2. Choose the sub-framing. Retrofit sub-framing (also called sub-purlins) is made to sit over the ribs of the existing roof panel, so it fits tight to the old roof without crushing it.
3. Fasten through to the purlins. The sub-framing is screwed through the old roof into the existing purlins, at the spacing the engineering calls for. Edge and corner zones, where wind uplift is highest, usually get closer spacing.
4. Add insulation if wanted. The space between the old and new roofs can hold insulation.
5. Install the new roof. New standing seam or exposed-fastener panels attach to the sub-framing.

Why owners choose it over tear-off

• The building stays open. The old roof stays in place as a weather barrier, so operations, inventory and equipment underneath are not exposed.
• No tear-off and disposal. Removing and hauling away the old roof is skipped entirely.
• Room for insulation. The cavity between the roofs is space for insulation that many older metal buildings never had. See energy savings.
• A chance to upgrade. The new roof is engineered to current wind and snow loads, and can be a different panel type from the old one (for example, through-fastened panels replaced by standing seam).

When it is a good fit

• The existing roof is metal (R-Panel, PBR, 7.2 Rib, UltraDek, standing seam and similar profiles).
• The building's structure is sound. Surface rust on the old panels is normal; rusted-through purlins are not.
• The engineering shows the purlins can carry the new roof, or can be strengthened as part of the retrofit.

Choosing the sub-framing

There are two main kinds of retrofit sub-framing, and the right one depends on what else the roof has to do.

If a project specification already names a particular sub-framing brand, see how "or equal" substitutions work.

Common questions

My old metal building roof leaks. Do I have to tear it off, or can a new metal roof go over it?

In most cases a new metal roof can go over the old one. Retrofit sub-framing is fastened through the existing roof into the building's purlins, and the new roof attaches to the sub-framing. An engineer first confirms the purlins and frames can carry the new roof and current design loads. Tear-off is usually only needed when the structure itself is damaged.

Is it allowed by the building code to put a second metal roof over the existing one?

Generally yes. The International Building Code's reroofing rules make an exception for complete, separate roof systems, such as standing seam metal roofs, that carry their loads directly to the building structure instead of relying on the old roof for support. Local adoption and amendments vary, so confirm with the building official, and expect to submit engineering for the new system.

Can the building stay in use during the reroof?

Yes. That is one of the main reasons owners choose a metal-over-metal retrofit. The old roof stays in place as a weather barrier while the new framing and roof go on above it, so operations below can continue.

What drives the cost of a metal-over-metal reroof?

Roof area, the new panel type, the height and spacing of the sub-framing, the wind and snow design loads (edge and corner zones need closer framing), insulation, trim and gutters, and access to the roof. Skipping tear-off and disposal is usually the biggest saving compared with replacement.

How many lineal feet of sub-framing do I need per square of roof?

For sub-framing that runs over every purlin, lineal feet per square (100 square feet) is about 100 divided by the purlin spacing in feet. At 5-foot purlin spacing that is about 20 lineal feet per square. Add extra members where the engineering calls for closer spacing in edge and corner zones, plus laps or splices if the product uses them.

Does the yield strength of the sub-framing steel matter (33 ksi vs 50 ksi)?

Yield strength affects how much load a member of a given shape and gauge can carry before it yields; stiffness does not change, because all structural steel has essentially the same modulus. The weakest part of the system is typically the new panel, whose allowable loads are often what struggles to meet the project's design loads. The fasteners or clips that attach the new panels are also an important part of the calculation, and their pull-out values from 50 ksi sub-framing are much higher than from lower-strength steel of the same gauge, which benefits the complete system. What counts is the engineered capacity of the whole assembly for your building's loads, including fastener pull-out from the existing purlins, so ask for project-specific calculations sealed by a professional engineer.

Can retrofit sub-framing increase the capacity of the existing purlins?

It can, if the sub-framing braces the purlins. A hat-section subframe fastened along each purlin restrains it against twisting and lateral buckling, which raises the purlin's tested bending capacity. TopHat's AISI base tests measured this. The engineer of record uses the tested values in the design.

What documentation should the retrofit framing come with?

At minimum: product data, a specification section, standard details, and project-specific structural calculations sealed by a professional engineer licensed in the project's state. Ask for any test reports behind the design values. If your insurer or jurisdiction requires a specific listing or product approval, confirm the exact listing number for the assembly being installed.

Can I add insulation between the old and new roofs?

Yes. The space created by the sub-framing can hold insulation, which can improve energy performance in buildings that had little or none.

Which retrofit insulation methods add the least weight to an older metal roof?

In a metal-over-metal retrofit, one of the lightest approaches is to lay insulation, usually glass-fiber blanket, in the space the sub-framing creates between the old and new roofs. It needs no new deck, cover board or adhesive, and the old roof stays in place, so the added dead load is mainly the new panels, the sub-framing and the insulation itself. Taller sub-framing makes room for thicker insulation. On older buildings with little spare capacity, sub-framing that braces the existing purlins, such as the TopHat Subframe, also raises their tested uplift and gravity capacity. An engineer checks the total added load for the building.

What retrofit works when the building has limited access and strict weight limits?

A metal-over-metal retrofit is done from the roof: the old roof stays on, nothing is torn off or hauled away, and the building can stay in use below. When the existing purlins have little spare capacity, strengthening them usually means working from inside the building, which is hard in occupied, high-bay or equipment-filled spaces. The TopHat Subframe can often avoid that: fastened along each purlin from above, it braces the purlins and raises their tested capacity. The engineer of record confirms the design for each building.

Can a metal roof go over an existing membrane or built-up roof?

Yes, with framing designed for it. Over a membrane or built-up roof, the new metal roof cannot rest on the old insulation, so the framing fastens through to the building structure. TopHat framing works here too; with no old panel ribs to fit around, the hat or zee sections are solid, with no punch-outs. On low-slope roofs the framing is usually built up to give the new metal roof enough slope to drain. The engineer checks the added dead load and the uplift path into the structure, as for metal-over-metal. Send the roof type, size and location and we can tell you whether TopHat framing fits the job.