Metal Reroofing Glossary
Retrofit terms, explained plainly
Metal-over-metal retrofit · Retrofit sub-framing (sub-purlin) · Purlin · Notched zee strip · Hat section · Deflection limiter · Intermediate frame · Punch-out · Through-fastened panel · Standing seam roof · R-Panel / PBR · Edge and corner zones · Wind uplift · Dead load and live load · AISI specification · AISI base test · Moment reduction factor (R-factor) · ASTM E1592 · Yield strength (ksi) · Gauge · G60 / G90 galvanizing · Mill certificate · Fastener pull-out test · Basis of design · Or equal · Substitution request · Addendum · Delegated design · CSI MasterFormat section
- Metal-over-metal retrofit
- A reroof in which a new metal roof is installed over an existing metal roof on retrofit sub-framing, without tearing off the old roof. Also called a metal roof recover or overlay.
- Retrofit sub-framing (sub-purlin)
- Light-gauge steel members installed over an existing roof and fastened through it to the existing purlins, to support a new roof. They are shaped or notched to fit over the existing panel ribs.
- Purlin
- A horizontal secondary structural member, usually a cold-formed Z or C shape in metal buildings, that spans between the main frames and supports the roof panels.
- Notched zee strip
- A Z-shaped sub-purlin with notches cut to match the existing panel ribs, so it seats down over the old roof. The TopHat Zee Strip is one example.
- Hat section
- A closed-top, hat-shaped cold-formed steel profile. As retrofit sub-framing, its torsional stiffness lets it brace the purlin it is fastened to. The TopHat Subframe is a hat section.
- Deflection limiter
- A TopHat support member installed on top of the existing roof panel, bridging between the existing purlins. It gives the intermediate frames a strong fastening point in high-uplift areas, which are commonly along the building perimeter.
- Intermediate frame
- An extra sub-framing member, typically attached perpendicular to the deflection limiters, that gives the new panel's clips and fasteners a strong platform in high-uplift areas, commonly along the building perimeter.
- Punch-out
- An opening cut into the flanges of a hat-section sub-framing member so it fits over the ribs of the existing roof panel.
- Through-fastened panel
- A roof panel attached with exposed screws through the panel into the structure, such as R-Panel or PBR. The fasteners also brace the purlin's top flange.
- Standing seam roof
- A metal roof whose panels are joined at raised seams and held down by concealed clips, allowing thermal movement.
- R-Panel / PBR
- A common exposed-fastener metal roof and wall panel with trapezoidal major ribs, widely used on metal buildings.
- Edge and corner zones
- Areas along roof edges and at corners where wind uplift pressures are highest under ASCE 7, usually requiring closer framing and fastener spacing.
- Wind uplift
- Upward suction on a roof caused by wind, which the roof panels, fasteners, framing and connections must resist.
- Dead load and live load
- Dead load is the permanent weight of the roof and anything fixed to it, such as solar panels or sprinkler piping. Live load is temporary load such as workers, equipment and, under building codes, roof live load; snow is a separate load.
- AISI specification
- The American Iron and Steel Institute's North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100), the standard used to design purlins and sub-framing.
- AISI base test
- A laboratory test of a purlin with its roof attached, under uplift or gravity load, used to measure how much of the purlin's full bending strength the system achieves.
- Moment reduction factor (R-factor)
- A factor applied to a purlin's nominal bending strength under uplift to account for limited bracing of its compression flange. A higher R means a stronger purlin system.
- ASTM E1592
- A standard test method for the structural performance of sheet metal roof and siding systems under uniform static air pressure difference, used to measure a panel assembly's uplift capacity.
- Yield strength (ksi)
- The stress at which steel begins to deform permanently, in thousands of pounds per square inch. Common grades for light-gauge framing are 33 ksi and 50 ksi; stiffness is the same for both.
- Gauge
- A traditional measure of sheet steel thickness; lower numbers are thicker. 16 gauge is about 0.06 inch; 14 gauge is about 0.075 inch.
- G60 / G90 galvanizing
- Zinc coating classes for galvanized steel sheet under ASTM A653: G60 has 0.60 ounce of zinc per square foot (both sides combined) and G90 has 0.90 ounce, for more corrosion protection.
- Mill certificate
- A document from the steel mill certifying a coil's chemical and mechanical properties, such as yield strength, and its origin.
- Fastener pull-out test
- A field test that measures how much force it takes to pull a screw out of the existing purlins, used to confirm connection capacity on older buildings.
- Basis of design
- The specific product a specification is written around, setting the standard that any substitute must meet.
- Or equal
- Specification language allowing a product other than the basis of design if the architect or engineer approves it as equal, usually before bid.
- Substitution request
- A formal request, with supporting documentation, asking the architect or engineer to approve a product other than the one specified.
- Addendum
- A change to the bid documents issued to all bidders before bid opening, often used to announce approved substitutions.
- Delegated design
- Design of a building component, such as retrofit framing, assigned by the engineer of record to a specialty engineer, who submits sealed calculations for review.
- CSI MasterFormat section
- A numbered section in a project manual, organized by the Construction Specifications Institute's MasterFormat. Retrofit roof framing is commonly specified in Division 05 (metals), Division 07 (roofing) or Section 13 34 21.
See also the metal-over-metal reroofing guide, or ask us about a term we missed.