Metal roofing in Alexandria, VA: the code text, section by section

Which code book applies to your building

Virginia sets one building code for the whole state, and Alexandria enforces it through its Code Administration office. The City adds the sorting step: its Permit Requirements page puts detached houses, two-family homes and townhouses of up to three stories in a residential category, and lists buildings of three or more dwelling units, apartments and condominiums, as commercial. A roof on one of the first kind answers to the residential code. A roof on one of the second kind answers to the commercial code, which words the same metal rules a little differently.

Not sure which side you are on? The Alexandria roofing guide covers the housing mix. For a condominium, the board's approval comes first.

What the code says about the panel itself

The residential section for metal panels is short, and the commercial section repeats it with small differences. Slopes are on the general metal roof page; this page reads the other parts.

Deck

Panels go on solid or spaced sheathing, except where the covering is designed to go on spaced supports. The commercial text says "solid or closely fitted deck" for the same case.

Metal and thickness

Table R905.10.3(1) names a standard for each metal. Aluminum follows ASTM B209 at a minimum thickness of 0.024 inch for roll-formed panels and 0.019 inch for press-formed shingles. Cold-rolled copper follows ASTM B370 at 16 ounces per square foot. Zinc has a minimum thickness of 0.027 inch. Galvanized steel is listed at ASTM A653, G90.

Coating

Table R905.10.3(2) sets minimum corrosion resistance by coating: aluminum-zinc alloy steel at ASTM A792 AZ 50, aluminum-coated steel at ASTM A463 T2 65, and galvanized steel at G-90. Paint on steel is read under ASTM A755 and goes over one of those coatings.

Underlayment

Section R905.10.5 sends metal panels to R905.1.1. That section lists metal roof panels among the coverings whose underlayment must meet the chapter's standards, with an alternative: a 4-inch self-adhering strip over every deck joint.

Seams, clips and fasteners

The building code never describes a seam. It sorts by slope and by test. The seam vocabulary comes from the Metal Construction Association (MCA), which describes standing-seam systems as vertical panels fastened with hidden clips or fastening flanges, and corrugated or tile-look panels as fastened through the face with exposed fasteners. On low-slope jobs the MCA says some panels need machine seaming: a seaming tool is rolled along the panels to crimp them together.

Attachment is where the code hands over to the manufacturer. Panels are secured as the chapter and the manufacturer's instructions say. The code names fasteners only for the gap case where no instructions exist: galvanized for steel, copper, brass, bronze, copper alloy or 300-series stainless for copper, and stainless as acceptable on any metal roof.

The National Park Service adds two cautions from the historic side. Steel nails in copper sheets are its example of galvanic corrosion, where dissimilar metals touching start to corrode. And it says sheet metals may fail if nailed too rigidly, because thermal movement fatigues the metal at joints.

For wind, the commercial code does name tests. Metal panel systems on a solid or closely fitted deck are tested to FM 4474, UL 580 or UL 1897. A structural panel that serves as both deck and covering is tested to ASTM E1592 or FM 4474 when it is standing seam, and to ASTM E1592, FM 4474 or UL 580 when through-fastened. Ask which listing the quoted product carries.

Metal over an old roof: replacement versus recover

The City's pages do not answer this question. The residential code does, by drawing a line between two words.

• Replacement. The code says it "shall include the removal of existing layers of roof coverings down to the roof deck." That is the default.
• Recover. A new covering over an old one is permitted when it follows the covering manufacturer's approved instructions. The text singles out complete, separate systems, with standing-seam metal as its example, that carry the roof loads directly to the structure and do not rely on the old roof for support. Those need not remove the existing covering.
• No recover. It is not permitted where the old roof is water soaked or deteriorated to the point it cannot serve as a base, where the old covering is slate, clay, cement or asbestos-cement tile, or where the roof already has two or more applications of any covering.
• Wood shingle or shake underneath. Metal panels and shingles may go over it, but where that creates a combustible concealed space the whole surface must first be covered with gypsum board, mineral fiber, glass fiber or another approved material.

Two cautions. The recover language says nothing about the deck, flashings or edges under a new covering, so the scope has to. And flat roofs follow a different route. The MCA describes retrofit systems in which light-gauge steel sub-framing is installed over the existing roof's structural supports and the new framing is engineered to keep the roof system's structural integrity. A roof like that needs an engineer's drawings, not just a panel order. The flat roof page covers membrane alternatives.

What Alexandria's own pages say

Three City documents bear on a metal roof.

For a historic address the Board's sheets then name metal. Its policy says original metal roofing is replaced in the same style: standing seam, flat seam or stamped shingle. A pre-formed, pre-finished standing-seam panel may stand in for a field-installed one only where the seams and the metal pan are the same sizes, and solid copper may replace painted standing seam. The roofing chapter lists standing-seam metal and stamped metal shingle among the materials found across the districts. The district map and thresholds are in the Alexandria guide.

Questions that put the code text into a scope

• Is my building residential or commercial under the City's sorting, and which code chapter does the quote cite?
• Does the scope treat the job as a replacement down to the deck or a recover, and which sentence of the code supports that?
• If a recover: how many layers are on the roof now, and is the old roof sound?
• What is the deck, and is the panel designed for solid sheathing, spaced supports or open framing?
• Which panel metal, coating standard and thickness are quoted, matched against the code tables?
• Which clips and fasteners are named, in which metals, and how do they let the panel move?
• Which wind test listing does the panel carry, and is it the commercial-code list?
• If the address is historic or an association building: has the permit, the Board or the association answered, and in writing?

A phone consultation starts from the address, the building type and what is visible from the ground. The wider materials page and the services page show the other systems. Call (571) 397-6072.