How Much Does an Aluminum Roof Weigh, and Does Oregon Framing Care?

How much does an aluminum roof weigh compared to what is on Oregon houses now?

Interlocking aluminum shingles weigh roughly half a pound per square foot of roof, or about 40 to 50 pounds per roofing square of 100 square feet. Asphalt shingles run about 2 to 4 pounds per square foot. Cedar shake sits around 2. Concrete tile runs about 6 to 11. Real slate, the material the aluminum slate profile is imitating, weighs 8 to 15 pounds per square foot. On a typical Portland roof of 22 to 26 squares, the aluminum adds somewhere around 900 to 1,300 pounds total. The asphalt it replaces weighed several times that. If the house carries two asphalt layers, which is common on older Multnomah and Clackamas County homes, the existing roof can be 5 to 8 pounds per square foot before a single flake of snow lands on it. Pulling that off and replacing it with half a pound per square foot is a net reduction in load, not an addition. The weight also does not change over the roof's life. Asphalt absorbs water and picks up moss west of the Cascades, where 40 to 80 inches of winter rain keeps north slopes damp from October to May, and a saturated mossy shingle roof is heavier than a dry one. Aluminum takes on no water and gives moss nothing to root into, so the load the framing sees on a wet February morning is the same as in August.

Does 1900s Portland framing care about roof weight?

It does, and that is where most of the weight questions we hear come from. Portland's Craftsman bungalows, foursquares, and Victorians were framed with full-dimension old-growth lumber, which is good, but they were framed for the roof of their day and often with rafters spaced wider and sized smaller than current tables would call for. Many have had a century of wet winters, some have had a dormer or a skylight cut in, and a fair number have sagged a little at the ridge. That is no reason to assume spare capacity for a heavy roof. What we see on the ground in the Portland install record, 374 roofs documented since 1998, is that the weight conversation usually starts because a homeowner wanted slate or tile for the look and someone told them the house could not carry it. Real slate at 8 to 15 pounds per square foot on a 1910 foursquare is a structural project before it is a roofing project. Aluminum slate or shake gives the profile at a small fraction of the load, which is why the question of reinforcement rarely comes up at all. The Willamette Valley farmhouses around Salem, Albany, and McMinnville have a related history. Additions, porches roofed at a different pitch, and a second asphalt layer laid over the first in the 1980s or 1990s are the norm. A roofer's job on those houses is to look at the deck and the rafters after tear-off and report what is there. A light roof does not fix a rotted rafter tail, but it keeps the repair scope to existing damage.

How does snow load change the math in Hood River and the Cascade foothills?

In the Willamette Valley snow is a minor part of the load picture. Multnomah and Clackamas Counties average about 19 inches a year, Marion 11, Linn 7, and most of it is gone in days. In Hood River County the ten-year average is 76 inches of snow with 55 freeze-thaw days, and the Mt. Hood foothills, the upper Clackamas, and the Cascade side of Lane County, which averages 31 inches, see real accumulation that sits. A roof structure carries dead load, which is the roof itself, plus live load, which in the mountains is mostly snow. The framing has a fixed capacity, and whatever the roofing material takes up is capacity the snow cannot use. A roof carrying 4 pounds per square foot of layered asphalt and then a wet Gorge snowpack is working much closer to its limit than the same framing under half a pound of aluminum and the same snow. Aluminum has a second snow-country trait worth stating plainly. Snow slides off a smooth metal roof more readily than off granulated asphalt, and on a steep Hood River pitch it can come down all at once. Snow guards above doors, walkways, and the meter base are part of a proper install in the Gorge, and the 13 roofs on record in Hood River since 1999 and 12 in The Dalles reflect that detail. Ice damming comes from attic heat loss, not roofing material, so a lighter roof does not replace eave insulation and ventilation.

Is a lighter roof better in Oregon earthquake country?

Western Oregon sits over the Cascadia Subduction Zone, and the state's building code reflects that. The general principle is simple: in a quake the mass at the top of a structure is what the walls and connections have to restrain, and less mass up high generally means less force on the framing below. That is why lighter roofs are generally favored in seismic regions and why heavy tile roofs draw extra attention from engineers in those regions. This is a general statement about mass, not an engineering claim about any particular house, and a licensed engineer is the person to ask about a specific structure. What we can say from the roofing side is that replacing two layers of asphalt, or an old concrete tile roof, with half a pound per square foot of aluminum is a reduction of thousands of pounds at the top of the house. On a Portland foursquare with an unreinforced brick chimney and a cripple wall below the first floor, that reduction is welcome even if it is not a substitute for a seismic retrofit of the foundation and chimney. Weight is also not what holds a roof on in wind; attachment is. The Interlock system fastens through the deck and interlocks on all four edges so there is no free edge for wind to catch, and ICC-ES ESR-1790 documents uplift resistance to 177.5 pounds per square foot. That matters on the Clatsop coast, where gusts reach 91 mph, and in the Gorge, where sustained wind is the everyday condition.

Can an aluminum roof go over my existing shingles in Oregon?

Often, and its weight is the main reason it is even a conversation. The Oregon Residential Specialty Code, like the model code it is based on, permits a new roof covering over one existing layer of asphalt shingles in defined cases and requires tear-off when there are already two or more layers, when the existing roof is water-soaked or deteriorated, or when the existing covering is wood shake, slate, or tile. Because aluminum shingles add so little load, going over one sound asphalt layer keeps the total roof weight below what many Oregon houses are carrying today with two layers of asphalt. Your county building department has the final say, and permits apply either way. The honest caveat is that going over the old roof only works if the deck underneath is sound, and you cannot fully know that until someone is on the roof. A Beaverton ranch with one clean layer and a dry attic is a good candidate. A Scappoose farmhouse with two layers and moss-lifted tabs on the north slope is a tear-off, and the code says so. On Tillamook and Lincoln County coastal homes, with 71 to 81 inches of rain a year, we assume deck repair is likely and price it as an allowance. The things that do not change with an overlay are ventilation, flashing, and ice protection. Valleys, chimneys, and skylights get new flashing whether the old shingles stay or go, intake and exhaust ventilation get corrected, and in the Gorge and foothills an ice-and-water membrane goes at the eaves. Interlock Metal Roofing (PNW) holds Oregon CCB #130826 and has been doing this work in the state since 1997, and the first job on any roof is telling you which path the house needs.

Common questions