The Wind Speed Your Roof Was Nailed for Is on File at Your Building Department

Why does a roof come off before a wall falls in?

Because wind pulls up on it rather than pushing in, and somebody wrote down the exact speed yours was built to take. That figure stands on file at your building department, free to anybody who walks in, and almost no homeowner has ever asked for it.

Air crossing your roof does what air crossing a wing does. It speeds up over the top, pressure on that side drops, and suction hauls upward on everything underneath. A storm doesn’t shove your roof off sideways. It lifts it.

Roofs are also where your insurer’s money goes. Roof damage drives an estimated 70 to 90 percent of what insurers pay out on homes after a catastrophe, by the Insurance Institute for Business and Home Safety’s count.

Seventy to ninety percent. Not walls. Not windows. Roofs.

Which is why a building code talks about uplift instead of push. IRC Table R802.11 lists rafter and truss uplift forces in pounds per connection, and every one of those figures is a strip of metal pinning your roof down against something trying to take it up.

Then comes what IBHS calls a cascade. Water gets into your ceilings. Loose pieces turn into projectiles. Whatever sat on top of your roof goes.

So, plainly, and it’s what we’d tell a neighbor. If you want to know how your house does in a storm, look at where the roof meets the walls.

What wind speed was your house built for?

Whatever somebody at your county office wrote in by hand.

Table R301.2 of the 2021 International Residential Code is a blank form, and footnote d tells your jurisdiction to go find a map and fill it in.

A national code, with an empty box where its most-quoted figure ought to be.

That’s deliberate rather than sloppy. Coastal Florida and inland Ohio get different storms, and one figure would be wrong at both addresses. R301.2.1 sends a builder to Figure R301.2(2) and to whatever ultimate design wind speed applies at your address.

Somebody looked that up before your house was framed. It waits on file at your building department, and almost no homeowner has ever asked to see it, which strikes us as the oddest fact in this whole subject. A figure that decided how your roof got nailed down, sitting in a drawer twenty minutes from your kitchen, free to anybody who walks in.

Texas shows how firmly it gets fixed. Texas Department of Insurance rules put a house built between January 2008 and September 2020 under the 2006 code, one built after September 2020 answers to the 2018 edition, and one going up after April 2026 answers to 2024.

Your house is stuck at whichever year it was permitted. Re-roofing under a newer code is one of very few things that changes that.

Is that a gust, or wind that keeps blowing?

A gust, and a short one. Section 1609.3 of the 2021 International Building Code pins that value to a 3-second gust, 33 feet off the ground.

Three seconds is what changes the meaning of everything else here, and it took us a while to see why.

A gust is a brief violent peak rather than an afternoon of blowing, and gusts are what tear things off. A structure can settle into a load that keeps coming. A gust arrives as a shock, and your roof edge only has to lose its grip once.

It also means you can’t set a code figure beside a hurricane category and read across. Saffir-Simpson rates a storm on its strongest sustained wind over one minute, and Category 1 runs 74 to 95 mph.

Two measurements of one storm, and a gust always says higher. Tim Marshall, an engineer who wrote a 2009 draft study for the Saffir-Simpson Scale Committee, pairs them directly, so 74 to 95 mph sustained goes with gusts of 96 to 115.

So a figure on your permit that sounds comfortably above a hurricane category may be describing that exact category. Part of that gap is unit rather than margin.

Tornado ratings run on gusts instead. NOAA’s Storm Prediction Center draws EF1 at 86 to 110 mph and EF2 at 111 to 135, both as 3-second gusts, which is the same currency your permit is written in.

Does what surrounds your house change anything?

A great deal. Trees and neighboring houses slow wind down before it ever reaches your eaves, and a code counts that.

IRC R301.2.1.4 sorts your site into one of three categories, decided by what stands around it. Exposure B is an ordinary street, meaning suburb, woods, buildings about the size of a house standing close together. Exposure C is open ground.

One wind, one house, different loading, purely because of what stands nearby. People skip this part, and in our reading it moves a figure further than anything else on a permit.

Watch what it does to a real one. Table R802.11 takes a 32-foot truss span at 24 inches on center with a 5:12 pitch, at an ultimate wind speed of 115 mph, and gives 194 pounds per connection in Exposure B and 398 pounds in Exposure C. A quiet season does not lower the standard your roof is held to, and Hurricane Andrew struck during a below-average year.

More than double, for identical houses. One stands behind other buildings and trees. One faces open ground.

A cutoff waits right beside those two figures. R802.11 lets a builder use ordinary nailing where uplift stays at or under 200 pounds per connection, which that Exposure B house clears by six pounds and that Exposure C house misses by nearly two hundred.

Six pounds. That is how wide the line is between a handful of nails on your roof and a strap.

So look honestly at what surrounds you now, and we’d start with whatever has changed since you moved in. A house built behind woodland, standing today beside a cleared lot, is in a different exposure than it was drawn for, and nobody comes back to check.

How often is a design wind speed meant to happen?

Rarely. Note 5 to Section 1609.3 puts a basic design wind speed at roughly a 7 percent chance of being exceeded in 50 years, which works out to a 700-year mean recurrence interval.

Seven hundred years. That is the storm your permit was drawn against.

McAllister, Wang and Ellingwood, writing in the Journal of Structural Engineering in 2018, list ASCE 7-16 return periods of 300, 700, 1,700 and 3,000 years across four risk categories. Your house falls into that second one. Hospitals and schools get the long ones, because a hospital has to be standing afterwards.

Two facts arrive together and pull opposite ways, and we have never found a comfortable place to stand between them. Your house is engineered against a storm nobody alive expects to see, and it’s engineered against that and no further.

Real houses do better than a category name implies, though, and that surprised us. Marshall found that at 96 to 110 mph sustained, the vast majority of wood-framed homes came through fine, with a small number losing gable ends or roof decking. Across 8,119 homes he studied after Hurricane Katrina, fewer than 15 percent took structural wind damage. Warning time has improved even as the buildings have not: tornado lead time has gone from about 3 minutes to 13.

Which puts roof coverings a long way ahead of structure in the order things go. Shingles first. Decking sometimes. Frame rarely.

Are shingles held to that same standard?

No. Asphalt shingles get tested on their own bench, nowhere near your house, under ASTM D7158, against classes listed in IRC Table R905.2.4.1.

So your house carries one wind figure, and shingles nailed to it carry another.

Which explains an outcome that baffles people. Your house comes through a storm dead sound and loses half its shingles, and both parts met their own requirement.

Mark Graham of the National Roofing Contractors Association wrote this up in Professional Roofing in 2018. Two test methods are in circulation, and either one can end up on your roof. ASTM D3161 blows air at a shingle with a fan and grades it Class A at 60 mph, D at 90, F at 110. ASTM D7158 measures uplift force instead, and takes Class D to 115 mph, Class G to 150, Class H to 190.

Which makes “what class of shingle are you putting on my house” a fair question, and we’d ask it out loud before anybody signs anything.

IBHS built a whole program on that gap. Its FORTIFIED Roof standard seals the deck rather than trusting a covering, and IBHS says sealing those seams cuts water getting in by up to 95 percent. It also swaps smooth nails for ring-shank ones in a denser pattern, which IBHS says nearly doubles a roof’s strength against wind.

Ring-shank nails have rings cut around the shank, so wood bites back when something pulls. Nobody looking at a finished roof can tell which kind went up there, which is why that question belongs in a conversation while a roofer is still standing in your driveway, rather than after a storm has answered it for you.

What should you do about your own roof?

Four things, none of them urgent. Our order, easiest first.

Call your building department and ask what design wind speed applied when your house was permitted. That is a real answer about your own address, and it costs one phone call.

Ask whether that figure is ultimate or nominal. IRC Table R301.2.1.3 pairs them, 110 with 85, 115 with 89, 120 with 93, 130 with 101, 140 with 108. An older permit or an insurance form may be speaking in that older scale, so comparing across the two looks like a change that never happened.

Walk your lot and look at what is still standing. If trees or buildings that sheltered your house are gone, an assumption behind its design went with them.

Then look at where your roof meets your walls, because uplift acts there, and that is where failure starts. Hurricane straps and clips are cheap bent metal doing one job, and a house that cleared that 200-pound line was allowed to go up without them.

One last thing, and it matters to us more than any figure above. A design wind speed is what a building was engineered against, not a rating it wears or a promise about any particular storm. Anybody quoting you a survivable wind speed as a guarantee is describing a sales pitch, not your roof.