Minimum Design Wind Load Step by Step - 16 psf and 8 psf on Projected Areas (ASCE 7-22 27.1.5)

Section 27.1.5 - the minimum design wind load

0:00Step five — the floor every analytical wind calculation has to clear. Every analytical wind calculation ends at the same checkpoint. Section 27.1.5 — and its Envelope Procedure twin, 28.3.6 — sets a floor: the MWFRS wind load cannot be less than sixteen pounds per square foot times the wall area, plus eight pounds per square foot times the roof area, both projected onto a vertical plane normal to the wind, applied simultaneously. The rule exists because Chapter 27's pressures can legitimately come out small — a low mapped wind speed, sheltered exposure B terrain, a favorable C sub P — and the code wants a hard lower bound tied to nothing but geometry. Two things trip people up: it is a projected area load, not a surface pressure — you flatten the silhouette onto a plane and push horizontally, you don't

0:47chase the wind around every roof slope — and it is checked per direction, so a comfortable margin one way cannot excuse a shortfall the other way.

Projected areas - the elevation views

1:01Both panels here look perpendicular to the wind — the same section along the wind convention as the earlier pressure diagram. The dashed line above the eave on the left panel is the projection plane; there is no wall up there, the eight psf acts on the flattened roof silhouette above it. The eighteen foot eave and the eight foot roof rise both come straight from this elevation.

Wind perpendicular to the ridge - wall strip + roof strip

1:24Looking along the ridge, the wind sees the forty eight foot long face. The silhouette splits at the eave: the roof's vertical rise is half the building width times the tangent of the pitch — sixteen feet times tangent twenty six point five seven degrees, eight feet. Wall projection: forty eight times eighteen, eight hundred sixty four square feet. Roof projection: forty eight times eight, three hundred eighty four square feet. Sixteen times eight sixty four, plus eight times three eighty four: thirteen thousand eight hundred twenty four plus three thousand seventy two — sixteen thousand eight hundred ninety six pounds.

Wind parallel to the ridge - the gable triangle is wall

1:55Looking across the building, the wind sees the thirty two foot gable end — and here is the subtlety worth a highlight. The triangle between eave and ridge is the gable end wall, a vertical wall surface, not roof. So the whole silhouette takes sixteen psf: thirty two times eighteen, plus one half times thirty two times eight, five hundred seventy six plus one hundred twenty eight, seven hundred four square feet of wall, and zero roof area, because the roof planes project onto that same triangle and are not counted twice. Sixteen times seven oh four: eleven thousand two hundred sixty four pounds.

The check per direction - and when the floor takes over

2:29The check, per direction: computed base shear against the minimum, both directions. Run this identical house in Exposure B instead — a suburban lot — and the case one shears drop to fifteen thousand seven hundred eighty three pounds perpendicular to the ridge, and nine thousand three hundred seventy six parallel to it. But the minimums do not move, because they depend only on geometry. Now section 27.1.5 governs in both directions. For light frame buildings in sheltered terrain, that is the rule, not the exception — which is why this check belongs in every submitted package, not just the ones where it looks close.