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The mapped basic wind speed for the site, at the risk category of the building.
Taken from the wind speed map in the adopted code, at the risk category the building falls in — the same site carries different speeds for a house and for a hospital. Working from a map drawn in metric? 115 mph is 51.4 m/s, and 45 m/s is 101 mph.
The terrain roughness upwind of the building, over the distances the code specifies.
Exposure is assessed per wind direction over an upwind fetch, and the governing case is the roughest wind can reach the building through. A house on the edge of a subdivision facing open farmland is Exposure C on that side however many neighbours it has behind it.
Average of eave height and ridge height, measured from grade.
For a roof pitched at 10 degrees or less, the eave height may be used instead. Measure from the grade the wind sees, which on a sloping site is the low side.
The shorter of the building's two plan dimensions.
It is the SHORTER side that sets the zone width, which is why a long narrow building has narrow zones running its whole length rather than wide ones. On an irregular plan, take the least dimension of the rectangle that encloses the roof under consideration.
The pitch of the roof plane being checked, in degrees.
A 4:12 pitch is 18.4 degrees, 6:12 is 26.6 and 12:12 is 45. Slope does not enter the velocity pressure arithmetic — it decides which coefficient figure you should be reading, and the note under the result says which family this slope falls in.
Read from the components-and-cladding figure in your adopted code, for this zone and effective area.
This calculator does not supply GCp and the value in the box is a placeholder. It changes with roof form, slope, which zone you are in and the effective wind area of the element being fastened — a clip, a panel and a whole purlin all see different coefficients on the same square of roof. Enter the magnitude; the sign is handled here, and the internal pressure is added to it.
Design uplift pressure in the selected zone
30.5 psf
The external pressure coefficient is the value you entered, not one this page supplies, so the answer is only as good as the figure you read off. This is a low-slope roof, so the low-slope coefficient figure applies and the corner zone wraps the full perimeter corner. The topographic factor is taken as 1.0, which is wrong on a hill, a ridge or an escarpment.
- Velocity pressure at mean roof height
- 25.82 psf
- Velocity pressure exposure coefficient
- 0.9 (Kz)
- Internal pressure component included
- 4.65 psf
- Corner and edge zone width, measured in from each roof edge
- 3.9 ft
- Corner zone plan area at each corner
- 15.21 ft²
They open the calculator with your figures already in it
Roof Uplift Zone Pressure and Zone Width Calculator: 30.46 psf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, components and cladding provisions as adopted locally — velocity pressure q = 0.00256 × Kz × Kzt × Kd × V² in psf with V in mph, and net pressure p = q × (GCp − GCpi)
- ASCE 7 terrain exposure constants used for Kz = 2.01 × (z / zg)^(2/α), held constant below zmin: Exposure B α = 7.0, zg = 1200 ft, zmin = 30 ft; Exposure C α = 9.5, zg = 900 ft, zmin = 15 ft; Exposure D α = 11.5, zg = 700 ft, zmin = 7 ft
- ASCE 7 directionality factor Kd = 0.85 for components and cladding of a building, and internal pressure coefficient GCpi = ±0.18 for an enclosed building. Topographic factor Kzt and ground elevation factor Ke are both taken as 1.0 here — Ke = 1.0 is always permitted, Kzt = 1.0 is only correct away from a hill, ridge or escarpment
- Zone width a = the smaller of 0.1 × the least horizontal dimension and 0.4 × the mean roof height, but never less than 0.04 × the least horizontal dimension nor less than 3 ft
- THE EXTERNAL PRESSURE COEFFICIENT IS NOT SUPPLIED HERE. GCp depends on roof form, slope, zone and effective wind area, and it is read from the components-and-cladding figure in the code edition your jurisdiction has adopted. The default in that field is a starting point to be replaced with the value your own figure gives, and is not itself a published coefficient
Inputs used
- Basic Wind Speed V (mph, 3-second gust)
- 115
- Exposure Category
- C — open terrain with scattered obstructions
- Mean Roof Height
- 19.5 ft
- Least Horizontal Building Dimension
- 39 ft
- Roof Slope (degrees from horizontal)
- 5
- External Pressure Coefficient Magnitude |GCp| for the Zone Being Checked
- 1
Intermediate steps
- Velocity pressure at mean roof height
- 25.82 psf
- Velocity pressure exposure coefficient
- 0.9 (Kz)
- Internal pressure component included
- 4.65 psf
- Corner and edge zone width, measured in from each roof edge
- 3.9 ft
- Corner zone plan area at each corner
- 15.21 ft²
Confidence note: The external pressure coefficient is the value you entered, not one this page supplies, so the answer is only as good as the figure you read off. This is a low-slope roof, so the low-slope coefficient figure applies and the corner zone wraps the full perimeter corner. The topographic factor is taken as 1.0, which is wrong on a hill, a ridge or an escarpment.
What this calculation does not cover
- Enclosed building assumed. A partially enclosed building carries a much larger internal pressure coefficient and a building with a large dominant opening larger still.
- Kzt = 1.0 assumed. Speed-up over a hill or an escarpment can raise the pressure by half again, and that is a separate calculation.
- Gives pressure, not fastener spacing. Turning pressure into a clip or fastener layout needs the tested assembly's own rated resistance and its safety factor.
Add the equipment this sizes
This result is a specification — 30.5 psf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations5
- ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, components and cladding provisions as adopted locally — velocity pressure q = 0.00256 × Kz × Kzt × Kd × V² in psf with V in mph, and net pressure p = q × (GCp − GCpi)
- ASCE 7 terrain exposure constants used for Kz = 2.01 × (z / zg)^(2/α), held constant below zmin: Exposure B α = 7.0, zg = 1200 ft, zmin = 30 ft; Exposure C α = 9.5, zg = 900 ft, zmin = 15 ft; Exposure D α = 11.5, zg = 700 ft, zmin = 7 ft
- ASCE 7 directionality factor Kd = 0.85 for components and cladding of a building, and internal pressure coefficient GCpi = ±0.18 for an enclosed building. Topographic factor Kzt and ground elevation factor Ke are both taken as 1.0 here — Ke = 1.0 is always permitted, Kzt = 1.0 is only correct away from a hill, ridge or escarpment
- Zone width a = the smaller of 0.1 × the least horizontal dimension and 0.4 × the mean roof height, but never less than 0.04 × the least horizontal dimension nor less than 3 ft
- THE EXTERNAL PRESSURE COEFFICIENT IS NOT SUPPLIED HERE. GCp depends on roof form, slope, zone and effective wind area, and it is read from the components-and-cladding figure in the code edition your jurisdiction has adopted. The default in that field is a starting point to be replaced with the value your own figure gives, and is not itself a published coefficient
Which documents these citations point at
Standards referenced: ASCE 7 (American Society of Civil Engineers, United States).
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