Attached Canopy and Awning Wind Uplift and Snow Load

ScopeWhere the governing case is usually uplift and the anchors are the whole design. A canopy is open underneath, so wind acts on BOTH faces at once -- pressure on top and suction below, or the reverse -- and ASCE 7 handles that with a NET pressure coefficient rather than the separate external and internal coefficients used on an enclosed building. That is what makes a canopy different from a roof. On a roof, gravity is the design case and wind uplift is a check; on a canopy the uplift case frequently governs outright, and it loads the wall anchors in TENSION and the connection in PRYING, which are the two load directions masonry and stud walls are worst at and the two that get detailed most casually because the canopy looks light. Both cases have to be run because the two loads come from different weather: the snow case governs the members and the deflection, the uplift case governs the anchors. A 12 ft by 20 ft canopy in a 115 mph wind sees about 28 psf of velocity pressure, 34 psf of net uplift, and 8,122 lb trying to pull it off the wall, against 5,040 lb of flat-roof snow pushing down -- uplift beats snow by 61 percent and it acts the wrong way, while the canopy's own 800 lb of dead weight offsets only a tenth of it. That is the number that decides whether this canopy is through-bolted with backing plates or lagged into a stud, and it is the number nobody runs before the second one blows off. LOAD ONLY, NOT A CANOPY DESIGN. It does not size members, connections, or anchors, does not compute the prying and eccentric moment at the wall, and does not check the wall or its backup for the tension it is being asked to carry, which on masonry is frequently the actual limit. Kz, Kzt, Kd, Cn, Ce, and Ct are all ASCE 7 values determined elsewhere, and pg is the ASCE 7-22 ground snow load for the risk category (7-22 has no Is), and a wrong Cn moves the answer by a factor. Fabric awnings behave differently again, and drifting or sliding snow off the roof above can far exceed the flat-roof value. ASCE 7, the structural engineer, and the AHJ govern.

Formula or tablearea = projection x width; q = 0.00256 x V^2 x Kz x Kzt x Kd; uplift pressure = q x Cn uplift and downward = q x Cn downward; force = pressure x area; pf = 0.7 x Ce x Ct x pg (ASCE 7-22, no Is); snow force = pf x area; net uplift = uplift force - dead load x area.

Source and editionVelocity pressure q = 0.00256 x V^2 x Kz x Kzt x Kd and the ATTACHED-CANOPY net pressure coefficients of ASCE 7 Chapter 29, cited by chapter and linked, with no ASCE table reproduced -- Cn is entered by the user. Flat-roof snow from ASCE 7-22 Chapter 7, pf = 0.7 x Ce x Ct x pg (Eq. 7.3-1, no Is), cited by chapter. Load only, not a canopy design. ASCE 7, the structural engineer, and the AHJ govern.

Free accessThe velocity-pressure and flat-roof snow relations are stated and cited to their chapters; every coefficient is a user-entered value from the applicable ASCE 7 provisions rather than a reproduced table.

What governsEstimate. AHJ and licensed professional govern.

API field namesprojection_ft, width_ft, wind_speed_mph, kz, kzt, kd, cn_uplift, cn_downward, ground_snow_psf, ce, ct, dead_load_psf, area_sqft, q_psf, uplift_psf, uplift_force_lb, snow_pf_psf, snow_force_lb, net_uplift_lb

• Net coefficient, both faces an open canopy uses a NET Cn rather than separate external and internal coefficientsASCE 7 Ch. 29
• Uplift usually governs and it loads the wall anchors in tension and the connection in pryingASCE 7 Ch. 29
• Drift not modeled sliding and drifting snow off the roof above can far exceed the flat-roof valueASCE 7 Ch. 7