Snow Loads¶

The Snow Load module works through ASCE 7 Chapter 7 for each roof region of a structure. You enter the site values once, describe each roof region, and choose which design cases apply to it. The summary on the right recalculates as you type and carries the code reference beside every value.

Key Features¶

• Site values entered once for the structure: ground snow load, the winter wind parameter (ASCE 7-22), and the risk category read from the structure
• Any number of roof regions, each with its own R-value and design cases
• Nine design cases per region, each switched on only where it applies
• Drift loads at roof steps and at parapets and projections, with leeward and windward cases and a diagram
• A summary and report with the equation and code reference for every value

Adding the Module¶

1. Click the three-dot menu (⋮) next to the structure name in the sidebar
2. Select Snow Load

The module opens with placeholder site values and one roof region. Only the Minimum Snow Load case is included until the region has a roof R-value.

Site Parameters¶

The ASCE Hazard Tool button opens ascehazardtool.org in a second window, where the site values can be read for the project's latitude and longitude. Enter them here when you have them.

Roof Regions¶

A roof region is a part of the roof with one set of snow design cases. A simple building has one region. A building with a high roof and a low roof, or a main roof and a canopy, has one region for each.

• Add Roof Region at the top of the page adds a region below the existing ones
• The three-dot menu (⋮) on a region's header offers Duplicate Region, which copies the region with all of its design cases, and Delete Region
• Expand all and Collapse all on the header open and close every design case of the region

Design Cases¶

Each region lists nine design cases. The first two are always calculated. The rest have an Include design case checkbox. Checking a box creates the case and opens it; clearing the box removes the case and its inputs.

Balanced Flat Roof Snow Load¶

The case shows Ct and the flat roof snow load pf from Equation 7.3-1. Most other cases build on pf or the sloped roof load ps. The minimum snow load, the drift heights, and the unbalanced load on narrow gable roofs are computed from pg instead.

Balanced Sloped Roof Snow Load¶

The case shows the slope factor Cs and the sloped roof snow load ps from Equation 7.4-1.

Minimum Snow Load¶

Shows the minimum roof snow load pm for low-slope roofs. In ASCE 7-22 (Section 7.3.3), pm equals pg up to the limit pm,max of Table 7.3-4: 25, 30, 35, and 40 psf for Risk Categories I through IV. In ASCE 7-10 and 7-16 (Section 7.3.4), pm is Is pg where pg is 20 psf or less, and 20 Is where pg is greater.

Unbalanced, Ice Dam, and Sliding Loads¶

Include these cases where the roof geometry calls for them. Each opens with the inputs its section needs: the eave-to-ridge distance and slope for unbalanced loads on hip and gable roofs, the eave condition for ice dams, and the upper roof for sliding snow. The results appear in the case and in the summary.

Drift Loads for Roof Steps¶

Each roof step is a row. Click Add Row, then enter:

The row reports the balanced load height hb, the clear height hc, the drift height hd, the drift load pd, and the drift width w for the leeward and windward cases. Values marked in the footnote are enveloped. Diagrams opens the figure for the case.

Drift Loads for Roof Projections and Parapets¶

Each parapet or projection is a row with a Location, the fetch length lu, and its height h above the roof. For a parapet, lu is the length of the roof upwind of it. For a rooftop projection, lu is the greater of the roof lengths upwind and downwind of it. The row reports the windward drift height, load, and width per Section 7.8.

No drift is required on a side of a projection shorter than 15 ft. ASCE 7-16 and 7-22 also waive the drift where the clear distance from the top of the balanced snow, hb, to the bottom of the projection, including its horizontal supports, is at least 2 ft.

Rain On Snow Surcharge¶

Include the case where Section 7.10 applies: the roof slope in degrees is less than W/50, and pg is not zero and does not exceed pm,max of Table 7.3-4 (ASCE 7-22) or 20 psf (ASCE 7-10 and 7-16). It shows the surcharge added to the balanced load: 8 psf in ASCE 7-22, and 5 psf in ASCE 7-10 and 7-16. The surcharge is not combined with drift, sliding, unbalanced, minimum, or partial loads.

Summary View¶

The summary on the right lists the general parameters, then a Design Summary for each region with the governing values of every included case, then a Design Detail that writes out each case with its equation, inputs, and code reference. The outline button at the top of the summary jumps to a region or a case.

PDF Export¶

1. Click the Export PDF tab on the module page
2. Review the report in the viewer
3. Click the Save button in the viewer toolbar to save the PDF

The module report has the same content as the summary, with the project header and your company information. The same pages appear in the project report.

Deleting the Module¶

1. Hover the Snow Load entry under the structure in the sidebar and click its three-dot menu (⋮)
2. Select Delete Module
3. Confirm with Delete Snow Load

Best Practices¶

• Take pg (and W2 on ASCE 7-22 projects) from the ASCE Hazard Tool for the project coordinates, and record the source in the structure notes
• Enter the roof R-value first; the design cases other than Minimum Snow Load stay disabled until it is entered
• Make one region per roof level or roof type, and name regions and drift rows for the locations on the drawings
• Include only the design cases the roof geometry calls for, so the report shows the loads that govern
• Check the drift widths against the lower roof dimensions. Where a drift is wider than the lower roof, ASCE 7-16 and 7-22 taper it linearly to zero at the far edge of the lower roof; ASCE 7-10 truncates it at that edge instead

Code Reference¶

All three editions cover snow loads in ASCE 7 Chapter 7.