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Shed Roof Metal Decking Calculator

Bill of Materials (BOM) generator for single-slope (skillion) roofs. Calculates sheeting, purlin spacing, and specific trim types (high/low eave) based on wall heights.

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About

Single-slope roofs, often called Shed or Skillion roofs, are architectural staples for modern homes and utility structures. While geometrically simpler than gable roofs, the structural load distribution requires careful attention to "purlin spacing" - the distance between horizontal supports.

This calculator generates a comprehensive Bill of Materials including the specific trim needed for the high side (Peak/High Eave) and low side (Drip Edge). It also checks your span against standard load tables for common steel gauges (29ga, 26ga) to warn if structural sagging is likely under snow or wind loads.

shed roof skillion roof lean-to calculator purlin spacing metal BOM

Formulas

Slope length (Ls) is calculated from the height difference (dh) and horizontal run.

dh = Hhigh − Hlow

Ls = √Run2 + dh2

Pitch = dhRun × 12

Purlin Rows = ceil(Ls / Spacing) + 1

Reference Data

GaugeThicknessMax Purlin Span (Residential)
29 ga0.014"24 inches
26 ga0.018"36 - 48 inches
24 ga0.024"48 - 60 inches
22 ga0.030"60+ inches

Frequently Asked Questions

On a shed roof, the top edge requires "High-Side Peak Trim" (often with foam closures) to seal the highest point. The bottom edge uses "Eave Trim" or Drip Edge to guide water into gutters.
For standard 29-gauge residential metal, purlins should generally not exceed 24 inches on center. Heavier 26-gauge metal can span up to 36-48 inches depending on local snow load codes.
Yes, a lean-to is structurally a single-slope roof attached to an existing structure. Ensure you account for flashing where the roof meets the existing wall.