Roofing Geometry & Sheet Overlaps
Analyze the trigonometric conversion from plan area to sloped area and learn why 'Effective Width' is the key to leak-proof roofing.
Plan Area vs. Sloped Area
One of the most common errors in roofing procurement is ordering sheets based on the floor plan area. Because roofs are pitched (sloped), the actual surface area of the roof is always greater than the area it covers on the ground. To find the true area, we use simple trigonometry:
The Slope Equation
Example: A $5$m horizontal span at a $15^\circ$ pitch becomes a $5.18$m sloped length.
Effective Width and Length
Corrugated or profiled sheets are designed to overlap to prevent water ingress.
1. Side Overlap: One or two "corrugations" are overlapped side-by-side. If a sheet is $1050$mm wide and the overlap is $150$mm, its Effective Width is only $900$mm.
2. End Overlap: When stacking sheets vertically along a long slope, the top sheet must overlap the bottom one by at least $200$mm to prevent wind-driven rain from being pushed underneath.
Standard Pitch Recommendations
- Minimal Pitch ($3^\circ$-$5^\circ$): Only for long-run continuous sheets without end joints.
- Standard Corrugated ($10^\circ$-$15^\circ$): The most common range for residential and industrial roofs.
- Steep Pitch ($> 20^\circ$): Used in areas with heavy snow to allow for self-clearing.
Frequently Asked Questions (FAQ)
What is a 'Barge' and 'Eave' overhang?
The Eave is the edge of the roof that hangs over the side walls, while the Barge (or Gable) is the edge that extends past the front or back walls. You must add $300$mm to $600$mm to your dimensions to account for these overhangs, which protect your walls from rain runoff.