Keep nominal roofing dimensions separate from effective coverage.
Does the whole product rectangle cover the same usable roof area?
Use the matching calculator
Use this decision sequence
- Locate the applicable effective or exposed coverage for the selected product and method.
- Use nominal dimensions only for the physical stock or an explicit overlap model.
- Avoid applying a second overlap deduction to a value that already excludes it.
- Round each required plane using the applicable layout and purchased unit.
Keep the quantities distinct
| Quantity or assumption | How to use it |
|---|---|
| Nominal piece size | Physical overall dimensions. |
| Effective coverage | Usable installed coverage under a stated method. |
| Entered overlap | Must not duplicate overlap already removed in the effective value. |
Worked comparison
An illustrative piece is physically 1×2 m but has supplied effective coverage 0.9×1.8 = 1.62 m². Ordering a 16 m² area by physical 2 m² would predict eight pieces; an effective-area ratio alone requires ten. A plane grid can require more still. Deducting the same overlap again from 1.62 m² would understate its already-effective coverage.
Check before using the estimate
Use product-specific supplied coverage; roof structure, fixing and weather detailing are outside this quantity calculation.
All dimensions, product properties, prices and specifications in this example are illustrative arithmetic inputs. Use the values from your measured plan, selected product data sheet and supplier quote. This guide does not choose construction specifications or certify safety.
Calculation and scope checked 2026-10-04. Methods and scope.