MEASUREMENTS AND PURCHASE DECISIONS
Account for both side and end laps in roof rolls.
Which two overlap directions affect a rectangular membrane order?
Use the matching calculator
Use this decision sequence
- Measure the actual roof plane along and across the chosen roll orientation.
- Use the supplied side and end laps for the selected product.
- Count courses using effective width after the first course.
- Count rolls along each course using effective length after its first roll.
Keep the quantities distinct
| Quantity or assumption | How to use it |
|---|---|
| Side lap | Reduces the width added by later courses. |
| End lap | Reduces the length added by later rolls. |
| Offcut reuse | Needs a separate compatible placement schedule. |
Worked comparison
A 12 m by 3 m plane uses 10 m by 1 m rolls with 0.10 m side laps and 0.20 m end laps. It needs 4 courses and 2 rolls per course under the no-cross-course-reuse model, giving 8 rolls.
Check before using the estimate
The calculation does not set lap requirements, select waterproofing details or assess safe roof access.
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.