Formula used
Screws per metre = fixing points × fixing rows × screws per point
Base screws = row screws + overlap screws + end screws
Final screws = ceiling(base screws × (1 + waste percentage ÷ 100))
Spacing mode converts the chosen spacing into fixing points. Each point multiplies by rows and screws. Exposure and wind factors then adjust the rate.
Rib mode starts with roof rib spacing. Alternate ribs use two ribs per fixing. Custom mode accepts a direct screw density.
How to use this calculator
- Select spacing, rib, or custom calculation mode.
- Enter the total roof ridge length.
- Choose fixing rows and screws per point.
- Add cap overlaps and ridge-end requirements.
- Apply exposure, wind, and waste adjustments.
- Enter packaging and price information.
- Select Calculate screws to view results.
Use the manufacturer’s fixing specification whenever available. Local wind rules may require closer screw spacing. Confirm structural requirements before beginning roof installation.
Worked example
A ridge measures twelve metres with two fixing rows. Screws are spaced every three hundred millimetres. One screw is used at each point.
The base rate equals 6.67 screws per metre. Overlap and end screws are then added. Ten percent waste increases the final order.
| Input | Example value |
|---|---|
| Ridge length | 12 m |
| Fixing spacing | 300 mm |
| Fixing rows | 2 |
| Screws per point | 1 |
| Waste allowance | 10% |
| Box quantity | 100 screws |
Frequently asked questions
1. What is a running metre?
A running metre measures length along the roof ridge. It does not measure surface area. Screw density is applied to that length.
2. Should ridge screws fix both sides?
Many ridge caps require fixing along both sides. That usually means two fixing rows. Always confirm the manufacturer’s installation detail.
3. How does fixed-spacing mode work?
The calculator divides one metre by the fixing spacing. It then multiplies by rows and screws. Smaller spacing produces a larger screw quantity.
4. How does rib-based mode work?
Rib mode calculates roof ribs within one metre. It can fix every rib or alternate ribs. The selected rows determine total screw density.
5. Why include overlap screws?
Ridge-cap joints may need extra fasteners. These screws are separate from regular running fixings. The calculator adds them to base requirements.
6. What waste percentage should I use?
Ten percent is a common planning allowance. Complex roofs may need a larger allowance. Unused screws can support future maintenance work.
7. What does exposure adjustment mean?
Exposure settings increase or reduce estimated screw density. They are planning factors, not structural design rules. Use certified requirements for final installation decisions.
8. Why are results rounded upward?
Partial screws cannot be purchased or installed. Rounding upward avoids an undersized material order. Box calculations also round upward automatically.
9. Can this replace engineering advice?
No, this calculator provides a material estimate. Wind zones and substrates affect fixing requirements. Obtain professional guidance for safety-critical roof work.