Underlay Minimum Pitch Depends on the Roof Covering
A roof-pitch calculator tells you the roof geometry. It does not supply a universal underlay minimum. Match the exact underlay to the roof covering and its current manufacturer instructions; one published example lists different low-pitch figures for profile tiles, flat tiles or slates, and plain tiles.
First get a measurement you can name
Measure rise over horizontal run on the roof plane, then use the roof pitch calculator to express that measurement as a ratio, degrees, or percent. Keep the source measurement with the result. Geometry is the first input; it is not an installation approval.
Then match three things—not just one number
Open the current sheet for the named membrane rather than a generic roof-pitch chart.
Identify the stated covering category before comparing a pitch figure.
Check ventilation, fixings, and any detail-specific instructions alongside the pitch statement.
A product-specific example
Klober’s membrane support page calls Permo Extreme a low-pitch product and names three different pitches by covering: 12.5° for profile tiles, 15° for flat tiles and slates, and 22.5° for plain tiles. Klober also says Butylon tape is required to seal nail penetrations for the stated low-pitch use. These are statements about Permo Extreme in the listed situations, not default limits for all underlays.
| Named product | Covering named by the source | Stated pitch |
|---|---|---|
| Klober Permo Extreme | Profile tiles | 12.5° |
| Klober Permo Extreme | Flat tiles and slates | 15° |
| Klober Permo Extreme | Plain tiles | 22.5° |
What else can change the answer?
For this same product, Klober describes Permo Extreme as a vapour-permeable HR underlay and says the roof requires ventilation at low and high level in line with the roof construction. The page also says a vapour-control layer does not remove that ventilation requirement. Those statements show why copying a pitch number alone is incomplete.
Can I use one underlay pitch figure for every tile or slate?
No. If the exact underlay, roof covering, construction, or current instructions are not known, the appropriate answer is not confirmed. Keep the calculator result as a measurement and ask the product documentation to answer the product question.
Sources and scope
- Klober Technical Support — Membranes — the product-specific Permo Extreme pitch statements, Butylon-tape note, and ventilation context used on this page.
This page is a reading aid for product documentation. It is not a roof design, code interpretation, or approval for a particular project.