Why roof pitch matters when designing an extension, and how to achieve a traditional appearance.
When you’re extending an older house, matching the existing roof tiles isn’t always straightforward.
Traditional Rosemary clay tiles might look lovely on your existing roof, but they may not be suitable for a single-storey extension, particularly where the roof needs to fit beneath first-floor windows.
At Pride Road Architects, we consider roof pitch early in the design process because it affects the roof covering, ceiling heights, rooflights and even the height of patio doors.
What’s the minimum pitch for roof tiles?
Roof pitch is measured in degrees, and different roof coverings have different minimum requirements.
| Roof covering | Typical minimum pitch |
|---|---|
| Traditional Rosemary clay plain tiles | 35° |
| Natural slate | Around 20–30°, depending on specification |
| Sandtoft 20/20 Flanders clay interlocking tiles | 15° with 100 mm headlap |
The minimum pitch depends on the product, headlap, roof length and exposure. Always check the manufacturer’s current technical guidance.
The geometry of a single-storey extension
Designing a mono-pitched roof is really a geometry exercise.
You’re working between the height where the roof meets the existing house and the height you need at the garden end.
If the roof sits below a first-floor windowsill, you need room for the roof construction and weatherproof flashing. We often allow around 150 mm at the abutment during initial design, although the final detail depends on the roof construction and flashing requirements.
At the garden end, you still need enough height for comfortable ceilings and full-height bifold or patio doors.
The deeper the extension, the shallower the roof becomes if those heights remain fixed.
It’s a balancing act, and sometimes the roof covering you originally wanted simply won’t work.
Don’t design right on the minimum pitch
If a roof tile is suitable for a minimum pitch of 15°, I’d be cautious about designing the roof at exactly 15°.
Construction tolerances can affect the finished pitch, so designing at 16°, for example, provides a little margin.
Rooflights also need consideration. VELUX offers specialist low-pitch roof windows and flashing systems for pitches between 10° and 20°.
However, the roof covering must still be suitable for the actual pitch. A low-pitch rooflight doesn’t make an unsuitable tile acceptable.
Our Bramhall extension: Sandtoft 20/20 Flanders
We recently visited a completed Pride Road Architects project in Bramhall, involving a single-storey extension to a detached house built in 1927.
The original house has traditional Rosemary-style clay roof tiles, but the new extension roof was designed at approximately 20°, too shallow for conventional Rosemary tiles.
We specified Sandtoft 20/20 Flanders clay interlocking tiles by Wienerberger.
These have the traditional appearance of small-format clay tiles but can be installed at pitches as low as 15°, subject to the manufacturer’s specification.
The homeowners and builder considered alternatives during construction but eventually returned to our original choice.
Seeing the finished extension, I was really pleased with how well the Flanders tiles complement the existing roof. Although they’re different products, their scale, colour and texture work beautifully together.
It’s a good example of achieving a traditional appearance without having to use exactly the same material.
What if the pitch still doesn’t work?
Sometimes the available height simply won’t accommodate a tiled mono-pitched roof.
A dual-pitched roof may allow steeper slopes by introducing a ridge, rather than taking the roof all the way up to the existing house. It can also create an attractive vaulted ceiling.
Alternatively, a flat roof with a parapet can provide a contemporary solution while maintaining good ceiling heights and accommodating rooflights.
Flat roofs still need falls for drainage and a properly designed waterproofing system.
Getting the roof design right
At Pride Road Architects, we explore roof forms as part of our architectural design workshops, considering how they affect the appearance, natural light and internal space of an extension.
Our Bramhall project shows that choosing the right roof covering can make all the difference to how a contemporary extension sits alongside an older house.
Planning an extension? Book a free online consultation at www.prideroad.co.uk.
