The minimum slope for a Flat Roof

Feb 26, 2020 in News
The minimum slope for a Flat Roof

Despite the name, all flat roofs need to be slightly angled in some direction. This is necessary for allowing water to run off of the EPDM membrane surface and towards an appropriate drainage system. But this slope doesn’t need to be extremely steep, and when covering a large property, the angle of the flat roof can be near invisible to the naked eye.

What is the most appropriate angle for Flat Roofing?

Normally, 1:80 (0.72⁰) is the recommended minimum angle for a slope, and this is universal across the vast majority of flat roofing systems, regardless of the materials that they incorporate. In order to achieve this drop, a slightly steeper angle of 1:40 (1.44⁰) should be the aim when installing a flat roof, as this accounts for surface deviations and material deflection among other things.

What are the main sloping systems?

The variations in angled slopes for flat roof systems depend on the types of drainage the property decides to install. Thus, the three main drainage systems that affect how you slope a flat roof are:

  • Scupper Drains: These are fitted at either end of a parapet wall, meaning the flat is typically angled in two directions from the middle.
  • Internal Drainage: Usually installed at the centre of the property, this drain transports water underneath the roof surface towards a nearby sewer. As a result, the flat roof is usually angled on all sides towards the middle.
  • Gutter Mounted: If the slope is only angled in one direction, a gutter and downspouts are fitted at the low end of the flat roof.

Why does the slope need to be effective?

Ensuring that your flat roof system is draining water sufficiently is vital to maintaining the long-term integrity of the material you use. If water is standing on your roof for more than 48 hours, then it can be described as a ‘Ponding’ roof. The damage this can do includes the following: 

  • Reduced material lifespan
  • Regular leakage
  • Loose debris
  • Insulation trouble
  • Potential bug infestation

Therefore, getting the angle of your slope correct is required for guaranteeing the long-term viability of your flat roof. For more advice, you can see our articles on Effective Draining Solutions and Guttering Solutions. Alternatively, feel free to Contact Us if you have any questions on roofing systems.