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How to Slope a Flat Roof for Drainage?

Learning how to slope a flat roof for drainage is critical to avoid water collection and subsequent structural damage, which could become much worse than a nightmare – experienced homeowners must be aware of such dreadful situations. While it may appear paradoxical, placing a modest slope on a flat roof is a frequent building method, which directs rainfall away from the roof's surface and towards specified drainage points.

Learn more: Efficient Roof Process

Here's how you can successfully learn how to slope a flat roof for drainage.

Evaluate Roof Slope

The first step is to determine the optimum slope for your flat roof. A 2% slope is ideal, which implies that the roof should have a ¼ inch of fall for every foot of breadth (or width). Calculate the roof measurements and indicate the tallest point in the centre. Then, near the edges, make a gently downhill slope. This directs rainfall toward the edge of the roof and into the gutters.

Select Suitable Roof Material

Next, you should select the appropriate roofing materials and underlayment. EPDM rubber or TPO roofing materials find wide usage for flat roofs due to their longevity and flexibility. Use tapered insulation boards to provide the necessary slope for enhanced drainage. Stack the insulation boards to attain the proper inclination, enabling efficient water drainage.

Roof Edges

Once your roofing components are in place, pay special attention to the roof's edges. Build a parapet wall or a perimeter border to keep the water out. Ensure the installation is done correctly and the edges are sealed to avoid leakage. In addition, have a qualified contractor evaluate your work to ensure the slope is constant and the drainage system works properly.


Understanding how to slope a flat roof for drainage is a meticulous procedure that necessitates a careful design and the use of appropriate materials. You can prevent water from collecting on your flat roof and safeguard your property from water damage by following these measures and providing the correct slope.

Hiring a professional for this work is recommended to provide a dependable, watertight roofing structure. Here's on OnlyRoofing you can find specialist roofers.