How to Calculate & Measure a Roof's Slope

Reading time: 5 minutes

Why roof slope matters

The slope of a roof isn’t just about looks. A roof that’s too shallow can hold water, while one that’s too steep might create limitations on the materials you can choose. 

A few of the key reasons to measure properly include:

• Weather resistance: A steeper slope helps rain, snow and leaves run straight off.
   

• Material choice: Every covering, from tiles to felt, comes with a minimum slope requirement.
   

• Energy and comfort: The angle changes how insulation fits and even whether solar panels will be effective.
   

• Safety: Knowing the incline in advance helps plan safe access and working conditions.

In short: if you’re repairing, replacing or designing a roof, the slope is the number that shapes a lot of the decisions that follow.

The basics: pitch vs slope

You’ll often hear people mixing up “pitch” and “slope.” While many builders use them interchangeably, it's important to note the key distinctions:

• Slope is normally given as a ratio, showing how much the roof rises over a set distance (the “run”)

  • For example, a rise of 100mm over 300mm of run is a 1:3 slope
     

• Pitch is often expressed as an angle in degrees

Different suppliers prefer different measurements, so it’s always worth checking what format is needed before you place an order.

Tools you’ll need

To measure safely and accurately, it’s best to have the following tools to hand:

• A reliable tape measure (a laser measurer works too)
   

• A spirit level or digital angle finder
   

• A notepad and pencil for jotting numbers down
   

• A ladder or loft access, depending on where you’re measuring

If you’re going up high, follow sensible precautions such as the HSE’s ladder safety advice. You could also take a look at our helpful guides on how to work on a roof safely and ladder safety regulations.

Method 1: Manual calculation

Here’s a step-by-step way to measure by hand:

1. Start by measuring a set distance inward from the roof edge or the run -  12 inches (300mm) is typical.
 

2. At that point, measure vertically from the base up to the roof’s surface. This then gives you the rise.
 

3. Compare the two, as the rise over the run is your slope ratio.
 

4. Want the slope in degrees? A quick calculation, arctan(rise ÷ run), will give you the angle.

This old-fashioned approach is quick, requires no special kit, and gives you a reliable result each time.

Method 2: Using a roof slope calculator

Not everyone enjoys maths. If you’d rather skip the sums, a roof slope calculator will do the groundwork for you. Type in the rise and run, and it gives both the ratio and the angle in a matter of seconds.

• For a standard pitched roof, use a sloping roof calculator.
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• If you’re dealing with an extension or garage, a flat roof slope calculator is more ideal, as it confirms whether the gradient is steep enough to shed water.

Online tools like these save time and remove guesswork, which is handy if you’re ordering materials to exact specifications.

Measuring from inside the loft

Sometimes simply climbing up onto the top of a roof isn’t practical or safe. If you can get into the loft, you can measure the slope from there instead. To do this:

1. Hold a spirit level flat along the underside of a rafter.
 

2. From the end of the level, measure vertically up to the rafter.
 

3. Use that figure as the rise, and the length along the horizontal level as the run.

It’s a great trick that lets you calculate slope without ever setting foot outside (which with the unpredictability of the UK weather, is a small win!).

Typical roof slopes in the UK

Knowing your measurement is one thing, but comparing it to common ranges gives you the context you need:

• Flat roofs: Technically not flat at all, usually 1:40 to 1:80 for drainage.
   

• Low-pitch roofs: Between 10° and 20°, suited to specific coverings.
   

• Standard pitched roofs: Most often around 30° to 45°, a balance of function and appearance.
   

• Steeper designs: Anything over 45°, sometimes chosen to create loft space or match a traditional style.

Common mistakes to avoid

It’s easy to slip up if you’re in a rush. To avoid this, here’s a few pitfalls to watch out for:

• Measuring once and assuming it’s right is generally not a recipe for success; always double-check.
   

• Don't forget that your materials will have minimum slope requirements.
   

• Mix up slope ratios with a degree pitch.
   

• Measure over tiles instead of the roof structure itself.

Final checks

Before you start ordering tiles or felt, ask yourself:

• Did I double-check both the rise and the run? A small error here can throw the whole calculation out.
   

• Does my figure match what’s typical for the roof type?
   

• Do I know whether the supplier wants slope or pitch?

If you’re unsure, a quick test with an online roof slope calculator can back up your manual numbers.

Roofing supplies from Travis Perkins

Whatever your measurement shows, you’ll need the right materials to match. 

Travis Perkins supplies everything from roof tiles and membranes to measuring tools and ladders, helping you tackle projects with confidence.