Roof Hip Calculator - How Long Are Your Roof Hips?

Tips & Advice
Roof Hip Calculator - How Long Are Your Roof Hips?

Your hip length is an important measurement. Not just when you are constructing a new roof, but also when laying tiles, working on a loft conversion or really any job that requires repairs or alterations to the front or back of your house’s roof. Measuring your hips length by hand can seem a bit tricky but fortunately there are a few methods that can this seemingly challenging task a walk in the park.

Below, we’ll go through two such methods that’ll provide you with an accurate estimation of your hips length in just a matter of minutes.

How to Calculate the Length of Your Roof Hips

Method 1: Calculate Your Roofs Hip Using Your Rafter Length and Half Span

For this method the very first thing you will need to know is the length of your common rafters. Learn how to calculate your rafter length. Once you have that measurement clear in your mind, next comes your half span. This is just half the width of your roof, from the edge of your eaves to the centre of your stud wall. With those two measurements in place you will have the lengths of two sides of a right-angle triangle that can overlay nicely across your roof with the hip being the third side.

Calculating Roof Hip Length Using Rafter Length and Half-Span

Note: When calculating your rafter length, be sure to add the overhang (or rafter feet) onto the final measurement before using it here. This will ensure you get a full length for your hip so that you can order timber long enough for appropriate plumb cuts.

If you’ve already read through our guide on calculating rafter lengths, then you can probably guess what comes next. Making use of Pythagoras’s famous theorem we can very simply determine the length of the hip (side c) by using the rafter length (side b) and the half-span (side a):

a2 + b2 = c2

As an example, assuming our half span was 2.5m (a) and our rafter length was 3m (b) our formula would look like this:

2.52 + 32 = 15.25

Then, we simple square root c2:

√15.25 = 3.9

Thus, making out approximate hip length 3.9m. When ordering it is always better to round up rather than down as longer timber can always be cut to size.

Method 2: Calculate without Your Rafter Length

Another, more complex, method for establishing your hip length still requires you to know the half span of your roof but this time we’ll be using the roofs pitch in the calculation rather than the rafter length. Your pitch can be determined a number of ways, which are explained in our guide for finding your rafter length.

Step 1

The first step of this multi-step process is to determine the rise of your roof per meter run. For this you’ll need a scientific calculator, if you don’t have one to hand there are also a lot of apps out there you can use instead. To figure this out, you simply to need to tan your roof's pitch. For example:

tan(40°) = 0.839

This means for every meter run your roof rises by 0.839m (or 839mm).

Step 2

Next, we’ll need the ‘plan’ length of your hip per meter run. This is essential a rough estimate based on a flat surface, as if your hip was running straight along the ground towards the apex – like you would see on a top over building plan. We’ll be using Pythagoras to determine this. Assuming side a and side b are both a meter that makes our plan length per meter 1.414m. This will be a constant figure regardless of the roof and simply demonstrates the length/m of any hip without a rise.

Calculating Roof Hips without Rafter Length

Step 3

With the rise per meter and the ‘plan’ length per meter we can determine the true length of our hip. Once again, put these figures into the Pythagoras equation and square the answer.

1.4142 + 0.8392 = 2.702

√2.702 = 1.643

Now taking your rise into account this would make the approximated true length of your hip 1.643m per meter run (TLH/m).

calculating roof hips diagram

Step 4

Remember when I said you’ll need your half span. The final step is to times it by your TLH/m for your final full length. Assuming our half span is still 2.5 that would mean:

2.5 x 1.643 = 4.108

This makes the final length of your hip 4.108m. If needed you can then add the length of your eaves overhang for a more complete estimate.

Next Steps

Now that you've calculated your hips' lengths, you can move onto the next phase of your project. Whether you need roof tiles, brand new loft insulation, or anything else, we're here to help. With over 40,000 roofing supplies at your disposal you’ll be sure to find the right products for you. If you have any questions about this or anything else roofing related you can contact our award-winning customer service team via our online chat or direct at 01295 565 565.

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