Do your own home math

When it’s time to replace your roof, the contractor isn’t the only one that has work to do. If you’re in the market for roof replacement, you’ll have to take the time out to educate yourself on a few different roofing components and their dimensions. Having this knowledge will help you determine where your money is going when you receive a cost estimate from the contractor you choose to work with.

 One example of a roofing part you’ll need to measure is a roof truss. A roof truss is typically a triangular structure that is designed to distribute the weight of the roof. The two parts of a roof truss and the “members” (straight lengths of wood) and the intersections that disperse the weight evenly down the length of each member. The truss is built by attaching the ends of the member to joints that connect to the intersections.

Learning how to estimate roof trusses and their measurements may seem like a tough problem but we put together a formula to help you easily figure out your roof truss calculations. The simplest and most easy to build roofs are usually “Open Gable Roofs”, and this is the type of roof that most homeowners have. To calculate the measurements of this type of roof truss, it’s best to use the Pythagorean Theorem. 

This old equation that you may remember from middle school will finally come in handy to help you during your roof replacement journey. We chose this equation because it allows you to reduce each truss to a pair of right angles triangles that are arranged back to back.

The first thing you’ll need to do is measure the “roof span”. The roof span is the distance between the outsides of the walls that will support the roof. Half of that distance is referred to as the “run”. The run forms the base of a right-angled triangle with the height that is equal to the rise of the roof. The rafters of the truss will be used as the hypotenuse (highest part of the triangle). 

The average roof will overhang the side walls of the home by just a small amount (12-18 inches). Keep this in mind as you are calculating the rafter length.

The “pitch” (highest point) of the roof is an important ratio when measuring the truss. A sample calculation would be: A roof that rises 1 inch for every 4 inches of its horizontal distance has a 1/4th pitch. The best outcome of the pitch depends on the covering of the roof. For example, if you use a heavy load of asphalt shingles, a minimum pitch of 2/12 is needed for proper drainage. Pitches should in most cases never exceed 12/12 or else the roof is dangerous to walk on.

Rafter Length from Rise

After you calculate the roof span, next you’ll have to determine the rise which is based on the roofing material you choose and other design options. This will also affect the length of the rafters. Now imagine the entire truss as a pair of back to back right angles triangles.

Equation:a2 + b2 = c2

A = span

B = rise

C = rafter length

Already having the rise will make it easy to determine the rafter length by loading the numbers into the equation chart. For example, a roof with a span of 20 ft and a rise of 7 feet needs rafters that are the square root of 400 + 49 = 21.2 feet. This does not include the extra length required for the overhangs.

Calculating Rafter Length from Pitch

If you don’t have the rise of the roof, you might know the pitch based on the manufacturers’ recommendations for the type of roofing you plan to use. Don’t worry because that is still enough information to calculate the rafter length using a quick and simple ratio.

Imagine the desired pitch is 4/12. This is equal to a right-angled triangle with a base of 12 inches (one foot) and a rise of four inches. The length of the hypotenuse of this triangle is the square root of the equation a2 + b2 = 12(2) + 4(2) = 144 in + 16in = 12.65in. Because the length of the span and rafter are measured in feet, you’ll want to convert 12.65in to 1.06 ft. The length of the hypotenuse of this small triangle is therefore 1.06 ft.

Suppose the base of your roof is measured as 40 feet. Now set up the following calculation: Base of triangle = base of roof. After plugging in the numbers, you get 1/40 = 1.06/x (x is the required rafter length). Solving for x, you get x = (40) (1.06) = 42.4 feet.

Now that you finally have the length of the rafter, you have two options for finding the rise. Take the time to set up a similar ratio or you can solve the Pythagorean Theorem equation. When you choose the second option, you know that the rise (b) is equal to the square root of c squared – a squared, where c is the rafter length and a is the span. This means the rise equals root (42.4(2) – 40(2) ) = root 1797.8 – 1600) = 14.06 feet.

When using this free calculation, it’s important that you have the right design for the right height. Make sure that if you have a different type of roof truss than the simple open gable design, you use a different equation. You want to make sure to calculate all the angles, heights, and dimensions correctly. This will make it much easier to understand which type of rafters and struts you will need and how you will need to combine them for each height. If in an open gable style, use this calculation for a wood roof, steel roof, or even shed roof trusses or a steel roof truss. Knowing these measurements will give you a headstart in replacing your roof.

TL; DR

How do I calculate Roof trusses?

Determining how many trusses your roof needs comes down to a simple calculation. Truss count is determined by the following equation:

Truss count = ((roof length * 12) / 24) + 1

The simplest form of this equation is to take the length of your roof and divide it by 2. For example, if your roof is 40-feet long, it will need a total of 20 trusses.

 How do you measure truss sizes?

You’ll typically need a buddy to help you measure truss size. Place two ladders at each end of the area you’ll be measuring. While your assistant holds the other end, pull a tape measure along the top plate of the house to determine the length and write down the measurement in feet. To determine the width, repeat the process on the width of the top plate.