how to determine pergola rafter spacing

How to Determine Pergola Rafter Spacing | Apply Unique Methods

A pergola is an outdoor structure of posts, beams, rafters, and lattice. It serves the purpose of provision of a shade in different usabilities such as a walkway and a sitting area. One can either attach a Pergola to a roof or install it as a free-standing structure. Among the many features of the Pergola, the rafters stand out the most. A seamless finish is ideal for maintaining an equal spacing distance from one rater to another. The spacing of the rafters plays a vital role in the look and feel of the Pergola. Achieving an equal spacing of rafters during construction can prove futile if one lacks the know-how technique. To attain the correct spacing, it is crucial to understand how to determine pergola rafter spacing.

As a matter of fact, that is what we will get to read on. We also explore the pergola spacing options, beam dimension, and a few tips to get you started on your pergola project. Let’s start by determining the pergola rafter spacing.

How to determine pergola rafter spacing

The rafters’ spacing dramatically affects the final image of the Pergola. For this reason, it is critical to observe an equal spacing distance from rafter to rafter. Here is an easy-to-follow guide on how to determine Pergola rafter spacing.

Step 1

Determine the distance running across the girders. Then, measure only the space that will be covered by the rafters.

Step 2

Measure the width of the rafter boards that you will be using for the building of the Pergola.

Step 3

With the number of rafters, you want to use, calculate the total distance they will occupy together along a horizontal axis. This becomes the width of the rafters when put together. For instance, if you use 6 by 4 lumber for the rafters, the calculation becomes 4 multiplied by the number of rafters you want to use for the project.

Step 4

To determine an equal spacing from rafter to rafter, take the total width of the space where the rafters will be put (Distance running across the girders) and subtract from it the entire width of the rafters when put together.

Step 5

This step involves dividing the difference from the previous process by the number of spaces between the rafters. To get the total summation of spaces between the rafters, subtract 1 from the total number of rafters used. The quotient is the distance that should be observed from rafter to rafter.

What are the different pergola rafter spacing options?

The two standard pergola rafter spacing options are 12-inch and 16-inch options from rafter to rafter. Let’s explore these two options and when to use them.

12-inch spacing

12-inch rafter spacing is used on smaller pergolas where the total weight load of the rafters won’t cause the beam structure to bend or sag. It is ideal for pergolas with 14 feet in width.

12-inch spacing is simple to set up due to the even integer number of the foot. The evenness or oddness of the spacing is a crucial determinant. If the width of the pergola is likewise an odd number, then there will be an odd number of rafters. An even number of rafters will begin at a distance of the Pergola of exact feet. It should be noted that the measurements do not include any extra inches.

16-inch spacing

When working on pergolas that require wider spacing, convert the total number of feet into inches and divide by 16. This helps ease the calculation process. If the quotient is an odd number, finding the center from which to start placing the timber is a breeze. After locating the center, place the other rafters 16 inches from the previously center rafter. In case of further improvement and tinkering, adjust the spacing to an eighth of an inch. This adjustment won’t cause any change in terms of visibility.

What are the dimensions of a pergola beam?

The pergola beams form part of the main support structure. The shafts come in different dimensions depending on the span feet. For all the wood beams, it is recommended to use hardwood timber and treat for best results. Let’s explore the dimensions for 10′, 12′, 14′, 15′, 16′, 18′ & 20 feet spans.

A 2″x6″ beam dimension spans 10 feet. However, this depends on the wood species type and the treatability of the wood.

A beam with a 2″x8″ dimension will span for a total of 12 feet, while a 13 feet span will require at least 2×8 timber size. The rest of the measurements have been summarized in the table below;

Beam Dimension(Inches)Span Distance(Feet)
2″×4″ 6′
2″×6″ 10′
2″×8″ 12′, 13′
2″×10″ 14′, 15′, 16′
2″×12″ 18′, 20′

How to determine Pergola rafter spacing: Tips

Now that you know how to determine pergola rafter spacing out of the way, here are tips to help you have an impressive pergola.

  • Adhere to the permits and building codes where you want to place the Pergola.
  • Have a solid work plan to help you select the right style of the Pergola that suits your specific needs in mind.
  • With a wide variety of lumber and construction materials to choose from, select your materials keenly to avoid mishaps during the construction phase. Follow the National Hardwood Lumber Association rules to choose the correct lumber and refer to a pergola rafter span chart to construct a top-tier outdoor structure.
  • For starters, cut all your wood into the exact sizes before starting the construction process. This action enhances a smoother workflow.
  • When setting the posts, it is recommended to use concrete. It gives the structure a longer-lasting result and boosts its durability. It is also advisable to account for the length of the goes into the ground to avoid having a shorter design.
  • Attach the girders to the posts using brackets for Pergola to boost the firmness of the structure.
  • Ensure that the posts are assembled correctly to enhance a better-looking pergola. You can use the string or the 3-4-5 method to measure the angle.
  • As much as this is optional, notching the beam of the Pergola enhances a pleasing design while providing a steadier and firmer hold for the structure. Also, use pergola rafter brackets to add strength to the structure.
  • Work with a spirit lever to ensure the straightness and alignment of the beams and posts to avoid skewness of the structure.
  • Accessorize your Pergola with decorations and furniture to achieve a superb outdoor finish.

Frequently Asked Questions

How many rafters are needed for a pergola?

The number of rafters needed for a pergola varies from one Pergola to another. The number variation is a function of the length or span of the beam. It also depends on the spacing option being used. (Either 12-inch or 16-inch spacing.)

Why are the dimensions of a rafter important?

The measurements of a rafter are significant since they determine the wood size used, the lattice’s thickness, and the live load. Additionally, they are substantial in that they aid in determining the rafter-to-rafter spacing.

How do you calculate the span length of a rafter?

The span length of a rafter is calculated by using trigonometric functions. They are measured as the distance between points of support along the size of the rafter.

How do you calculate the span length of a pergola beam?

To calculate the span length of a pergola beam, you should use at least 2′′x6′′ size woods for a pergola beam for a span of 10′, 2′′x8′′ for a span of 12′, 2′′x10′′ for a span of 14, 15′, and 16′, and 2′′x12′′ for a span of 18′ to 20′.


For a pergola to offer its intended purpose, it is ideal to ensure that each part of the structure has been installed per the construction instructions. With equal spacing of the rafters, one enjoys adequate design shading while also boosting the value of their home. If built well, Pergola adds to the aesthetic touch of the outdoor garden. The rafters are typically placed 16 to 20 inches apart. Maintaining uniform spacing between all of the rafters is the most crucial spacing requirement. Install the front and back rafters, securing them to the girder’s face using the support posts.

Ensuring the correct spacing between rafters is a sure way of confirming that the Pergola will be aesthetically pleasing and durable. A hidden advantage is that one also gets to adhere to the construction codes of the geographical location when one follows such simple but crucial instructions. Now that you know how to determine pergola rafter spacing, it is time you’ve prepared for the summer heat by building one beefier Pergola.



Leave a Comment

Your email address will not be published. Required fields are marked *