While the rafters don't necessarily have to be evenly spaced across the width of the pergola (it's okay for one or more rafters to be shifted a couple/few inches one way or the other, particularly if the home has a bump-out that makes even spacing tricky), most people prefer to have exactly the same spacing between all the rafters.
Following are step-by-step instructions for how this is done for traditional pergolas up to 24 feet in width, meaning the pergola does not have a middle post on the width side. (If your pergola has a middle post on the width side, please contact us for alternative instructions.)
You can also refer to the video at the bottom of this page. (Please note that all measurements and calculations shown in the diagrams on this page are for illustrative purposes only. Your pergola kit likely has different dimensions and a different number of rafters.)
Step 1: Measure the Length of the Beams
Use a measuring tape to measure the length of the beams included in your pergola kit. All beams should be the same length (within 1/16 of an inch, which is negligible). Make sure to measure the beams, not the rafters (which are usually the same dimensions). There will only be 2 beams for attached pergola kits or 4 beams for free-standing pergola kits, whereas there will typically be many (6+) rafters.
Please note: If you plan to cut the beams down to a smaller length, make sure to do that before measuring the length of the beams.
For illustrative purposes, let's say that the beams are 192.0" in length.
Step 2: Choose Your Desired Overhang
Decide how much of an overhang (if any) you want from the outside edge of the outermost rafter to the end of the beam, not counting the end caps (but taking them into consideration when deciding on the overhang).
The traditional pergola installation instructions assume a 12" overhang, but you can make the overhang anywhere between 0" (i.e. the outside edge of the outermost rafter is perfectly flush with the end of the beam) and 36" (though we recommend limiting the overhang to ~28" to prevent the ends from sagging a bit), as illustrated here.
When deciding on the overhang, keep in mind that there will eventually be an end cap on both ends of the beam. If you've selected either Scrolled or Beveled end caps, the end caps will add an additional 9" (for 2x6" beams) or 12" (for 2x8" beams) of length to each end of the beam. So if you select a 16.0" overhang (as shown in the example image below), there will actually be a total overhang of 25" on each end when counting the end caps as well.
Step 3: Mark the Center Point of the Outside Rafter
Put a mark on the beam exactly 1 inch further in (i.e. towards the center of the beam) from the desired amount of overhang. This marks the center point of the outermost rafter (since the rafter is 2" wide, the center point of the rafter is 1.0" towards the beam's center).
Using our example of a 16.0" overhang, the mark would be placed 17.0" from the end of the beam (as illustrated below).
Step 4: Mark the Center Point of the Other Outside Rafter
Put another mark on the other end of the beam that is exactly the same amount of space from the end of the beam as the mark you made in step 3. This marks the center point of the other outside rafter on the other end of the beam. You now have marks on both ends of the beam showing the center points for the two outside rafters.
Continuing on with our illustrative example, you will now have two marks on the beam (17.0" from both ends of the beam).
Step 5: Measure the Distance Between the 2 Marks
The next step is to simply measure the distance between the two marks. All the remaining rafters (which are referred to as "inside rafters") will need to go between these two marks, with an equal distance between each rafter.
In our illustrative example, the distance between the 2 marks would be 158.0" (which is simply 192.0" - 17.0" - 17.0").
Step 6: Mark the Center Points for the Remaining (Inside) Rafters
This is the trickiest step, but it's really not all that complicated. Just make sure to follow these instructions carefully and double/triple-check your work.
To calculate the distance between each rafter's center point, divide the distance between the two marks on the beam by [number of remaining rafters left to place + 1]. Make sure not to forget the +1 part of this equation.
In our example, we're assuming the pergola kit has 8 total rafters (2 of which have already been placed as the outside rafters, and 6 of which still need to be placed as inside rafters). In this case, you would divide the distance between the two marks on the beam by 7 (which is 1 more than the number of inside rafters remaining). So the calculation is: 158.0" / (6 + 1) = 22.6".
You would use this calculated number to put marks on the beam for each of the remaining (inside) rafters. Just like the marks for the outside rafters, each of these new marks indicates the center point for the inside rafters.
Step 7: Verify the Center Marks, Make Outside Marks & Place the Inside Rafters
Count the number of marks you've written on the beam to make sure the total matches the total number of rafters included in your pergola kit and re-measure the distance between each mark to make sure they are all the same.
Once you're satisfied that your marks are correct, make additional marks (perhaps in a different color) exactly 1 inch to the left and to the right of each mark you made previously. These new marks indicate the outside edges of each rafter (since the rafters are 2" wide). You can now place each of the rafters on the beam and secure them in place using the fasteners provided.
