with American products
w/ American products
All of the pergolas featured on this page have a 28' x 20' pergola roof covered by shade purlins, not including the end caps on traditional-style pergolas (which extend 12 inches beyond the ends of the 28' beams and the 20' rafters, unless you opt for flat end caps). So for all of the 28-foot by 20-foot pergolas on this page, the beams and shade purlins are all 28 feet long (as they run the 28-foot width of the pergola), and the rafters are 20' long (as they run the 20-foot depth of the pergola), not including end caps.
The posts (i.e. columns) of a traditional 28' x 20' pergola can be placed anywhere from 0" to 36" in from the outside edges of the beams and rafters (not including the 12" end caps), as shown in this diagram. Most customers choose to place the posts 12 inches in from the ends of the beams/rafters as shown in this CAD drawing, which puts the outside edges of the posts 26 feet apart width-wise and 18 feet apart depth-wise. But again, you can position the posts anywhere from 0 to 36 inches in from the ends of the beams/rafters to create your desired overhangs. It's also worth noting that the overhang can be different on one side than it is on the other sides, if needed or desired.
For modern-style 28x20 pergolas (which never have overhangs), the posts are always in the corners. So for modern pergolas, the outside edges of the posts will be 28 feet apart width-wise and 20 feet apart depth-wise.
To make our pergolas as affordable as possible, we only manufacture the beams, rafters, purlin holders and shade purlins in 2-foot increments (or 4-foot increments above 20 feet). But one or both of the width/depth dimensions of your deck, patio or other outdoor space may be slightly shorter than 28 feet by 20 feet. Using a basic circular saw or miter saw, you can cut any of the pieces down to the exact length required. Thus, the 28x20 pergolas on this page can easily be turned into 27.625-foot by 19.75-foot pergolas (or whatever specific dimensions you need).
Please be careful when ordering your pergola. A 28x20 pergola is not the same as a 20x28 pergola.
The attached 28x20 pergolas on this page will attach to your home on the 28' side and come 20' out from the wall. For free-standing 28 x 20 pergolas, the shade purlins (which create shade underneath the pergola) must run the 28-foot width of the pergola. So to maximize the pergola's shade, you will want to orient the pergola such that the evening sunlight comes from the 28-foot width side.