Really? Nobody has any advice?

If it's coming off your house, I think they should run straight forward from the wall.

perpendicular to the house

i'd go in your backyard on a sunny day and play around with a piece of 2x6. you'll see how it has to be arranged, and how large, to get shade where you want it.

Thankd, nnigrt. The problem is the pergola is already under construction and I think the contractor is putting the slats on in the wrong direction. I was hoping for some confirmation from someone. :(

there's no standard "right" way, it depends on your goals re: shade, air flow, etc. and what your designer drew up. What do your plans call for?

Revolutionary Gardens, our goal for the pergola is filtered shade. The pergola is within an L-shape space so the slats would be perpendicular to the house no matter which way they went. Do you think slats should run north/south or east/west?

tliv1965: if the slats are simply 2x2's, it doesn't matter which way they run. Shade will be the same whether they are EW or NS. If the slats are rectangular in cross section and angled, then EW and leaning over to the north will provide most shade. There are a multitude of different slat sizes, shapes and installation layouts and trying to give you a definitive answer without additional information is like providing an answer to "How long is a piece of string?"

the reason why I'm saying it comes down to the design as to whether they're putting the purlins (slats) on the right way is structural. Generally with a pergola the posts support two beams (or one beam if attached to the home). Rafters go perpendicular across the beams, then purlins are usually perpendicular to the rafters (so parallel with the beams) or occasionally at an angle - but they still get supported by the rafters.

Impossible to say what's "right" without knowing what we're looking at. Solar Texas has good info for you though as well.

The purlins are 2x8" and running north south. They aren't creating much shade. I think we'll have to plant vines or something that will block more sun. Thank you for taking the time to explain everything. It's much appreciated.

You could also attach a pretty or bight canvas over the top for more shade.

If your goal is shade(as is the case with most pergolas) you want the boards perpendicular to the path of the sun. Think of it like this. Lets say you're using 10' 2x6(1.5x5.5") boards. Example 1: Run the boards perpendicular to the path of the sun(N to S) and tilt the boards at 45 degrees. The maximum amount of shade that a single board can provide(when the sun is at 45 degrees in the sky) would be it's height x length, or 4.58 sq ft. Example 2: If you ran the boards parallel to the sun(E to W) with no tilt, then the maximum shade would be provided when the sun is directly overhead(width x length), and that amount would be 1.25 sq ft. Huge difference. There are other considerations such as existing geometry of house or deck or whatever...where you want to put the shade, and during which season and time of day. Existing trees and other shade structures as well as the height of the pergola are also key components to consider before building.

Much depends on the seasonal and latitudinal context and your desires. You can run them parallel, perpendicular, diagonal, and any angle in between. You can even 'tilt' them for more coverage/less coverage depending on the aforementioned. There is no right way. In farther Northern/Southern latitudes, the sun never gets "overhead" which may affect your choice. Also, think about Summer/Winter, and time of day. Line them up to maximize/minimize whatever effect you are after.