How much shade does 14 inches of space
top members give?
HI! I have been out of town, so sorry for the late response to your question.
A 14" space between top members of a pergola is not going to provide a lot of shade. I don't know of a formula for calculating an exact amount of shade provided by a certain spacing. Sorry. If shade is what you need and you like the look of a pergola, you can put a shade cloth over the members. Shade cloths can be manual (simply tie to the wooden members) or automatic (moves on a track). This is very attractive. You can also install another level of top members going in the opposite direction to help narrow the spacing. You can place the members at whatever distance you choose, even as close as 6" which would provide more shade than a 14" spacing.
Thanks for your submission! Nancy