Wanting to build a pergola to cover a deck that will be connected to my house ledger board.

Constraint is the deck size and not wanting to put a post in the middle of the deck.
- planning cedar wood
-Two 6x6 posts 10 feet apart with a double 2*10 beam.
-Rafters (16inch on center)- House to beam distance 19 feet
Wanted to get people's thoughts on the rafter sizes to ensure I don't get any sagging. Was hoping 2*10 to match the beams. Thoughts?

1 Answer 1


A unloaded 19' long cedar 2x10 will sag about a little less than 0.2" which is L/>1000 and I would expect that to more than fit your needs.

HOWEVER: 19' long 2x10's will roll over in the middle meaning that they will twist in the middle with the appearance of the top tending to roll to one side as if attempting to roll completely over. The way to mitigate this is to add blocking. It is typical to add full depth blocking at 8' oc which would require two rows of blocking for your case.

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  • Would this be lessened with Purlins? - the wood slats that run perpendicular to the rafters either 2 x 2 or 1 x 2? Or actual blocking between the rafters ?
    – KKJ
    Apr 17, 2020 at 2:29

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