Planning to add a pergola to my deck for shade and growing grapes and need help deciding if I should extend the posts of the pergola to the ground on a concrete base or if I can just attach it to the deck by replacing the existing corner posts used by the deck railing and just beefing it up with additional blocking.

Photos showing existing deck and planned pergola post placements

What the pergola will look like - except using 4x4 posts

Some project specs:

  • Deck is 12x10’; attached to the house and supported by two 6x6 posts set on 16”(?) concrete pillars; the posts at the base of the stairs are set in the ground which presumably provides a small amount of additional support for that corner
  • I will use 4x4s for upright posts (expecting some warping and cracking) because in my opinion the 6x6’s look too beefy for my small deck and I'm trying to keep the added weigh down
  • I will use 2x6s for the cross beams and rafters
  • This will be about 800lb of added weight, including the weight of the grape vine and any accessories, so 200lb per post
  • The pergola will not be attached to the house, but will be ~2 inches away from the house at the closest point

Location: Greater Philadelphia Area

UPDATE (05/26/2020): Decided I'm comfortable with skipping the concrete base. For the four corners I will be using this style of blocking and tying everything together with plenty of structural screws. Its might not be necessary but I am also considering adding a sister board to strengthen the cantilevered portion of the outer joist. The outer joists did not have joist hangers connecting them to the ledger board for some reason so I will be adding that as well as securing the joists under the posts with DTT ZMAX Galvanized Deck Tension Tie Kit.

I am sticking with 4x4 upright posts and 2x6's for everything else, the trellis top will get covered with shade curtains and later grapevines so I don't care too much if they look "puny" as long as they are solid and strong. Toja Grid makes extensive use of 4x4s in their pergola kits so that gives me confidence in the strength of (properly secured) 4x4s.

  • If you "just attach it to the deck by replacing the existing corner posts" I would be concerned about the outside posts that sit on the cantilevered corners. You could put a pier block and post under them for support. Probably best for the back ones as well. You could build your pergola around your deck to the ground, instead of on your deck.
    – Alaska Man
    May 23, 2020 at 20:01
  • There's this, which isn't very helpful : "consult an engineer with your structure plans" : Can I build a pergola on an existing deck, without digging more footers? ... "likely putting entirely too much stress on the ledger board."
    – Mazura
    May 23, 2020 at 21:33
  • You win the prize for most questions I've linked where none of them are duplicates.... and where all but one of them has an 'accepted' answer. And very few or zero upvotes, which is how I got the tenacious badge, so it's cool ;)
    – Mazura
    May 24, 2020 at 0:54
  • 1
    Use 6x6s for a pergola, they're needed for the aesthetic. If you use 4x4s it'll look weird. May 24, 2020 at 18:18

2 Answers 2

  1. Posts need bolts not screws.
  2. Outside joist board not sported by cantilever.
  3. Stair stringers held up by nails and lacking brackets.
  4. Doggo is helping.

4x4s will crack and warp, and look like they're not sufficient for the job, because they aren't.

How do I vertically connect two 4x4s on a deck to build a roof?

But you're not splicing (I hope) so it's, can you use an oversized post and make it too long. Sure. - Can you put what is essentially a roof over that. No, columns (which is what you're going to turn the post into) are to be 6x6 minimum. How should I connect two beams to a post at a corner?

That all being said, it's a pergola go nuts. I got some pretty good mileage out of structurally using some 4x4s : What size posts for elevated Kids play house?

Just use at least what the correct bolts for the posts should've been (and nuts and washers, all of the same material as required). Stainless steel for cedar, galvanized for treated.

[Two] 5/8" [through] BOLTS [MINIMUM] w/ MINIMUM 2" x 2" x 3/16" WASHERS, [each of them 1-1/2" from the top and bottom of the ledger board]. – Chicago Deck Code, PDF page 45

And select your lumber carefully; still wet and oozing pressure treatment? no. Cedar, yes.

  • #2 is kinda a kicker; you're going to ask it to carry some of the new load, which is itself supported by the ledger, which is only supported by all the other joists, none of which are properly attached to each other, because: 5. No joist hangers.
    – Mazura
    May 23, 2020 at 21:17
  • Also note, your deck is "attached to the structure and [is] a fire escape" - Using old roof footings for a ground-level deck. Alterations you make to an already shoddily constructed deck w/o correcting those problems puts you from negligence into gross negligence.
    – Mazura
    May 23, 2020 at 21:29
  • I see what you mean by the outside joist boards not being sufficiently supported and shouldn't be relied on to take a load. Would adding another 2x6/2x8 joist (whatever fits without having to remove decking) specifically to support the corner posts be an option? Can you expand on the type of bolting I would use?
    – ScrapeHeap
    May 23, 2020 at 22:54
  • @ScrapeHeap - "BOLT SIZE AND NUMBER SUFFICIENT TO TRANSFER LOAD" .... 6C DETAIL – GUARDRAIL POST W/ JOIST NORMAL TO EDGE (page 44) - 5/8" [through] BOLTS [MINIMUM] w/ MINIMUM 2" x 2" x 3/16" WASHERS – Chicago Deck Code, PDF
    – Mazura
    May 24, 2020 at 0:12
  • If this wasn't cantilevered and sat on actual 6x6s, there'd be a bracket with two bolts supporting the ledger board. And then three more into the ledger board. Chicago takes this pretty freaking seriously after a bunch of people died in numerous separate porch collapses.
    – Mazura
    May 24, 2020 at 0:33

I would build it around, not connected to, your deck.

Bury your posts 4 feet in the ground in-cased in concrete, Use a post protectors around them for longer life. ( With the post protectors you do not have to use concrete, i would. )

This will insure that racking is minimal with the lack of room for knee bracing that you have and what little racking there may be would not be transferred to your deck, which could weaken/loosen your deck over time.

Added bonus: You could make it bigger, wider, taller; extend it out from the house/deck further. Think Larger deck in the future, more grapes.

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