I am posting here after a long time and this place is totally different! Apologies in advance if I post in the wrong forum.
I am redoing my deck this year, removing old PT decking boards and replacing them with cedar. Due to the sloping of the lot, the deck is about 4 feet of the ground. I was planning on adding a pergola over part of the deck. The pergola would be 8’ x 8’square, 8” high. I was thinking of going with 6” x 6” post. Any suggestions how to secure the post to the deck? Can I bolt them to the existing frame (2 x 10 joist on 16” centers) or do I take them all the way down into their own concrete footings?
Thanks
Replies
I'd say it pretty much depends on how solid your deck is, and whether you can easily devise a way to anchor the posts to it in a way that will resist sideways flex.
LB
Oddly, last week I walked over to a house just put up for sale (repo) next to one I was woking on. There was a deck with a "pergola" on top of it with (I think) 6x5 posts (4). They were anchored above the joists to brackets that surrounded the posts and came up maybe 6 inches. They were bolted down (flange) and then bolted to the 6x6's. I walked up and gave the posts a push. Not any movement. The apparatus up top took care of the sway and the post bases did their job holding it down. Check Simpson (Strongtie), thses brackets didn't seem special made.
Adding a structural detail...
like a pergola should by all rights be structured to add integrity as opposed to adding load. Decks are often engineered for the loads they will handle but what you're proposing would impact the load capacity you might find in such an environment as a neighborhood barbeque. Do you want to risk your pergola falling on everyone because it strained the existing decks capacity to the point that one Jenny Craig dropout tipped the scale just a tad too far causing failure? Take the support all the way to the ground and as you mentioned, use concrete beneath the posts for support. You can integrate the posts in your deck's structure quite easily once you've stripped the decking. For the peace of mind and not needing to get an engineer to sign off on it for liability reasons, you're time and money ahead to do this right. With just the dead weight of the pergola along with elements like high winds, people leaning against posts, and whatever other 'stressors' you might not foresee, you won't have future issues if you spend the time and a little more money now to do the job right.
However, a pergola actually reduces the number of people who can stand on the deck (and we were required to size our deck beams based on people standing shoulder-to-shoulder in a foot of snow).
If the weight of a pergola is going to threaten the structure then the deck should be rebuilt.
(There is a danger, though, of the pergola being torn off in a high wind and causing collateral damage. Whatever anchorage is used needs to be able to withstand substantial uplift, and this may mean somehow reenforcing deck planking fasteners to resist being torn loose.)
not enough info
Without knowing how stout your existing deck is, and without knowing how extensive the overhead structure on your pergola will be, it'd be best to recommend that you carry the load of the pergola down to earth.
Especially if the deck is attached to the house via a ledger board. If you can analyze that connection and remain within loading limits, fine. But it's best to not overload a ledger board connection.
Honestly, spread over the cost of the project, upgrading two posts from 8' to 12' and the other two posts from 8' to 16', the cost differential is minimal. And by carrying the load to earth a lot of the loading concerns can be eliminated. If you add knee braces of some sort where the pergola beams join the pergola posts, racking and lateral wind loading can be addressed.
If you carried the pergola posts to an on-grade or below-grade footing depending on your local requirements, then there would be no problem through-bolting the pergola posts to the deck structure. The synergy would improve the rigidity of the overall structure.
The open construction of a pergola isn't going to catch much wind, and the weight is minimal, actually reducing the number of occupants/sq.ft., as DanH mentioned.
I used a "rod-and-ring" setup to anchor a pergola last year.
You run a 5/8" all-thread rod up thru a centered hole in the bottom of the post, and secure it with a nut in a centered cross-hole that's drilled perpendicular to the hole for the rod, about 6-8" up from the bottom of the post. The bottom of the post gets another nut that tightens the ATR securely within the post, and you let the rod be long enough to pass down into whatever framing members are sturdy enough. Add what you might need belowdecks in the way of framing anchored with Simpson connectors that are appropriate. The rod passes thru with another washer and nut to pull everything tight.
The ring part is simply a 1" section of 3" PVC or ABS pipe that is sandwiched between the post bottom and the deck. It serves as a rot/corrosion resistant spacer that allows air and drying between the post bottom and the deck. The weight is on the ring, not the rod, so reinforce the decking section that the ring rest on.
Pergola on raised deck
Thank you all for your feedback. You have confirmed the doubts gnawing at the back of my mind. I will put the post on their own concrete footing.
Wind has become a big factor here in Edmonton. Things changed about 5 years’ backs with a freak windstorm (I am not including the 87 tornado). Since then wind storms in summer have become an issue. Wind gust over 50 kph are becoming more common.
Also the weather has been unusually dry. The soil in my backyard is mainly clay and without water develops fissure. Check out this article that came in the Edmonton Journal today about how the structural stability of houses and buildings is being impacted as the soil dries out.
http://www.edmontonjournal.com/Danger+from+below+threatens+homes/2927488/story.html
BTW, would 12" diameter footings suffice for a 6 x 6 post? As per code I will have to 48" deep on these footings.
Thanks again.