Using posts/columns to support a deck and gazebo
September 5, 2021 8:04 PM Subscribe
I'm working on a design for a low deck (~18"/45cm above grade) with a gazebo above the deck, and I'm trying to figure out the best way to support both the deck and gazebo.
The gazebo is going to be wood-framed with an asphalt shingle roof and thus heavy, meaning I don't want to build the deck and just set the gazebo on top. It seems like I should be able to run 6x6 posts from concrete footings up to the beams for the gazebo, and be able to support the deck from those same posts.
The problem I'm trying to work through is how to make the connection from the posts to the main deck supports. Simply bolting them to the posts seems like a bad choice--the bolts won't shear, but I'd be putting a lot of stress on the wood around the bolt hole. Notching the posts also seems like a bad idea as I don't want to thin the posts out 8' below the roof--racking stresses would be a big worry, I think. I could cut the posts and place the deck beams on top of the posts and then attach the gazebo posts to the top of the deck beams--that feels a little janky, but I don't have any other ideas.
I've failed to find designs that illustrate this problem, and don't know how to frame it for a search engine.
Opinions?
The gazebo is going to be wood-framed with an asphalt shingle roof and thus heavy, meaning I don't want to build the deck and just set the gazebo on top. It seems like I should be able to run 6x6 posts from concrete footings up to the beams for the gazebo, and be able to support the deck from those same posts.
The problem I'm trying to work through is how to make the connection from the posts to the main deck supports. Simply bolting them to the posts seems like a bad choice--the bolts won't shear, but I'd be putting a lot of stress on the wood around the bolt hole. Notching the posts also seems like a bad idea as I don't want to thin the posts out 8' below the roof--racking stresses would be a big worry, I think. I could cut the posts and place the deck beams on top of the posts and then attach the gazebo posts to the top of the deck beams--that feels a little janky, but I don't have any other ideas.
I've failed to find designs that illustrate this problem, and don't know how to frame it for a search engine.
Opinions?
This design from Mother Earth News uses notches on the posts for the perimeter joists and then joist hangers for the radial joists.
posted by doctord at 8:58 PM on September 5, 2021
posted by doctord at 8:58 PM on September 5, 2021
Response by poster: The beams will be carrying joists, yes. And I'll also be using post bases to attach to the foundation.
I hadn't thought of using something like the flange hanger; in the designs I've been working on, the posts are inset from the corners which had me thinking that the beams would be cantilevered, but now I'm thinking that moving the posts to the corners actually works aesthetically and structurally.
posted by Ickster at 9:02 PM on September 5, 2021
I hadn't thought of using something like the flange hanger; in the designs I've been working on, the posts are inset from the corners which had me thinking that the beams would be cantilevered, but now I'm thinking that moving the posts to the corners actually works aesthetically and structurally.
posted by Ickster at 9:02 PM on September 5, 2021
It would be great to see a sketch of your plan. Is this a free-standing structure? How large? It seems like your opportunity to fight racking is with the upper structure, where the wind will push the vertical post levers, since you don't have the space below-decks to install diagonal bracing.
posted by bullatony at 6:52 AM on September 6, 2021
posted by bullatony at 6:52 AM on September 6, 2021
I don't like putting the bottoms of structural wooden posts anywhere near the ground, because wood is a vegetable and soil eats vegetables.
The deck out the back of our place is built on an array of concrete stumps, each stump concreted into a somewhat pyramidal hole roughly 400mm square at a base between 450 and 600mm below ground (the stumps were all the same height so the holes had to be of varying depth to accommodate a slight grade).
Each stump has a reinforcing rod that's extended upward through the top of the concrete. That goes through a hole in a galvanized steel ant cap, then up between the paired 45x120mm treated pine members we used for bearers, then gets hammered over the top of one of the pair to make an inverted square J to hold the bearer down to the stump. A pair of lag screws placed maybe 150mm either side of each stump rod holds the halves of the bearer together. The hammered-over stump rods alternate between bearer halves as you go down each row of stumps. I poured enough boiled linseed oil over the reinforcing rods to get them thoroughly wet after they got hammered down, because water plus treated pine can be unkind to steel.
45x90mm treated pine joists run across the bearers, held down with diagonally opposed pairs of galvanized steel joist straps at the intersections. My builder mate told me that one of these at each joint is plenty but I'm happier with two.
The posts that hold up the roof each had a 120x20mm rebate cut into the bottom and got bolted into the side of a bearer, right next to a joist and very close to a stump so that the transferred weight of the roof doesn't try to flex the bearer. The joist layout puts one right next to each post, so another bolt can hold the post and joist together.
This is plenty strong enough to bear the weight of any amount of roof, but it doesn't have much resistance to racking as-is, so there needs to be diagonal bracing built into the rest of the framing to deal with that. We used crossed diagonal pairs of 6mm galvanized steel wire rope, run between 8mm galvanized steel eye bolts and tensioned with turnbuckles, run from the joint between the post and the roof beam on one side to maybe 700mm down the next post on the other side. Another pair of crossed wire ropes goes horizontally from corner to corner, just below the roof beams. This yields really good rigidity and makes a nice support for ornamental vines as well. I had to tighten all the turnbuckles about a year after the structure first went up to deal with a tiny bit of shrinkage, but haven't needed to adjust them since.
posted by flabdablet at 8:42 AM on September 6, 2021 [1 favorite]
The deck out the back of our place is built on an array of concrete stumps, each stump concreted into a somewhat pyramidal hole roughly 400mm square at a base between 450 and 600mm below ground (the stumps were all the same height so the holes had to be of varying depth to accommodate a slight grade).
Each stump has a reinforcing rod that's extended upward through the top of the concrete. That goes through a hole in a galvanized steel ant cap, then up between the paired 45x120mm treated pine members we used for bearers, then gets hammered over the top of one of the pair to make an inverted square J to hold the bearer down to the stump. A pair of lag screws placed maybe 150mm either side of each stump rod holds the halves of the bearer together. The hammered-over stump rods alternate between bearer halves as you go down each row of stumps. I poured enough boiled linseed oil over the reinforcing rods to get them thoroughly wet after they got hammered down, because water plus treated pine can be unkind to steel.
45x90mm treated pine joists run across the bearers, held down with diagonally opposed pairs of galvanized steel joist straps at the intersections. My builder mate told me that one of these at each joint is plenty but I'm happier with two.
The posts that hold up the roof each had a 120x20mm rebate cut into the bottom and got bolted into the side of a bearer, right next to a joist and very close to a stump so that the transferred weight of the roof doesn't try to flex the bearer. The joist layout puts one right next to each post, so another bolt can hold the post and joist together.
This is plenty strong enough to bear the weight of any amount of roof, but it doesn't have much resistance to racking as-is, so there needs to be diagonal bracing built into the rest of the framing to deal with that. We used crossed diagonal pairs of 6mm galvanized steel wire rope, run between 8mm galvanized steel eye bolts and tensioned with turnbuckles, run from the joint between the post and the roof beam on one side to maybe 700mm down the next post on the other side. Another pair of crossed wire ropes goes horizontally from corner to corner, just below the roof beams. This yields really good rigidity and makes a nice support for ornamental vines as well. I had to tighten all the turnbuckles about a year after the structure first went up to deal with a tiny bit of shrinkage, but haven't needed to adjust them since.
posted by flabdablet at 8:42 AM on September 6, 2021 [1 favorite]
Concrete stumps are also available with threaded exposed reinforcing rods, if you'd prefer a more positive hold-down than hammering and bending will get you.
posted by flabdablet at 9:11 AM on September 6, 2021 [1 favorite]
posted by flabdablet at 9:11 AM on September 6, 2021 [1 favorite]
Simpson has a number of specialized gazebo connectors (roof framing & floor framing), which are worth looking at if you're still in the design phase.
posted by aramaic at 9:59 AM on September 6, 2021
posted by aramaic at 9:59 AM on September 6, 2021
Response by poster: This is actually a square gazebo design rather than the more traditional hex/octagon. I'm definitely installing the posts above-ground with an air gap between the concrete footer and the base of the post (not sure whether I'm digging and pouring or going with something like a diamond pier, which is a question of cost vs. labor and a bit of a concern about tree roots).
posted by Ickster at 5:59 PM on September 6, 2021
posted by Ickster at 5:59 PM on September 6, 2021
6x6 is a pretty damn chunky post. You could easily afford to notch out both sides 3/4" deep to bolt doubled bearers into place and they'd still be super strong. That joint would certainly outperform anything involving cutting all the way through the posts at deck level.
I'd still use crossed diagonal tension braces high up on the posts in the vertical plane, and corner to corner under the roof in the horizontal plane. The difference in rigidity with and without them is night and day and there is no way that a diagonally braced structure is ever going to generate enough racking motion to overstress either the bearer-to-post joints or the post-to-foundation joints.
posted by flabdablet at 6:53 PM on September 6, 2021 [2 favorites]
I'd still use crossed diagonal tension braces high up on the posts in the vertical plane, and corner to corner under the roof in the horizontal plane. The difference in rigidity with and without them is night and day and there is no way that a diagonally braced structure is ever going to generate enough racking motion to overstress either the bearer-to-post joints or the post-to-foundation joints.
posted by flabdablet at 6:53 PM on September 6, 2021 [2 favorites]
Consult the prescriptive code for decks, please! Beams must bear directly on posts, they cannot be bolted or nailed to the side. The gazebo itself may also be regulated by the code for structures, if it is a walled structure.
posted by agentofselection at 4:30 PM on September 7, 2021
posted by agentofselection at 4:30 PM on September 7, 2021
This thread is closed to new comments.
I'm going to assume that you're using something like a Simpson post base to attach the posts to the foundations. Simpson (and other manufacturers) also make things for other wood connections that you'll probably want to use at some point in the deck or gazebo construction anyway. One thing might be to have the post interrupt the beam and put a concealed flange hanger on either side to attach the beam(s) to the post. Another might be to sister another piece of wood below the deck level to the post and have the beam bear on that, plus an angle on either side of the beam to keep it upright. I've only linked to representative Simpson product, you might need something a bit different.
Are the beams directly carrying the decking, or is there another layer of joists above them?
posted by LionIndex at 8:37 PM on September 5, 2021 [1 favorite]