Posts to Support a Gazebo

[Kent Belasco]

I am building a gazebo in my yard. It is a 12 foot octagon. I will be drilling 9 post holes to support it, and my grade slopes so the back of my gazebo will be supported by a post, above the concrete footing by 4 1/2 feet.

Here is my question. I plan on drilling 12" diameter post holes at 42" deep using concrete form tubes. I will also be using 6"x6" composite posts to support the gazebo. Is it best to install the post in the post hole and then fill it around with cement, or is it better to fill the footing tubes with concrete and install a bracket to attach the 6x6 post, sitting on top of the concrete? Which is stronger and more secure, given that the back end will be 4 1/2 feet above grade (the front will be about 2 1/2 feet above grade)?

Thanks for any help you can provide.

[Jim Becker]

I always prefer to use brackets mounted to the footers. But you should also check with your local jurisdiction if there are any code requirements on this for your area. In many places, things like decks/gazebos, require their approval and maybe even inspections...as well as adherence to setbacks.

[Kent Belasco]

Thanks Jim,
My local building superintendent is indifferent on posts vs. brackets. What is your logic on brackets, are they stronger, longer lasting, or more flexible for removal if need be?
Kent

[Jim Becker]

If you put the post IN the concrete, it's pretty hard to replace in the future and also requires you to be extremely "exact" in setting the post. Using a bracket gives you some adjustability (even if it's with a heavy hammer...) should you need it and makes for easy repairs in the future. Oh, and putting the post in the concrete means you also need a much longer post...and that likely costs more money than a metal bracket.

[Thomas Thompson]

Kent, the metal mounting brackets are fast becomming the accepted norm in the construction industry, used even when supporting immense loads such as large decks 20' off the ground due to the slope of the property. They accomplish several things; allowing you to gain some side to side movement should your footing be off a tad & you need it to achieve plumb, they keep your pressure treated (doesn't apply to you) posts from direct contact with the ground, and probably most important, they save you $$$ since you're no longer paying for approx. 4' extra length per post to get you below the frost line.

Make sure you embed the correct size threaded L bracket into the center of each concrete footing should you choose to go this route.

[Ben Grunow]

Encasing the posts in concrete, if it is part of the design of the gazebo (thinking you might have purchased plans), will add strength to help keep the structure square and plumb during high winds. Eliminating that will require additional braces in the framing.

Probably reading into the question too much, but structures need to be built to withstand high winds. Seems obvious but I have seen some gazebos that are far away from their homes after a good hurricane.

Ben

[Jake Helmboldt]

Kent, whose brackets were you intending to use? When I contacted Simpson about this type of application a couple years ago they said they didn't have an applicable product due to the wind issue previously mentioned.

Anyone else have input on a good product?

jake

[Greg Deakins]

concrete will eventually rot the post, if it is holding the moisture from the post within it. Your best bet would be to dig your hole a little deeper, put a six inch or so footing of concrete at the bottom, and bury the post.

[Jerry Olexa]

I agree with using brackets. Imbedding the posts in cement will cause later rotting. The only time I did imbed in cement was @ my cabin where I was on a fairly extreme slope and needed extra strength for that particular post(s). 98% of time, I use brackets

[Joe Chritz]

It looks like you got your answer but here is mine.

It is "better" to use an insulating post bracket of some kind on a concrete footer. However, setting the post in concrete is just fine if the post is treated correctly.

Many hundreds of thousands of pole buildings are built just that way and last decades.

Many factors such as wind, snow loads, earthquakes and the like have to be taken into account and usually are only known by an engineer or building offical. If the building offical doesn't care then go with whatever is easier.

If you can drill for the bracket after the concrete is hard it will be much easier to get the post in the correct position.


Joe

[Carroll Courtney]

I agree w/Joe.My grandpa had a barn that lasted his lifetime.Since its going to be a stand alone bldg,I would be afraid of the wind for above ground fastners.I have fence poles that are the 4x4 treated that have been in the ground for about 10yrs and are still solid.

[Rob Wright]

Kent,

I agree with both Ben and Joe. I know - there are contradictions in the replies. I do agree that the concrete may cause rot of the treated posts if not installed correctly, however substationaly side shear strength is gained by installing the posts below ground to combat lateral forces such as wind without having to install addiitonal bracing.

The brackets installed on top of the footing area easy to install, allow for adjustment, and will ensure that rot of the post does not occur. Bracing will be required to stop side sway. All of the side load (wind Load) in this situation is carried at the post to joist connection since the bottom of the post is considered a "pin" connection in structures.

Try this:
Hold pencil vertical in your closed fist with the point at the bottom of your pinky. push the eraser end to try and rotate your fist. Now hold the pencil tip between your thumb and index finger and push on the eraser end. There is a noticable difference in the amount of force required to move the penceil out of plump. The concrete and earth surrounding the posts acts much in the same way as the first situation in stabilizing the rotational forces in an impebeded post situation. To get the same amount of rotational resistance, the structural connection would be much greater than what is sold at the local BORG.

To throw the last bit of information into this problem, and to further muddy the water even further, you will have a an 8 sided structure that will structurally behave quite differently than a standard deck attached to a house. The posts will likely act together as a system, along with the framing. Since this is a sloping ground sititaion, perhaps you may want to just embed the two or three longest posts on the downhill side. The other posts being shorter do not have as great of a moment applied to them, and therefore the joist connection and mounting plate may be able to handle the force without a lot of additional bracing. Alos easier to get all 8 posts near where they need to be. As always - just my $.02 and good luck with a successful project.

[glenn bradley]

He's using composite posts.

[Steve Kohn]

When I build my pergola I had to contend with and existing deck and concrete pool area. I was also concerned about wind load and cross-bracing since the sides were going to be partially open. I simple buried 4 of the posts in the ground and put the remaining posts onto brackets bolted into the concrete patio slab.

The pergola has been there for 5 years with no shift, heave, or wind movement.

[Dave Falkenstein]

In my neck of the woods, posts are placed on top of the concrete support pier, with a Simpson metal J-bracket imbedded into the concrete to nail (not screw) to the post. We also use a piece of 2X redwood on top of the post as a bug-free interface.

Are you sure you want to use composite posts? I was under the impression that composite material had very little strength, and should not be used as a supporting member for anything. If I were in your shoes, I'd be talking to your material manufacturer about the application. I'd hate to see your gazebo fall down in a strong wind.

[Brian Ross]

I also question the reason for composite posts. I added onto a deck two years ago. I built the original deck 20 years before that and the 6x6 support posts set in concrete were as good as the day I put them in.I would use 6x6 PT posts and ensure that the treatment is for below grade. I always dig the holes on the larger size to give me some room to move the posts around to ensure that I have an octagon. Once the hole is dug I put 8 inches of concrete in the bottom and let it harden. I have built a few 12 footers and have a template made up to position the location of the posts.. I make my template out of spruce and run strapping from one end point of the octagon to the next point beside it until you have a continous band around the outside of the Octagon. This ensures that the points of your template does not move while placing the posts. I have also built them on concrete piers but the ones with the posts in the ground are there to stay.

Brian