# Securing a two-post pergola!



## Danno15 (Jun 14, 2014)

Hi everyone,
I'm looking to build a two-post pergola along a fence line and need input on footings. There will be two 6x6 posts spaced approx 15ft apart with a 16ft 2x6 running on each side of the beam. Then I'll be using 24in 2x4 rafters down the beams. Since it will only have the two posts, do I need to sink the posts into the footing or use special post bases?


Dan


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## Robpo (Mar 30, 2014)

Danno15 said:


> Hi everyone,
> I'm looking to build a two-post pergola along a fence line and need input on footings. There will be two 6x6 posts spaced approx 15ft apart with a 16ft 2x6 running on each side of the beam. Then I'll be using 24in 2x4 rafters down the beams. Since it will only have the two posts, do I need to sink the posts into the footing or use special post bases?
> 
> 
> Dan


How high and what is the other end attached to. Make it clear so we don't have to guess what you are doing.


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## wkearney99 (Apr 8, 2009)

And what does the local permitting authority have to say about it?


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## Danno15 (Jun 14, 2014)

Sorry! Two posts, 6x6's will be about 8ft high. Attached picture is similar to what I'll do. In this picture, they use the post base but I'm concerned whether it'll stand the test of time. I live in the Pacific Northwest!









Dan


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## Robpo (Mar 30, 2014)

Danno15 said:


> Sorry! Two posts, 6x6's will be about 8ft high. Attached picture is similar to what I'll do. In this picture, they use the post base but I'm concerned whether it'll stand the test of time. I live in the Pacific Northwest!
> View attachment 86006
> 
> 
> ...


I can't think of any good options.
Pressure treated 6x6"s in cares in concrete footings. 
galvanized steel in concrete footings, wrapped with cedar held off the concrete.
Best I can come up with.


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## Daniel Holzman (Mar 10, 2009)

I like the design, pretty elegant. I would not make that out of PT lumber, a design like that seems to call for douglas fir, redwood, or western red cedar, or a similarly attractive wood.

As to the base. There is very little load on the pergola normally. The worst case would be high winds, which would cause rotation around the base. A bracket such as shown in the picture is not really designed to resist rotation, it is considered a pinned joint, and is primarily intended to resist shear forces. Typically the structural design would incorporate diagonal bracing to resist rotation, but a single post pergola is not going to have diagonal bracing (unlike a deck), so there will be little resistance to rotation.

So here is my suggestion. Although embedding wood directly in concrete is normally not recommended due to rot issues, it can be done. The trick is to select a type of wood that is naturally resistant to rot when in contact with concrete underground. Western red cedar and redwood meet this criteria. The other key is to be careful casting the concrete to make the concrete as impervious as possible, which means you should use a dry, high strength mix, and vibrate the concrete to insure maximum density (maximum density normally means minimum porosity). See this article for more discussion about wood posts embedded in concrete http://www.ehow.com/how_7890917_set-concrete-preserve-wood-rotting.html.

A typical rule of thumb for embedment of piles is that the pile should be somewhere around 40% as deep as it is high, so if you had an 8 foot high post, you would want to bury it at least 3-1/2 feet deep. This will develop good resistance to rotation in most soils. The hole should be at least twice as wide as the post, so for a 6x6 post, you can probably use a 12 inch sonotube form. Bring the concrete to the surface, and dome it up a little so water does not puddle next to the post.

Your post will not last forever, but what does, so you should be able to enjoy your project for a long time.


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