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Forum topic by Thorxes posted 06-10-2021 07:31 PM 325 views 0 times favorited 13 replies Add to Favorites Watch
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Thorxes

34 posts in 249 days


06-10-2021 07:31 PM

Topic tags/keywords: pergola foundation concrete dimensions

I’m getting prepped to build a 12×12 Pergola (free standing) for my wife, and had a couple of questions:

1 – I’m in Northern Colorado and plan on setting the pergola on 4 concrete foundation posts that will be 36” deep into the ground using 12” sono tubes for the 6×6 posts I plan on using.. To make sure any movement is minimized, I was thinking about feeding rebar through the sono tubes at 2 locations (12 inches from the bottom and then 24 inches from that bottom), making an X at each location that would extend laterally beyond the tube a few inches. Is this advised, overkill, appropriate, etc.?

2 – Does anyone have a link to reference material outlining the dimensions needed for the lumber given the overall size/span of the pergola? It would be nice to have an overall reference, but right now I’m interested in whether 2×6 lumber would be appropriate for the “joists” running overtop and connecting the 6×6’s, for a 12×12 pergola, or if 2×8’s are required based on that size.


13 replies so far

View BlasterStumps's profile

BlasterStumps

2026 posts in 1559 days


#1 posted 06-10-2021 07:33 PM

Why not pour a slab for it to sit on?

-- "I build for function first, looks second. Most times I never get around to looks." MIke in CO

View JackDuren's profile

JackDuren

1600 posts in 2079 days


#2 posted 06-10-2021 08:05 PM

Mines 2×8 I wouldn’t do anything smaller..

View SMP's profile

SMP

4159 posts in 1025 days


#3 posted 06-10-2021 08:16 PM

Check the code in your area. There are also “calculators” on some of the sites, maybe Simpson Strong ties? Then go with whichever one meets code requirements.

View Thorxes's profile

Thorxes

34 posts in 249 days


#4 posted 06-10-2021 08:44 PM


Mines 2×8 I wouldn t do anything smaller..

- JackDuren


Yeah I was thinking the same – but was just curious if there was a reference document for all the sizes/relationships.

Curious – how did you attach the beams up top to the ledger at the house? Joist hangers?

View RClark's profile

RClark

139 posts in 3305 days


#5 posted 06-10-2021 09:13 PM

My pergola is 10’X14’ measure between the posts.

Posts are 6X6, joists (rafters?) are 2X8, and those 2X8s span the 14’ dimension. 54 2X2 purlins on top. All material is cedar. The posts are set 4 feet in the ground with packed gravel under and around the posts. Top cross members are 6X6, joined to the post with mortice and tenon, bolted through the tenon.

I’d go with 2X8s across the top. Two reasons:

- Strength across the span. You’re not planning to walk on top, but still…

- Proportion. 2×6 would tend to look small up there.

Couple pics:

Pic of one post with cross member fitted during construction.

-- Ray

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Thorxes

34 posts in 249 days


#6 posted 06-10-2021 09:15 PM



Why not pour a slab for it to sit on?

- BlasterStumps

The ground is too slanted for that. Going to just roll with breeze for the ground.

I’m also going to use 6×6’s that are a little taller on the low side to level everything off since we don’t want a pergola that’s too tall. Ideally we’d go with 8ft, but with the slant, one side will be slightly “taller”.

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Thorxes

34 posts in 249 days


#7 posted 06-10-2021 09:18 PM



My pergola is 10 X14 measure between the posts.

Posts are 6X6, joists (rafters?) are 2X8, and those 2X8s span the 14 dimension. 54 2X2 purlins on top. All material is cedar. The posts are set 4 feet in the ground with packed gravel under and around the posts. Top cross members are 6X6, joined to the post with mortice and tenon, bolted through the tenon.

I d go with 2X8s across the top. Two reasons:

- Strength across the span. You re not planning to walk on top, but still…

- Proportion. 2×6 would tend to look small up there.

Couple pics:

Pic of one post with cross member fitted during construction.

- RClark

I thought the 2×6s would look small too – that’s why we were going to push for 2×8’s either way, but it just spurred the question in my mind as to at what point would 2×8’s be a ‘necessity’ to support the load.

View squazo's profile

squazo

240 posts in 2765 days


#8 posted 06-11-2021 11:12 AM

The rebar thing sounds unnecessary. Besides it would rust away in no time.

View RClark's profile

RClark

139 posts in 3305 days


#9 posted 06-11-2021 11:41 AM


The rebar thing sounds unnecessary. Besides it would rust away in no time.

- squazo

+1.

I was going to mention this above. I, too, think the rebar is unnecessary. There just isn’t going to be the force on that cylinder of concrete that will cause a cracking issue.

Edit to add: I don’t think rebar sticking out the side of the cylinder will add any stability between the cylinder and the surrounding soil.

-- Ray

View tvrgeek's profile

tvrgeek

1941 posts in 2769 days


#10 posted 06-11-2021 01:21 PM

reference document is called “the building code”

I agree, in this case anesthetics is the important parameter. 2×6 would look weak. Also, work out the sun angles and you may want the deeper boards to manage the afternoon sun angle.

Don’t think the rebar in the footings would be any advantage. Just worry about wind lift loads. Not exactly sure of your local climate, but when I lived in Boulder, we had to deal with 100 MPH coming out of Left Hand. Even on an open frame, surprising forces. I would be more concerns with diagonal braces between the posts and upper beams. If on a deck, you might be able to bot bracing between the posts and joist supports. Racking is the usual cause for failure. That, and attaching it to the house. Safer if it is free-standing as any wind loads don’t get transferred.

View JackDuren's profile

JackDuren

1600 posts in 2079 days


#11 posted 06-11-2021 01:49 PM

Mine is on 12×36 deep holes.I went 4” above ground level to keep out of water.. I just used 6×6 post ancors from Home depot. $20 each I believe. I wanted no direct contact for rot… now I can mulch right up to the top of th 4”.

View gdaveg's profile

gdaveg

202 posts in 322 days


#12 posted 06-11-2021 03:18 PM

Use incised PT posts rated for ground contact. You can wrap in Cedar to make them look nice.

Would tend to use 14-inch Sono tube. The 6×6 post is about 8 inches on the diagonal leaving just 2 inches of cover from post corners to the soil. Concrete shrinks and cracks.

-- Dave, Vancouver, WA & Tucson, AZ

View PCDub's profile

PCDub

272 posts in 1364 days


#13 posted 06-12-2021 03:00 AM

In southern lower Michigan, frost level is 42” to avoid heaving of buried posts. Are you sure that 36” in Colorado is up to code?

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