I am putting up a small pergola- 4′ x 8′, made of western red cedar with 4 corner posts that are 4x4s. I am planning on setting it on 4 concrete footings and mounting it with metal tie-ins. I don’t think I need forms larger than 6″ in diameter… but most I’ve seen in big box stores are 8′ or 12 ” Any reason to use larger forms? Other than building forms out of plywood, any other ways to do this?
Thanks,
Glaucon
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Replies
Sonotube goes as small as six-inch diameter.
Not sure about your conditions, (wind, and freeze depth), but you may well need more than the six-inch diameter to get your needed strengths.
Edited 7/24/2008 6:38 pm ET by Jigs-n-fixtures
A pergola this size is really an arbor. The overall weight of the structure will be ~120 lbs, mostly in compression. I'm using cedar for its rot resistance. The major issue, I think, will be lateral stability against wind shear, especially with vines growing on it. Most of the sources that I could find about such structures note that footings don't even need to be below the frost line, since mild heaving of an arbor or pergola is not so much of a concern as it would be for a larger, or fully anchored structure.I think if I tie the 4x4s to the footings, and reinforce the purlins where they meet the uprights, I should be OK. I'd just like to minimize the amount of digging and concrete.Glaucon
If you don't think too good, then don't think too much...
More like 120# per post, but the 6" will do fine. I would sink the posts in the ground and forget the crete. That way you have wind/racking resistance without adding diagonal kickers in the upper structure.Not sure what you mean by minor heaving. Depending on your soil, water, and frost conditions, you can see one corner walk right up all the way out of the ground while the others don't move at all.Regardless of whether you bury the post or set crete, you need to get below frost depth or you are likely to have wasted your time building this.
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Western red cedar does not do well buried in the earth.
It doesn't do well if kept wet /not allowed to dry/drain out after rain.
Eastern aromatic cedar (closet lining) does better especially if it is heartwood.
I vote for the 6" sonatubes and anchors with a 1/4" space between the bottom of the post and the metal post anchor for drying and wicking prevention.
The vines and plants may take a toll over the long run as they hold organic debris and moisture to the wooden structure. Due vilagence is needed to preserve the WRC pergola through the years.
.............Iron Helix
Get you materials from a good local lumber yard. You'll be surprised that most if not all the materials will probably be less money, plus the material is always better. HD and Lowes are ok for some things but I only get lumber there if I need it quick and other places are too far away or closed.
The cheapest, hillbilly forms I
know of are 6" chimney pipes.
>> but most I've seen in big box stores are 8' or 12 "<<
Oh come on.... Go for the 8' ones. BE A MAN!
:-)