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Old 02-14-2006, 05:05 PM   #1
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Installation of pergola posts


I am ready to put up a pergola in my backyard in southern Arizona, but have some questions about the erection of the pressure-treated 4x4 posts. The soil here ranges from caliche-covered rocks to pea-sized gravel to sand and dust with a minimum of bio-mass. No topsoil to speak of. I don't know where the frost-line is (and have not been able to find a source of info by web-search), but it can-not be very deep . . . it's mid-February and it has not yet frosted this year.

I have heard some different ways to stabalize the posts, but need some advice on pros and cons.

1) Place the posts in the hole and simply backfill with "soil", tamping it down well. This seems easy, and using pressure-treated lumber in our dry climate, there should not be much rot or insect damage for some time, and even then the removal and replacement should not be difficult.

2) Place the posts in concrete, and fill the hole with concrete, surrounding the posts' bottom and sides. However, moisture will follow the timber down into the concrete well, and hasten rotting of the timber, at which time, removal of the old timber and concrete will be substantially more difficult.

3) Place the posts on top of some gravel (for drainage), and then fill the rest of the hole with concrete. Eventual removal of the timber and concrete will be essentially the same as option 2.

4) Fill the hole with concrete and top with a bracket for holding the post completely above the ground-line. This seems to be the easiest, and replacement of the posts (if ever needed) would be simple, but I am concerned with the hinge-effect at ground level where the post is fastened to the bracket, and the chance of it being damaged by wind.

Anyone out there got any advice? The holes are dug (to 24 inches), the pre-mix is on site, and I have 8-inch sure-forms ready for a pour.

Thanks!

Harvey

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Old 02-14-2006, 06:39 PM   #2
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Installation of pergola posts


Quote:
Originally Posted by HDelahay
I am ready to put up a pergola in my backyard in southern Arizona, but have some questions about the erection of the pressure-treated 4x4 posts. The soil here ranges from caliche-covered rocks to pea-sized gravel to sand and dust with a minimum of bio-mass. No topsoil to speak of. I don't know where the frost-line is (and have not been able to find a source of info by web-search), but it can-not be very deep . . . it's mid-February and it has not yet frosted this year.

I have heard some different ways to stabalize the posts, but need some advice on pros and cons.

1) Place the posts in the hole and simply backfill with "soil", tamping it down well. This seems easy, and using pressure-treated lumber in our dry climate, there should not be much rot or insect damage for some time, and even then the removal and replacement should not be difficult.

2) Place the posts in concrete, and fill the hole with concrete, surrounding the posts' bottom and sides. However, moisture will follow the timber down into the concrete well, and hasten rotting of the timber, at which time, removal of the old timber and concrete will be substantially more difficult.

3) Place the posts on top of some gravel (for drainage), and then fill the rest of the hole with concrete. Eventual removal of the timber and concrete will be essentially the same as option 2.

4) Fill the hole with concrete and top with a bracket for holding the post completely above the ground-line. This seems to be the easiest, and replacement of the posts (if ever needed) would be simple, but I am concerned with the hinge-effect at ground level where the post is fastened to the bracket, and the chance of it being damaged by wind.

Anyone out there got any advice? The holes are dug (to 24 inches), the pre-mix is on site, and I have 8-inch sure-forms ready for a pour.

Thanks!

Harvey
Option 4 is what I would do. Make sure that the bracket and all hardware you use on greentreated is a.c.q. approved. Not to sure what you mean by hinge effect of the post??? As far as the 24" that seems a little on the shallow end but I'm up north. Call your inspector and ask him how deep to make your holes. Is the pergola free standing??? If you do option 1 because it is easier go deeper with the posts....

bob

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Old 02-14-2006, 07:38 PM   #3
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Installation of pergola posts


Thanks for your quick reply, Bob. The "hinge-effect" I am concerned about is the ability of the post/footing to resist the lateral pressure of wind, since the only fastening (in the post, at least) is a couple of bolts through the bottom 4 or 5 inches of the post. It seems that there would be a significant weakness there over a post that is sunk into a hole in the ground. A friend suggests that my concern would be answered by "decorative" diagonal bracing at the roof of the pergola, but I am not totally convinced.
(By the way, I only went down 24 inches because that's about as far as I could reach with the tuna-fish can I was using to pull loosened soil out of the hole I was digging. I tried to look up how deep the frost line should be, but all references that I found simply said to go below the frost line, without saying how far down that is in my area! I will contact an inspector as you suggested.) - Harvey
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Old 02-14-2006, 09:53 PM   #4
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Installation of pergola posts


Quote:
Originally Posted by HDelahay
Thanks for your quick reply, Bob. The "hinge-effect" I am concerned about is the ability of the post/footing to resist the lateral pressure of wind, since the only fastening (in the post, at least) is a couple of bolts through the bottom 4 or 5 inches of the post. It seems that there would be a significant weakness there over a post that is sunk into a hole in the ground. A friend suggests that my concern would be answered by "decorative" diagonal bracing at the roof of the pergola, but I am not totally convinced.
(By the way, I only went down 24 inches because that's about as far as I could reach with the tuna-fish can I was using to pull loosened soil out of the hole I was digging. I tried to look up how deep the frost line should be, but all references that I found simply said to go below the frost line, without saying how far down that is in my area! I will contact an inspector as you suggested.) - Harvey
If your not totally convinced then set the posts in the ground, but 2' is unacceptable in my book. Bare minimum for me 8' high structure set posts 4' deep with gravel at the bottom. Your friend is right adding bracing going in opposite direction of joists (2x6) helps. Still don't know if it is free standing or attached to house??? That will make a big difference...

Bob
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Old 02-15-2006, 01:13 AM   #5
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Installation of pergola posts


Hi, Bob

I was able to find a Pima County Building Code newsletter from June 1995 which specifies that under 4000 feet the frost line is 0 inches, so that is not an issue here. The only problem I see is setting enough of the posts in the ground that they will not be knocked over by winds, but 4 feet under for 8 feet over? My understanding is that even power poles only have 1 foot under per 10 feet over, but I'm sure that there is a minimum hole depth. Although the pergola will be adjacent to the house (a manufactured home), I have been cautioned about attaching anything to the structure itself, but maybe a couple of stabilizing lag screws . . .
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Old 02-18-2006, 06:43 PM   #6
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Installation of pergola posts


go with the posts in the ground as far as you can dig them. get some post hole diggers. start laying out your holes and dig. brace the top corners anyways. gives you a place to habg plants from. Depends on the house siding whether you attach or not . if easier than digging a couple more holes. do it. if it means alot of carpentry work then dig. Make sure poles are square and plumb. Put the roof system on .sit back and ENJOY. If you put the posts on top of concrete you will always be concerned. As would I. Post a pic when done? Nick
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Old 02-20-2006, 08:34 AM   #7
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Installation of pergola posts


I'd go with your option #3. It's better having them in the ground. I do all mine 4' below grade but we have frost and winter here. For you, 3' - 4' should be fine.
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Old 02-20-2006, 08:42 PM   #8
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Installation of pergola posts


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I'd go with your option #3. It's better having them in the ground. I do all mine 4' below grade but we have frost and winter here. For you, 3' - 4' should be fine.
It sounds like he failed going deeper then 2'...
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Old 09-23-2006, 01:32 PM   #9
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Installation of pergola posts


This is the best looking pergola that I find so far on the market at such a great price from Costco.com. The craftmanship is "great to every detail". You can set it up in a few hours with its predrilled holes.

I heard that the MSRP is $3,999.99, but Costco is running a special deal on this high-end pergola for $2,599.99 (shipping included).

Definitely recommended. Visit the website address at

http://www.costco.com/Browse/Product...rodid=11160592

<img src="http://content.costco.com/Images/Content/Product/126545.jpg" />
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Old 10-04-2006, 01:11 PM   #10
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Installation of pergola posts


I live in the Dallas area of Texas and our frost line here is about 24 inches. I built an arbor a few years ago and the depth of my holes was around 3 feet with 6 inches of gravel at the bottom, it also was non-attached, used option #4 above, and it was still standing solid when I moved out.
I'm currently in the process of designing a pergola for our new home. I also saw the add in Costco, then looking through my old issues of Wood magazine, I came across plans for pretty much the exact same pergola that Costco has. It's in the April 2002 issue 141. I called around, and the material cost of the cedar with delivery is about $800. I figure I'll do it myself, save a few bucks, and have something to brag about when people come over.

Last edited by DWrath; 10-04-2006 at 04:24 PM.
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Old 10-06-2006, 12:29 AM   #11
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Installation of pergola posts


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I live in the Dallas area of Texas and our frost line here is about 24 inches. I built an arbor a few years ago and the depth of my holes was around 3 feet with 6 inches of gravel at the bottom, it also was non-attached, used option #4 above, and it was still standing solid when I moved out.
I'm currently in the process of designing a pergola for our new home. I also saw the add in Costco, then looking through my old issues of Wood magazine, I came across plans for pretty much the exact same pergola that Costco has. It's in the April 2002 issue 141. I called around, and the material cost of the cedar with delivery is about $800. I figure I'll do it myself, save a few bucks, and have something to brag about when people come over.

Yeah if you want to build one yourself, definitely save a lot. However, cedar is a soft wood, and won't last as long as the imported premium shorea, which is much denser and tougher. Plus, shorea does not have knots like cedar. You'll see the difference between a $800 pergola and a $4000 pergola right away.

Interesting that Costco just added a gigantic coupon $600.00 to this pergola. It looks like they try to sell all of them before the winter comes.

http://www.costco.com/Browse/Product...rodid=11169519
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Old 10-06-2006, 06:14 PM   #12
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Installation of pergola posts


Yes, I agree cedar in general does have more knots than premium shorea. As far as whether is will last as long as western red cedar outdoors for this application, they are about equivalent. Both, if you don't stain or finish them will develop a gray (patina) coloration. I guess it depends on the look you're going for. Though if I were choosing the species of wood and cost was not too much of a limiting factor, I'd go with Balau, and if cost was not a factor, teak. BTW, I'm a huge Costco fan and have always been impressed with the quality of the products they carry which represent a solid value.
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Old 10-11-2006, 01:07 AM   #13
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Installation of pergola posts


to noproblem:
Is that the pergola in Costco.com has the stones under each post? Is that helpful for stablizing the whole structure?

Instead of digging and filling, why dont we use any thing heavy to stablize the pergola if digging is not a part of setting up process, really.
Sorry about unrelated information, but i'm going to set up a pergola and have the same problem with him.

Mike,
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Old 10-11-2006, 06:46 AM   #14
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Installation of pergola posts


That would work if you have a sound foundation, like laying it on a concrete slab. The problem would be if it was just placed on the ground, that "anything heavy" stabilizer would be begin to settle into the ground and the pergola would become crooked.

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