How to Set Structural Posts for a Garden Arbor
If you’re planning on building an arbor structure of your own, bear in mind that it’s going to need some legs to stand on. And those legs, or posts, represent a super-critical structural element of any arbor system. They’ve got to be installed correctly, or your whole arbor project will tank. Seriously. This article is dedicated to outlining one thing, and one thing only: How to set structural posts for a garden arbor.
How to Set Structural Posts for a Garden Arbor
This past January, I wrote an article here which more or less synopsized the arbor/trellis building project which I undertook in my garden last fall. In that article, I provided a basic overview of the project, as well as a number of progress pics and finished product shots. I also promised to write a series of how-to articles specifically about building/installing some of the structure’s primary elements. So that’s what’s going on today – this one’s about installing structural posts, one of the three main types of structural element in an arbor/trellis system like mine. And although anyone with even a passing interest in such garden structures will hopefully still find this article interesting, it’s definitely going to provide some really basic, practical nuts and bolts information for those of you who are thinking about tackling a project like this in your own gardens. (I’m simultaneously publishing a separate article here about installing another one of the three pimary structural elements of my arbor/trellis system: the ledger board. To read about installing a ledger board, click here. And if you’d like to cheat, you can click here to read about building and installing the third main structural element of an arbor/trellis system: the girders.) So what you’re going to get with this article right here is a basic step-by-step on how to set structural posts for your arbor/trellis, and my recommendations of some of the materials and tools which I believe will be helpful to you when you start building. I’ll also recommend a couple of books that’ll help you come up with a plan for your structure.
Ok, here you go – here’s how to set those posts.
1) Determine the nature of your structure. You’ve got to have a handle on what you’re building before you build it. I talk about this, and about the need for a plan, or drawing, extensively in my January article. My structure consists of two main components: an arbor, and a trellis. The trellis part rises vertically from the ground, close to, and parallel with, the side of my house, and it attaches to the arbor at the ledger board, which serves as the juncture, or tie-in point of the trellis, the arbor, and the side of my house. By virtue of this tie-in point on the side of the house, my arbor/trellis system is not considered a free standing structure. It’s attached to, and partially supported by, the house itself. Again, you’ve got to know what you want to build before you do anything else. I envisioned, designed, drew, and built a structure which attached to my house, covered a big blank expanse on that part of the house, and extended, canopy-like, out into, and over, part of my garden.
2) Determine the height of the posts. How tall do you want your structure to be? Whether you’re building a structure like mine (supported by a ledger board on one side, and structural posts on the other), or an entirely free standing structure, you need to know how tall you want it to be. So you’ll need to know how tall your support posts need to be. You’ll need to know what length of post will be sticking out of the ground. The top of your arbor structure is going to happen at a point at or near the tops of your support posts.
3) Determine your structural posts’ locations. If you’ve planned properly for your arbor/trellis and are on your way to building and installing it correctly, then you’ve probably got an existing working drawing you’ve either created yourself, or one you’ve obtained from another source. Your drawing will specify dimensions and locations of your support posts. There are a number of means for physically determining the actual, field locations of your structure’s support posts (utilizing carpenter squares, string lines and/or stringer boards, or a theodolite/transit level). Whatever your means, I make the following suggestion: establish the locations of the two end, or terminal posts, and install them first. (Item numbers four, five, and six here – the next three on this how-to list – will outline how to dig the holes and set the posts – both terminal and intermediate – for your arbor.) Once the terminal posts are in place, you can tightly stretch a string line or stringer board(s) between them (attached to the face of each terminal post) to keep the intermediate posts in an exactly straight and consistent line. Mark off the distance between, and location of, each planned intermediate post on the string line or stringer board. Drop a plumb line from each location mark on the string (or stringer board) to the ground. Presto! You’ve got your intermediate post locations in perfect line with your terminal posts. This critical step ensures that you’ll dig your post holes in exactly the right locations.
4) Dig your post holes. The rule of thumb for digging post holes is as follows: About one third of the total length of the post should be in the ground, and the diameter of the hole should be a minimum of three times the width of the post material.
I’ll first use the three main structural posts of my arbor as an example of how I implemented (and extended) these excavating requirements. I knew that I wanted the tops of my posts to stick up 8 feet out of the ground. I knew that if 1/3 of the overall post’s length was going to be anchored in the ground, I’d need to start with a board 12′ in length and bury 4′ of it in the ground. And since the material I used for my posts was 6×6 treated structural wood (actual dimensions 5.5″x5.5″), I knew the diameter of the hole would need to be about 18″. But there were some other considerations associated with setting massive posts like these which led me to excavate holes that were 5′ deep and 24″ in diameter. After spending many years in the real estate development/building/manufacturing/construction industries, I’d acquired some pretty decent construction knowledge by the time I built and installed my arbor/trellis system. Plus, I had the added benefit of talking with Manny, the leader of the installation crew which, at the time of the start of my arbor trellis project, had just completed the installation of my new wood fence. I knew (and Manny confirmed this) that a 6″ layer of gravel at the bottom of any post hole would allow for the drainage of water away from the buried post material. And additional concrete (still dry mix at this point) added to a depth of about 6″ below the bottom of the buried end of the post would prevent any sinking or settling of the new post (again, Manny had practical experience with this kind of thing and gladly offered me this advice before I started my arbor/trellis project). Further, the 6×6 posts I was using as structural support were each extremely heavy. And the horizontal framework of the arbor’s top would add to the weight and increase deflection (wobbling/swaying) tendencies of the already heavy lumber. So my three arbor structural post holes, which my good friend Zen dug singlehandedly (Zen is a stud, no doubt about it), were extra deep and extra wide – 5′ deep and 24″ in diameter. Again, this was done to add 6″ of drainage gravel and 6″ of still-dry concrete mix below each post, and an encircling mass of concrete 24″ in diameter around each post, in order to keep this extremely heavy lumber from settling/sinking and deflecting.
The eight structural posts for the trellis component of my structure were different from the three arbor posts I installed, both in terms of the dimension and length of the boards used, and the nature of the holes I dug to anchor them. These support posts were used to provide the basis of the structural framework of the lattice material upon which my future vines would begin their respective vertical climbs. Each of these support posts were, at their tops, firmly attached to the arbor/trellis system’s ledger board. So each was anchored in concrete at and below ground level, and attached essentially to the side of the house at its top, as well. Deflection would not be a concern here. And the 2×4 boards I used were 10′ in length. Since 7.5′ of each board is what’s sticking out of the ground, only 2.5′ is below ground. Again, deflection is not a concern here because of the top and bottom anchoring methodology I employed. I will mention, however, that the holes I dug to set these posts (into which concrete would be poured) were still approximately 3′ in depth. They were small in diameter because of the comparatively slender 2x4s I used, but they were still deep. The reason for this depth was to achieve a solid concrete base poured to extend down beyond my area’s frost line. I take no chances with the depth of post holes and the ability of alternately freezing and thawing earth to heave posts up out of the ground. I’ll note here that although the ground’s substrate on my land is sand (which does not retain groundwater and, therefore does not freeze and heave in freezing temperatures), I had excavated this particular area of my garden (back in the spring/summer of 2020) and replaced the sand with organic garden soil and potting soil to a depth of approximately 24 inches. This garden/potting soil substrate does freeze and heave in freezing temperatures, hence, my decision to dig these particular eight post holes to the depth of 3 feet. When digging post holes, always refer to your municipality’s building code for frost line excavation requirements and/or suggestions.
A note about the grade of your land. The procedures described in this step assume that the grade of your land is level. The ground in which I set my arbor’s structural posts was perfectly level from side to side, and from front to back. The length of the material you’re planning to use for your structural posts, and the corresponding required depth of the holes in which you’ll set those posts, will vary with, and will need to be adjusted in order to compensate for, any sloping of your land that occurs along the length and/or width of your planned arbor/trellis structure.
5) Set your posts. This is typically a two-person operation, so make sure that you’ve got a helper. Now that your holes have been excavated, it’s time to set the posts. Again, for large, load bearing posts, I recommend digging the holes a foot deeper than the depth required to bury one third of the post’s length below grade. (Again, for my 12′ posts, I dug 5′ deep holes rather than 4′ deep holes.) This is to accommodate first a 6″ layer of gravel, then a 6″ layer of dry concrete mix upon which the post will set. Once these layers are added, set the post into the hole. Use the string line, stringer board or theodolite/transit level to check above grade height for the terminal posts, and the string line or stringer board (which you’ve strung from the face of one previously set terminal post to the other) to check the above grade height and linearity of the intermediary posts. You may need to lift each post out and add concrete to bring it up to the proper height, or slightly “corkscrew” the post down into the layer of concrete to lower it if it’s sitting up too high. Once the post is in the right spot and at the right height, hold a high-quality 4′ or 6′ construction level against all four sides of the post to make sure it’s perfectly plumb. Have your helper hold the post steady in this exact position (or if you’re using a stringer board, clamp the post to the stringer board to keep it in position), and move on to the next step.
6) Finish filling the holes with concrete. Once the bottom of your post hole is lined with a layer of gravel for drainage, and a layer of dry concrete mix (as described above), and your perfectly plumb post is in place at the right height and location, it’s time to add the concrete. The concrete you want to use for setting posts of this type is fast-setting dry concrete mix designed specifically for setting posts in the ground. I used Sakrete brand fast-setting concrete mix. The beauty of this type of product is that it requires no pre-mixing with water whatsoever. Once the post is in place in the hole, add the dry concrete mix to a point of about halfway to the top of the hole. The product I used came in 50 pound bags, and required the addition of one gallon of water per bag. When you’re doing this, keep track of the number of bags of dry concrete you’ve added to the hole (including the 6″ inch layer that the post is resting on) in order to get to the halfway point. If 10 bags of concrete took you to the halfway point, add ten gallons of water directly to the concrete in the hole. Allow the water to seep down into the dry concrete. Then add more dry concrete until the hole is nearly, but not quite filled, again keeping track of how many bags of concrete you’ve used. Add the corresponding amount of water. And again, let the water seep down into the dry mix. At this point you can add enough dry mix until the hole is slightly overfilled. Add the corresponding amount of water and, with your hands or a trowel, form the concrete into a mound sloping up from the outer lip of the hole to the post. Once the concrete dries, this sloped mound will allow precipitation to drain down and away from the base of the wood post. When you’re filling these holes with concrete and water, you need to act quickly. This type of concrete, while extremely easy to use, does dry and harden quickly.
The above six steps basically outline the long and short of structural post setting. They represent the methodology I used to set all of my arbor/trellis system’s structural posts. It was a methodology that worked perfectly for me, and is one that I believe will work perfectly for you.
Some Important Items to Note:
- Before doing any excavating of any kind on your property, it’s critically important to contact the appropriate utility marking service(s) for your area to have any and all utilities/utility service lines located, identified, and marked. Don’t do any digging at all until you get this done!!!
- Before beginning any construction project, contact your municipality’s building department. Before doing any work, make sure to obtain any and all required building and/or construction permits.
In addition to my six step post setting plan, I’m offering a few product recommendations below. As I already mentioned, I used Sakrete Fast Setting Concrete Mix to set my posts. And I used the two tools advertised here not only during the process of setting my structure’s posts, but throughout the entire fabrication, construction, and installation process. And I’ve used them for countless other construction projects over the years. They’re great tools, and they’ve proven invaluable to me.
You’ll also notice that I’m recommending two books. I’ve recommended them in the past, and will continue to do so. Although I designed my arbor/trellis system myself and developed my own construction plan and working drawings, I enthusiastically recommend these books to those of you who want to use excellent existing tried and true designs/plans/drawings. I’ve read through them in the past and found them both to be intelligently written and beautifully illustrated with highly useful drawings and/or photographs.
Sakrete Fast Setting Concrete Mix proved to be an incredible product. It was remarkably easy to use, and it set up quickly and with great strength. Just pour it in the hole and add water. No mixing required. Love this stuff! It worked perfectly for setting all the posts for my arbor/trellis system. I can’t offer ordering options for this excellent product here on The Renaissance Garden Guy, but you can click right here for more information and purchasing options.
The Johnson 48″ aluminum box level is an absolute job site must. This unit is accurate and remains permanently and perfectly straight and true due to its precision milled, aluminum construction. It perfectly accommodates dozens of different applications during the construction of an arbor/trellis system. I’ve got an older yellow Johnson box unit which I used almost constantly during my project. It was infallible. This new orange version has got the same excellent quality, precision, and accuracy as my older yellow model. Click the #advertisement link to learn more or to order it here, directly from Amazon.
The Johnson 72″ aluminum box level brings the same level of durability and precision as the 48″ unit to your jobsite, but in a sometimes necessarily longer tool. I’ve got an older yellow version of this 72″ box unit. When checking long spans, or when Zen and I needed to read it from two different locations, the unit came in incredibly handy. Perfect accuracy over a longer run. This current version brings the same level of craftsmanship and accuracy to the table as my older yellow model. To learn more, or to order it here, directly from Amazon, please click the #advertisement link.
Trellises & Arbors: Over 35 Step-by-step Projects You Can Build, from Sunset Books. This is a really terrific book with lots of beautiful ideas for arbor and trellis structures. I wish I had a picture of the book to include here, but my memory of its excellent content is the best I can do right now. What I remember most about it was the fact that it has really easy to follow instructions with excellent pictures. These can be used “as is” or can be easily adapted to fit the conditions in your garden. An excellent book with lots of great ideas. To learn more, or to order it here, from Amazon, please click the #advertisement link.
Trellises & Arbors: Over 35 Step-by-step Projects You Can Build, from Sunset Books
Click here to learn more or to order
#advertisement
Trellises, Arbors & Pergolas: Ideas and Plans for Garden Structures, from Better Homes & Gardens. This is an awesome book. Although it’s been years since I’ve seen it, I remember it had loads of ideas and a lot of actual construction drawings to go along with them. I would feel totally comfortable building an arbor from any number of the designs and drawings I’d seen in this book. Again, all I’ve got are my memories of its excellent content. Sorry, no pic. Great book with great ideas and excellent detail. To learn more, or to order it here, directly from Amazon, please click the #advertisement link.
Trellises, Arbors & Pergolas: Ideas and Plans for Garden Structures, from Better Homes & Gardens
Click here to learn more or to order
#advertisement
So now you know how to set structural posts for a garden arbor, and that pretty much wraps this one up. And, I’ve just simultaneously published, here on The Renaissance Garden Guy, the sister article to this one. As discussed at the beginning of this article, the sister article’s about installing a ledger board for a garden arbor and trellis system like mine. It’s pretty concise with some practical photos and useful diagrams. These two sister articles cover the installation of what I consider to be two of the three primary structural elements of a non-free standing garden arbor and trellis system. The installation of the third element, the girders, or outriggers, will be covered in an article I’ll be publishing here really soon (but, as I’ve mentioned, you can cheat, and click here to read that one right now, too). My whole plan with the timing of these instructional articles was to get this information into your hands toward the end of winter here in the Northern Hemisphere, so that you could start planning and building your own structures in the spring. For those of you who do decide to build your own arbors and/or trellises this gardening season, please remember to tell me about your structures and share your pictures. I’d love to hear about them, and I definitely want to see what you’ve come up with. And I’m sure the rest of The Renaissance Garden Guy readers and subscribers would, as well.
Cheers, and Happy Gardening!
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It is important to learn step by step process. Thanks for the information
Thank you, Rick. I hope the steps were easy to understand, and that the information was helpful. I tried to make it as useful as possible to any folks wanting to build something like this in their own garden spaces.
Really interesting post!
Thanks very much – I appreciate that! I tried to make the info understandable and helpful to those who want to build something like this on their own properties. I tried to build mine so it’ll be there for a long time. Thanks again!
I love the bulky wooden posts!
Thank you! I really loved using the heavy lumber for that component of the structure. I felt it would lend a real sense of permanence to the arbor, and create some drama in that section of the garden. Glad you like them!
This is going to last forever!
It’s definitely not going anywhere!