How to Install Girders for a Garden Arbor
Girders comprise one of the three primary types of structural element in a garden arbor. So when you’re building your own arbor, it’s critical that you “Get Your Girder On” the right way. Keep it right here to learn how to install girders for a garden arbor.
Greetings all, and welcome to the “This is the Last Time I’m Going to Write About My Arbor Until It’s Got Vines Growing All Over It” article. Some of you who’ve been following along here since I wrote the first article about this thing back in November (and who’ve also likely been putting up with my social media updates about it since even before that time), are probably ready to slap me. Or worse. For the rest of you (including any readers just tuning in), however, a quick explanation is in order.
As part of the overall design plan for my garden, I started building and installing a fairly substantial arbor and trellis system at the end of last summer, and I completed the project in very early December. I’ve written four articles (not including this one) about it here. The first of them was about the completed vertical trellis component of the structure. The second article consisted of pictures of the entire completed structure, and my emphasis on the need for overall planning and design. These first two articles were pretty much basic overviews of what had been built, and why. The third and fourth articles were about implementing two of what I consider the three major structural elements of the system – the support posts and the ledger board, respectively. (Click the highlighted links above to read each corresponding article). Which brings me to this, the fifth and final article, which is about the third structural element of my arbor/trellis system – the girders. A number of readers here have been asking for some “How-To-Do-It” info about building something like this. And that’s exactly what this article, and the two before it are: do-it-yourself guides for building and installing the three main structural elements of an arbor/trellis system. And seriously, people really have been requesting this info. So, I’m asking anyone who’s been thinking about slapping me for writing too much and too often about my arbor to please reconsider.
Alright, let’s talk about those girders, or outriggers as they’re sometimes called. In the case of my arbor/trellis system, which is not free standing, I typically refer to these structural components as “outriggers” (I’ll explain all of this in the first photo caption below), but going forward, I’ll just use the term “girders” in order to keep everything simple. Girders are absolutely necessary structural elements of any arbor – you literally can’t build an arbor without them. So, let’s get down to the brass tacks (or galvanized steel lag screws): What are arbor girders and what do they do? And how do you make them, and then install ’em? I’m about to tell you. And show you. Plus, I’m also going to lay some excellent product recs on you, in case you don’t have everything you need to get these bad boys done. Here you go…
What are Arbor Girders?
Simply put, a girder is a horizontal structural beam which is used to support a load, or weight. In the case of a structure like a garden arbor, which typically has some sort of rafter/crossmember (crossrafter) assembly, or canopy, installed horizontally at its top for the purpose of providing an attachment “matrix” for overhead vining plant growth, girders provide the sole basis of horizontal structural support. The canopy is attached to, and sits on top of, the girders. Think of the girders in an arbor as something like the floor joists in your house, except instead of a floor sitting on top of them, the arbor’s canopy does.
Note: Before beginning any construction project, always check with your municipality’s building department, and obtain any and all necessary building and/or construction permits.
How to Fabricate and Install Girders for a Garden Arbor
1) Consult your drawing/plan. Since you’re doing this whole arbor thing correctly, you will already have created/obtained a working construction drawing or plan. (Read my second arbor/trellis system article to understand the importance of this step.) Your drawing will specify the nature (dimensions, profile, materials, etc) of your girders.
2) Get your materials together. To make and install girders like mine, you’re going to need the following:
- Pressure treated 2×10 boards.
- Miscellaneous pressure treated dimensional boards (2x4s, 2x6s, etc).
- 3/8″ x 3-1/2″ long galvanized hex head lag screws.
- 3/8″ x 1″ galvanized washers.
- 3″ exterior construction screws.
- Clear silicone exterior caulk.
- Exterior wood sealant.
- Exterior wood glue.
- 1″ diameter wooden dowel.
- Note: Obviously, the lumber material you’re planning to use per your own construction drawing will ultimately determine the non-lumber items on your materials list. Again, this is a list of what I used for my girders.
3) Get your tools and equipment together. Here’s what you’ll want to use:
- Sturdy ladders, scaffolding, or stages.
- Cordless drill.
- Chop (miter) saw, preferably with sliding capabilities.
- Worm drive saw or circular saw.
- Jig saw.
- Belt sander.
- 3/16″ or 5/32″ wood drill bit.
- 1/2″ or 7/16″ wood drill bit.
- 1/32″ wood drill bit.
- 1″ spade/paddle drill bit.
- 9/16″ socket wrench.
- 9/16″ hex drive.
- Carpenter’s square.
- Speed square.
- String line.
- 48″ or 72″ box level.
- Torpedo level.
- 8″ capacity c clamps.
- Adjustable pipe clamps and lengths of steel pipe (my clamps acommodated 3/4″ steel pipe).
- Painter’s tape.
- Claw hammer.
- Carpentry pencils.
- Tape measure.
- Caulk gun.
4) Get a helper. Girders can be long, heavy, and cumbersome. Mine were. You’re going to need help wrestling with these. Find someone you can trust to be your second pair of hands.
5) Prep your girder boards. Again, the type and size of board you choose for your girders, and what you do with each one in terms of design/profile, attachment details, etc, is going to be determined by your own design and construction drawing. My design and drawing called for the use of 10′ long, 2×10 pine, exterior grade construction lumber, and dictated all of my board preparation details.
- Overall length. My girder boards were cut to 9’9″ in length. This allowed them to extend from the ledger board at the side of the house to 22″ beyond the structural posts at the front of the arbor.
- End profiles. The profile of each girder board as it occurs at the front of my arbor is often called a “knife edge” profile. This profile is totally arbitrary and depends entirely on personal taste. I liked this look, and it was easy to repeat on the smaller boards I used for the rafters and crossmembers of the arbor’s canopy structure. The “tip” of each board is 3-7/8″ long from the top of the board to the 125 degree, open mitered cut at the bottom of the profile. To achieve this profile, I measured 3-7/8″ down from the top edge at the end of the board and marked that spot. I then set the miter angle on my DeWalt 12″ sliding compound miter saw to 55 degrees. I then flipped the board over so that my mark was now 3-7/8″ up from the bottom of the board. I then lined up the blade of the saw with this mark and made the cut, which reulted in an obtuse cut of 125 degrees. The sliding feature of the saw allowed this profile to happen. (See Figures 1, 2, and 3 below.) The end of each board which dies to (terminates at) the back of the house, and is fastened to the ledger board, has a rabbeted, or “bird’s mouth” profile. This profile allows each board to sit with two edges – one vertical and one horizontal – in contact with the ledger board, and allows one vertical edge to back up almost flush against the siding of the house. The cuts to achieve this profile were accomplished using both a worm drive saw and a jig saw. Because the worm drive saw’s blade radius didn’t allow for a completely clean cut and left some “ridging,” I used a jig saw to free cut as much of the ridging away, especially in the tight inside corner of each cut. Any protrusions compromising the cleanness of the profile as a result of this slightly tricky cut were sanded out with a belt sander. (See figures 4, 5, and 6 below.) Had I designed and built my arbor as a free standing structure, the “knife edge” profile would have been used on both ends of each girder, and the girders would have been fastened to posts, rather than to the ledger board on the side of the house.
6) Position the girders and fasten them to the posts and the ledger board. In the case of an arbor structure like mine, with one end anchored to a building by virtue of a ledger board, this positioning/fastening part is literally twice as easy (or half as difficult) as doing it with a free standing structure because you can start the process by immediately setting one end of each girder, at exactly the correct height, directly onto the ledger board. But in any case, you’re working from a drawing, right? So your ledger board is already installed at the correct height and is perfectly level, or (in the case of a free standing structure) you know how high up on the support posts you want your girders to sit. This is because you did all of this arbor stuff the right way and started out with a drawing. To make things a little easier for you (even though you do have a drawing), here’s the methodology I used for my arbor, and definitely recommend.
At this point, it’s important to note that the methodology outlined below applies to the construction of a structure identical or closely similar (in terms of design, dimensions and nature of materials used, etc) to mine. If you’re using a different design and different materials in the construction of your arbor, the drawing which you’re working from will reflect these facts and will inform your subsequent fabrication and installation methodology.
- Start with one girder on one end of the arbor structure, pin it, and run a stringer board. But make sure it is the outermost girder. Set one end of the girder on the ledger board and have your helper hold it in position. Then, holding it level from the ledger board out to the post, and making sure it’s square to the ledger board, clamp the girder to the post. At this point, it’s advisable to mark the girder with the locations of the future lag screws which will permanently affix it to the post. Once these marks are made, use the 1/32″ drill bit to drill a pilot hole (at each mark) through the girder into the post. Then “pin” the girder – which is now in a perfectly level and square location – to the post by driving in a temporary 3″ construction screw. This will be your level benchmark for setting the rest of the girders. You can now clamp a stringer board, pushed immediately up against the underside of this first girder, and held perfectly level, to this first post and to the next one. Clamp the level stringer board to the next post. (Figure 7 below, which is also this article’s featured image, shows the use of a stringer board, and shows some of the girders which have been pinned in pace.) This will allow you to set the next girders in an exactly level position on the next post(s). Since your ledger board is perfectly level, and the first girder you’ve pinned to the first post is level, and the stringer board you’ve just clamped into place is level, every successive girder you install will be level from front to back, and side to side.
- Clamp the rest of the girders into place against the posts. With the front ends of the remaining girders resting in perfectly level position on the perfectly level stringer board, clamp each one to its corresponding post.
- Permanently fasten the back ends of the girders to the ledger board. Always make sure each girder is square to the ledger board. Even though they’re all clamped in place at their front ends to the posts, there will be enough play along each girder to slide its back end slightly along the ledger board until it’s square. Since we started the positioning process with the outermost girder first, and the posts used are 6×6 boards, each with an actual dimension 0f 5-1/2″ x 5-1/2″, in order to correctly position and fasten the next girder, we’ll install two 5-1/2″ wide kicker boards (one made from 2×6 treated material, the other from 2×4 treated material) directly to the ledger board (one to it’s face and one to it’s top edge, using 3″ exterior construction screws) between the first girder and the next one, which we’re about to fasten in place. Since the first girder was held square to the ledger board, the next girder, held tightly to these 5-1/2″ kickers, will also then be square to the ledger board. The girders can now be permanently screwed directly to these kicker boards using the 3″ exterior construction screws (always drill a 1/32″ pilot hole first). In this way, the girders are permanently, and squarely, attached to the ledger board. The drawings I used for this fastening detail are represented below in Figures 8 – 11.
- Pin each girder in its correct place directly to its corresponding post. The girders are now exactly where they need to be. Pin them to their corresponding posts, just as we did above with the very first girder. Now, every girder is permanently affixed (albeit pinned rather than lag screwed to its post) in the right spot- perfectly level and square, front to back, and side to side. And they’re all permanently attached to the ledger board at their back ends. We can now remove the stringer board and all of the clamps, and move on to the step of lag screwing them – in truly permanent fashion this time – to the posts.
- Lag screwing the girders permanently to the posts. Using beefy, rustproof lag screws to anchor girders to vertical structural support posts is one way to ensure that each of those girders will be able to faithfully, and permanently, execute its intended duty. Since at this point, each girder is already pinned by 3″ construction screws, in its correct and permanent position, to its respective post, we’re essentially just replacing each construction screw with a lag screw. We’ll work on only one screw location at a time, leaving each pinning construction screw in place until we’re ready to remove it and install the lag screw in its place. So, starting with the first location, back only the first construction screw out (again, leaving the others in place). Since there’s already something of an existing pilot hole in place, you’ll need to basically just lengthen it. We’re using 3/8″ x 3-1/2″ galvanized hex head lag screws. So extend the pilot hole using the 3/16″ or 5/32″ wood drill bit. Countersink this hole to a depth of slightly more than 1/2″ using the 1″ wood spade/paddle bit. Slip one of the lag screws through a 3/8″ x 1″ galvanized washer, and then start the the lag screw’s threads into the pilot hole. Using a 9/16″ hex drive insert in your cordless drill, drive (at low speed) the lag screw into the pilot hole. Once the drill meets some resistance, switch to the 9/16″ socket wrench and tighten almost to the point where the screw will no longer turn. Do not snap the head off of the lag screw by overtightening!!! Repeat this entire process, one screw location ata time, until the girders are completely lag screwed to the posts. Refer to Figures 12, 13, and 14 below for a closer look at this fastening detail.
- A note about positioning and fastening girders to posts on free standing arbor structures. If the arbor you’re building is free standing, then your girders will be fastened to posts on both of their ends. In this case, you’ll obviously use the stringer board/levelling/clamping/pinning/lag screwing methodology which I used for positioning and fastening the front ends of my girders, all the way around your structure. Front and back.
7) Plugging the countersunk lag screw holes. The 1/2″+ deep countersunk holes can now be plugged with wooden dowel plugs. I used a 1″ diameter wooden dowel and cut it into individual plugs, each approximately 1/4″ thick. Since the dowel material I used wasn’t pressure treated, I soaked each of these 1/4″ thick plugs in a container of Thompson’s Water Seal for approximately 15 minutes. I then set them out on waxed paper to dry. Once dry, I applied exterior wood glue to their sides and backs, and to the rims of the countersunk holes in the girders, and tapped the plugs into the holes with a claw hammer.
8) Caulk. Once everything is screwed into place, and all the glue on the plugs you just installed is dry, you can caulk all girder fastening points with clear exterior silicone. I caulked all the heads of the construction screws fastening the 5-1/2″ wide kickers to the ledger board and the girders themselves to the kickers.
9) Canopy design considerations. Once the girders are in place, the sky, literally, is the limit when it comes to the type of canopy, or “roof” you want for your arbor. Since you’ve been working from your design and drawing all along, you already know what your arbor’s canopy is going to look like, and how you’re going to build it. Your girders are in place – so have at it! For mine, I opted for a “stacked” appearance achieved by using comparatively smaller boards – 2x4s for the rafters, and 2x2s for the crossmembers. I fastened the 2x4s to the girders by setting them, full profile-to-full profile (edge-to-edge with no bird’s mouth) and through-screwing them. I then fastened the 2×2 crossmembers to the rafters in the same way. By not doing a dado cut (bird’s mouth) at the joinery points of these boards, they look like they’re “stacked” in crisscrossing, perpendicular fashion. This made for a more time consuming install, but it was the look I was after. I keep imagining this open network of boards swallowed up by thick, green, leafy vines. I can’t wait!
Stuff to Help You Build Your Own Arbor
I want to finish this article up, and the topic of arbor/trellis building as a whole, with some product recommendations. First, I’ll recommend two really terrifice books filled with great designs and plans, and great how-to info. I’ve recommended them before, and I’ll do it again. If you want to build your own arbor, but have no clue about how to even start, either (or both) of these books are for you. Even if you’ve got your design and drawings ready for the structure you want to build, these books are still great references. Next up, I’m recommending some never-give-up tools and tried and true equipment. Planning on building anything in your home or garden? These are must-haves. Finally, there are a few building materials here that I’ve used year in and year out with zero-failure results that I’m recommending wholeheartedly. Remember, as I’ve mentioned time and time again, I’ll never recommend anything which I haven’t used or haven’t read and researched. I have absolute faith in every single product I recommend here.
If any of these recommendations look interesting to you, click the Amazon product icon and have a look. Feel free to order anything you like, conveniently from right here. It’s totally private. I’ll never have any idea of what you look at or what you buy. That’s between you and Amazon. So take your time, and have a look around.
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. Click the #advertisement link to learn more, or to order it here, directly from Amazon.
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 this book’s excellent content. Sorry, no pic. But, again, this is a great book with great ideas and excellent detail. To learn more, or to order it here, conveniently and 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
I use Makita’s 18 volt, 2 amp cordless drill/hammer drill/driver for everything, including the assembly of my arbor. This kit also comes with an impact driver, which is helpful when dealing with tightly threaded nuts and bolts, and tough threaded fastener penetration situations. This is a great, rugged combo by a matchless manufacturer of cordless drills. I’ve used Makita cordless drills my whole adult life. To learn more, or to order this great set here, directly from Amazon, please click the #advertisement link.
Makita 18-Volt 2-Amp Lithium Ion Brushless Cordless Driver/Drill/Impact Driver Set
Click here to learn more or to order
#advertisement
My Skilsaw 7-1/4″ worm drive saw is one of the most versatile saws available. It’s incredibly powerful and very lightweight and can be used in a variety of cutting applications. I couldn’t have completed my arbor without this iconic piece of equipment. To order yours here, directly from Amazon, or to learn more, please click the #advertisement link.
This Craftsman 21″ x 3″ belt sander is remarkably powerful, and super easy to control and adjust. Its low and close profile, combined with simple tensioning and tracking controls, allows me to sand material in difficult to reach profiles and always tricky corners. A terrific piece of equipment which performs beautifully year in and year out. Click the #advertisement link to learn more, or to order it here, directly from Amazon.
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 level unit which I used almost constantly during my arbor/trellis project. It was infallible. This new orange version has got the same excellent quality, precision, and accuracy as my older yellow model. Learn more or order it here from Amazon by clicking the #advertisement link.
The Pony 8″ C Clamp is a must have for any woodworking project. Time-honored and proven, Pony is the best name in clamping products. Learn more, or order it here, from Amazon, by clicking the #advertisement link.
Pony’s adjustable 3/4″ pipe clamp is a must have for any arbor/trellis building project. With these clamps, your material clamping length is limitless. Pony clamps are the best: they’re powerful and there’s absolutely no slip whatsoever. To learn more, or to order this unit here, from Amazon, please click the #advertisement link.
GE Supreme Silicone Sealant is 100% weatherproof and is water and rain resistant within 30 minutes of application. It’s the best exterior silicone caulk I’ve ever used. Click the #advertisement link to conveniently order it here, directly from Amazon.
I’ve used a variety of Thompson’s WaterSeal products for exterior wood projects for longer than I can remember. It’s simple to apply, and it seems to last forever. As your pressure treated outdoor structures – like fences and arbors – age, hitting their surfaces with a treatment of Thompson’s breathes new life into them. Again, a tried and true product. Order it here, from Amazon, by clicking the #advertisement link.
Thompson’s WaterSeal Multi-Surface Waterproofer Stain
Click here to learn more or to order
#advertisement
Alright gang, that’s the last you’re gonna hear about my arbor. Again, I wasn’t trying to beat a dead horse here. I just wanted to get the last of my “How to Do It” articles out there for those of you who’ve been asking. And now, after getting this last one under your belts, you should have a pretty good basic idea of how to get the structure of an arbor together. But remember, if you’re seriously thinking about building one of your own, make sure you’ve got a design, a plan, and a working drawing in your pocket before you even think about picking up a hammer. This is absolutely critical. The practical information I tried to relay in these articles is not meant to take the place of a real, honest-to-goodness construction plan. I just wanted you to know how I handled some of the basic design and construction issues/necessities involved in building an arbor. Sometimes, seeing something on paper isn’t always the same thing as seeing it in the real world, and when you pull something out of your imagination, put it on paper, and then try to get it up out of the ground, the story can get lost in translation. Construction is tricky that way. So even if you’ve got a plan, and you’re ready to hit the ground running, a little practical advice from someone who’s been around the track once or twice can be a good thing.
Cheers, and Happy Gardening!
The Renaissance Garden Guy is a participant in the Amazon Associates Program. As an Amazon Associate, I earn from qualifying purchases.
Additionally, The Renaissance Garden Guy is a participant in the Bluehost, SeedsNow, and A2 Hosting affiliate programs. I earn a fee/commission each time a visitor clicks on an ad or banner on this site from one of these companies and makes a subsequent qualifying purchase.
Please click here to view The Renaissance Garden Guy Disclosure page.
You could be designing arbors and selling the plans! You have taken us from start to finish and really shown us every aspect of this project. I am looking forward to seeing your arbor covered in flowering vines.
Thank you so much, Kevin – how kind of you to say so! I’m glad you enjoyed the article. And I’m also definitely looking forward to seeing vines and flowers totally engulfing it – thanks again!
I am impressed by you. You have the vision, the planning, the skill… just an incredible talent. I can appreciate the time & hard work you’ve put in.
You also have interesting friends with remarkable talents, who are so deeply thoughtful.
I’m enjoying your life. 😅😊
Thank you so much, Kelly. Your words are so incredibly kind – I appreciate this more than words can express. I’m so glad you’re finding my efforts interesting and entertaining. Knowing this makes me very happy. And I do have some interesting, thoughtful friends with remarkable talents – you are certainly one of them. Thank you once again!
This is incredible. So much information and wonderful plans of how to complete a project like this. I doubt anyone reading this would not be able to do it. I really enjoyed reading this as well and learned a few things. Well done:)
Thank you so much, Louise! I’m so glad you enjoyed the article. I tried to provide some basic construction details for other arbor builders to reference for use in their own projects. I’m really happy that you liked it. Thanks again!
Wow. So informative! Nicely done!
Thank you, Colleen! I’m happy that you enjoyed it and I’m definitely glad it was comprehensible. I wanted to provide basic details to anyone attempting something like this. Once again, many thanks!
From a non gardener like me I can say that …..
If anyone still has questions on how to build and install Girders for a Garden Arbor after this article I will be very surprised 😊 it is absolutely an amazing article, can’t get more complete and better described then this 👏👏👏 Happy Gardening!🌼🌺I almost want to try 😊
Thank you, Roxxy! I’m so glad you liked the article, and I’m glad it was understandable. And if you do build an arbor of your own, I’d love to are what you come up with – I bet it would be amazing! Thanks again for your kind thoughts!
Your Garden Arbor is so beautiful!! The craftsmanship is awesome!! I know anyone that needs details about building a arbor could do great by using your examples!! I can’t wait to see it covered!! Great work!! Sir!!
Thank you so much, Sharon! I tried to demonstrate my own methodology for building some of the basic components of a structure like this, so anyone contemplating one could borrow some of the basic techniques and concepts. I’m so glad you liked it!
Very informative and detailed.
Thank you. I’m glad you think so. I tried to present the info in the most helpful and comprehensible way I could. I’m hoping this article will be helpful to anyone planning to build an arbor. I’m glad you liked it.