The Arched Bridge
Posted by joeabbott on October 14, 2012
I was looking through all the drafts I’d created but never posted and came across a short article I started when building the small, arched bridge in our backyard. The link to the original blog entry on the arched bridge is here, but I had wanted to write an addition to augment that post. I searched the web and wasn’t able to find a description that worked for me, so I figured I’d create one for those of similar interest who came after me.
To be fair, my bridge very much benefitted from this article on How to Build a Footbridge. Now, one might review that article and come to the conclusion it’s fairly explanatory and should have been all I needed, but I wanted a bit more handholding, as that post didn’t lay it out the way I wanted. So, here goes!
Planning the Site
Planning the site for us was easy but for everyone requires two details: the span you want to span and ensuring you have the footings you’ll need to hold it up. Another detail may be the width of the bridge but I’m assuming the path it’s part of will define that detail.
For our bridge, we really weren’t in need of spanning a particular distance: we were putting in a path and where the land was steep, we thought it would be a colorful addition to the yard. So, for us, the main consideration was “how long do you want it?” And, with dimensional lumber coming in 8’ nominal lengths, that became our distance.
For those covering a small creek, a dry streambed, or other natural “obstacle”, you’ll want to build the bridge long enough to span the area in question and then go a bit further for footings. The footings are the support infrastructure you put in place (or use if there’s something pre-existing) to hold up the bridge.
For the footings, you’ll want to make sure your bridge is sitting on solid earth, stone, pavers, or similarly solid material. You want to avoid anything that will slough off or erode and jeopardize the safety of the bridge. I used pressure treated (PT) 4×4 lengths for footings and had a small challenge with our placement. On one end, on the downhill side, the earth was soft and it was questionable whether it was placed well enough to hold up over time. But, so far it’s looking solid!
The Overall Look
Because we chose an 8’ overall length, the span (the arched opening) was the overall length minus the length of the footings. I chose 18” footings based on the article (How to Build a Footbridge) and so that left me a 5’ span. The article also suggested a 12” height … which I almost didn’t get based purely on the cost of a 2×12 PT board! But, the cost between the 2×10 and 2×12 wasn’t that much and when you started to get narrower boards, the arch really started to diminish.
So, for our bridge, 26” wide (the width of the path), 8’ long (the standard size of a board sold at our home center), and 12” high (the cost I could afford). Now some people may use other factors to determine their look, but that’s what we used.
Side Supports – Stringer
The thing I disliked most about the article I was using is that it didn’t give me a picture like this:
This tells me everything I need to know! I see the height of the PT 2×12 lumber, I see the length, the 18” footings are indicated … but then it shows the path of the lower arc and gives me a couple ways to create it. I can either draw a parallel line to the outer arc that’s 7 9/64 wide, or I can get a string and a pencil to draw an arc with a 95” radius! And, for the purely mathematically minded … yes, some of the numbers are approximate but, in building a garden bridge, we can get away with approximations.
And that picture is really all you need to know to start building your bridge.
When I built the bridge in Sketchup, it stood up perfectly, no tipping, not troubles. In real life, I was having all sorts of problems. While solid footings and the treads will hold the shape of the bridge just fine, I ended up adding a couple of inner supports that I could put into place when I built the substructure in my garage before moving it to the installation site.
As you can see in the image to the right, the inner supports are really just a couple of 2×4 cut-offs that I used to ensure my bridge would hold shape and the side supports would stay parallel while I installed the treads. To be clear, you could just use the clamps to hold things together and, when the treads were on, drop the clamps and remove the (temporary) inner supports. I chose to drive a few decking screws in to hold things right and avoided the hassles of clamps.
The right length for the inner supports? Easy! We’d decided on the side supports being 26” wide (to line up with the path width), so just subtract the width of the two inner side supports (1.5” each) and I had 23” long inner supports. The placement for them was a bit subjective … I just sorta eyeballed about 2’ from each end.
I chose 2×6” lumber for my treads. I wanted something that looked solid and, to my eye, 2×4 boards were a little delicate looking. Also, I had a couple 2×6 boards in the lumber rack so I didn’t have to buy as many from the home center.
For length I wanted a little overhang but I wasn’t sure what was the right amount. The article in question would have had me build the treads the same width as the path (but to install the stringers … side supports … 4” closer). I wanted the bridge just slightly wider than the path but didn’t think I needed (or wanted!) 2” overhang on each side. So I went with 1”. I would have probably been fine with 2” overhang (it was a solid “chunky” bridge). It starts to come down to what you want or think looks good.
Assembling as I did isn’t necessary, but I do enjoy working in the garage more than the backyard. Tools are handy, it’s dry and clean, and the lighting is always the same. And, I had a flat surface so I could get everything lined up and squared away. And, as long as I was doing a bit of work in the garage, I wanted to do as much as I could.
So, after putting in the inner supports, I screwed on the supports for the footers. This is actually a little controversial.
Proper construction has the footings installed solidly into the earth, and then the structure you wish to place on top of it either set in place or attached to the footings. Here I was attaching the footings right to the thing I was installing … meaning I really didn’t have footings! Not proper ones.
But, the point of the footings for this simple bridge was to give a solid something to sit on, a broad area to diffuse the load, and something that would hold up to time. So, a chunk of PT wood that was a foot-and-a-half long and three inches wide fit the bill.
The last thing I did before moving it was to tack on a couple treads on each side. What this helped me do was to keep the whole thing from wracking as I moved it uphill, dug out the area to set it, worked on getting things level, and whatnot. It also meant that I’d have to address the final fitting of the treads during installation, but that was fine.
Before installing, I moved the subassembly (the side supports, the inner supports, the footings, and a couple treads on each side) out to the backyard. It was easy if I stepped inside the subassembly and lifted it by the side supports. A bit bulky, but not very heavy.
While working around inquisitive chickens, I dug out the hillside, ensured it was level, and dealt with burying the footings far enough down to make the first tread even with the path. The keen eye can see the downhill side of the supports being questionable, but they’ve held up to time and the elements. And, yes, to chickens.
For spacing on the threads I used a couple decking screws. I was attaching the treads with 3” screws, two on each side, so I always had a bunch of screws handy. Before installing I’d place a screw between the installed tread and the one I planned to screw down, made sure the outside edge was even with the prior tread, and then drove the screws holding the new tread in place. Sometimes the threaded screw would give me troubles removing it from between the treads, but it was never that big of a deal.
I then installed the treads from both ends.
At the top of the bridge I came to a problem I knew I’d have: the width of my tread wasn’t right to support the correct spacing. Here I figured I wouldn’t get lucky and intended on just ripping down a 2×6 to the proper width … and that was a plan that would have worked great. However, I guess I was planning on having a single board at the very top but what happened was the gap was a bit wider than the tread … meaning I’d have to rip down two 2×6 boards for the tread layout to be balanced.
But then I noticed that, if I used a couple of 2×4 boards instead of ripping a 2×6, it would come out nearly perfectly! And so, at the very crown of the bridge, you’ll see 2×6 treads giving way to a couple of smaller 2×4 treads! While it might not satisfy a purist, it worked out very nicely for me.
And that’s it. That’s how you build a simple arched garden footbridge. I hope you’ve enjoyed and picked up a few tips to help you out or avoid other problems. Drop me a note in the comments section if you have a question or something isn’t clear. A big thanks to the folks at Handcrafted Garden Bridges … the author of the article I referenced at the start of this post. Their tips helped a lot and they also have other resources on their page you should check out … especially if you plan on installing a handrail!
Thanks for dropping in.