Introduction: Simple Garden Bridge With Concrete Footings
I decided to make a simple arched bridge for my backyard stream and if anything is worth doing, its worth OVER doing. So, I decided to make some hefty concrete footings for the bridge rather than just set it on the ground. The bridge span will be about 7 1/2 feet.
Concrete mix - Approximately five 80 lb bags
2x8 board - qty 2
2x12 board - qty 2
2x4 board - qty 11
1/2" rebar 24" - qty 4
Gravel - 2 bags
2x1x8' firing strips
Concrete Mixer (optional)
Step 1: The Forms
The forms are simple enough. I wanted the footers to be pretty hefty so I planned on 34" x 10" of concrete about 8" thick. I cut down the 2x8 boards and assembled simple box forms with deck screws.
Step 2: Site Preparation
I picked a relatively narrow and straight section of the stream for simplicity. I started by removing the decorative border pavers in the area that the footings would go. At this point I eyeballed the two spots on opposite sides of the stream to line them up. I dug out a big enough area to set the forms into the holes with a little wiggle room for adjusting the placement to line up the footings with each other. The yard is sloped away from the back wall and sideways toward a drain block in my side wall so the hole I dug on the back side of the stream is about 2" deeper than the hole on the front side to accommodate this. They are about 6" and 4" deep. After I placed the forms in I used two 10' steel square tubes to check whether they were level and also check how square they were. I could have used a couple straight 2x4 but those are hard to come by from the big orange and blue stores. I considered the laser level but its really hard to see mine in broad daylight. The steel tubes seemed the easiest way to align everything at the time. The two sides were fairly square but not very level. I used a bag of gravel per side to level them out compared to each other and then leveled them each out so the finished footings would be flat to the ground. I tamped the gravel down after adding little amounts each time. Every time I made an adjustment I double checked all parts for level and for square. It took about and hour of tweaking before I was satisfied with it.
Step 3: Pouring the Concrete
A concrete mixer is not required but boy oh boy does it help. I've had a small one for about 7 years that my wife bought for my birthday. I've used it 3 times total since I got it. It is the definition of the phrase "Its better to have a tool and not need it than to need a tool and not have it." If you don't have one a bucket would work just fine. It ended up only taking 5 bags of concrete so I could have mixed it in a bucket or wheel barrow. A quick note about the amount of concrete I used. I used the brand name concrete calculator website to determine how many bags I needed and it came up with 8 bags. I'm not sure what kind of rounding safety factor they build into their calculator but the calculator spit out about 40% more concrete than I needed. I'm sure that was unintentional on their part...yeah, unintentional. I wanted the footings to match the color scheme of the rest of the yard so I tinted the concrete with Quikrete brand Terra Cotta color liquid. It took about 1/4 bottle per bag. The resulting color is fairly subtle. Before pouring the concrete in the molds I wet them down a little to help keep the water in the concrete mix from wicking out into it too quickly and help separate the forms when the concrete cures. I poured half of each form and then laid the rebar in the middle. I'm sure this was overkill but I thought better safe than sorry. The rebar is supposed to help keep the concrete from cracking if the ground settles a little underneath. After the forms were full I used a trowel and float to smooth the tops. I'm not the greatest concrete guy and I'm notoriously impatient to the top is not terribly smooth, but its good enough for government work. Finally I covered the forms with trash bags to help keep the concrete damp while it cured...it was 105F outside at the time so I didn't want it to dry out too quick.
Step 4: Removing the Forms
After 2 days I removed the forms. I used a concrete scraper stone to take the sharp edges off the corners slightly. I set my 4' level on both footings to check level and the bubble is within the lines. There was a minor lean but luckily they both lean the same direction so it won't be wiggly. I checked for square with the tape measure and the footings were only about 1/8" out over the 8' span. Pretty good if I do say so myself.
Step 5: Building the Arch
As I alluded to before, if there's one thing you can count of from the orange and blue hardware stores its wet warped boards. So when I brought home the 2x12's I first set them on the floor of the garage and placed the left over concrete bags on them for a couple days to help straighten them out. This isn't necessary if you buy better wood.
For drawing the arch I used a method similar to this youtube video (not my video). Its easier to watch someone do it than to explain it. But simply put you use 3 nails to define three points along the arch and then use two overlapping boards to span the nails and draw the arch. It helps when creating large arcs that would otherwise be difficult to swing with a compass or string line.
After the line was drawn I used a circular saw to cut it out. The arch was shallow enough to use the circular saw set at the minimum depth necessary to cut through the board. I started out thinking I was going to make the arch with a 6" rise but the arch would be 5" thick if that make sense. But after I cut out the bottom arch I remembered some other tutorials I had seen where people then glued the bottom round part to the top of the board to make a an arch. In hindsight if I would have know I was going to end up going this route I would have made the rise taller.
Once I cut bottom arch I ran both pieces through the table saw to square off the edges since 2x12 studs come with rounded edges. I then put the pieces together as shown in the drawing. Now my arch has a 6" rise but the whole thing was 11 1/2" thick. It looked ridiculously tall for a garden bridge. I cut about 3" off the bottom section so the arch is only about 8" thick now.
After I glued and clamped the pieces together I notched out the ends where they would rest on the footings. I spaced the footings so that I would have 5" of overlap on each end of the bridge so the notch is 5" long x 2" deep.
The final width of the base of the bridge is 30". I used the wood that I used to make the concrete forms to make the cross beams connecting the two arches. I should mention that the arch segment I drew was not perfectly symmetrical because my layout method was a little sloppy. So when I put the two sides together exactly how I had cut them out they didn't line up. I had to "mirror" them by flipping one over before attaching them together with the cross beams. This would make sure the treads would be even from side to side.
Step 6: The Deck
The deck treads are plain ol' 2x4. I bought 96" boards so that I could cut them in three 32" lengths and not have any waste. This would give me 1" overlap on each side. I needed 32 treads total, so 11 2x4's.
Step 7: Paint and Assembly
I waffled back and forth about the color of the bridge. I wasn't sure if I wanted it to be a focal point or not. I felt like I would eventually punctuate the otherwise monotone look of my yard with some vibrant pots and plants so the bridge could be more of a subtle color. I thought a darker brown would keep it somewhere between blending in while still being noticeable. Now when I was in the store I saw a color called Berry Brown. And in the store it looked more of a maroon. What I ended up with was very Barney purple. "I love you, you love me, we already spent $50 on a gallon of paint so we're going with it." I gave all of the parts of the bridge two coats of flat Barney skin. I then went over all of the visible parts and the tops of the deck with a coat of semi-gloss of the same color.
Before attaching the deck treads with deck screws I carried everything to my backyard to assemble it on my patio. It would have been two heavy to carry the whole assembled bridge from my garage to the backyard. I did attached the two kickplates on each end before carrying it out of the garage. Before I started screwing the boards down I marked a couple of random points along the top of the arch to give me some reference points to make sure i wasn't getting cattywampus as I added the board. I used two leather letter stamps as spacers to keep an even 3/8" gap between the boards as I screwed them down. I had dry fit all of the boards while still in my garage to make sure this gap would look good but to also make sure I wasn't going to end up with any partial boards that I would need to cut down. I moved from one end to the other. I stopped before the last four boards and discovered that despite my best efforts I was about 3/8" cattywampus so I used minor adjustments on the last four boards without the spacers to correct this without making it obvious.
After I got everything assembled I asked my wife and daughter to help me set it in place. The first step up onto the bridge is 6" so I want to make some stepping stones to make this a little less severe. I'll probably just make some more faux flagstones for that.
Participated in the
Stone Concrete Cement Contest