Building a flat wooden bridge can really be great fun and involve the entire family as a project. We have a small stream running to our lake that in most areas is about ten feet wide. The embankments are about three feet high on each side so we chose one of those areas for our bridge. I decided to make three large beams as supports for the walkway. I chose six, two inch by twelve inch, by fourteen foot, pressure treated planks for the support beam materials. Each of these planks are quite heavy for one person to handle so I moved them one by one to the place where they would ultimately bridge inspections end up at the stream. Next, I dug two trenches, one on each side of the stream, to construct footings to support the three wooden beams. Using the largest, flattest, stones I could find, I created a flat rock ledge in each trench. The rocks being an average of three inches thick would support the beams with no problem. As each end of the bridge needed to be buried for a smooth transition to the ground on each side of the stream, the pressure treated lumber would allow me to just bury the rock footing and the end of the beams with earth holding it in place. Dragging the 2 x 12 planks across the water was no fun as the September weather had already cooled our mountain stream but I got it done.
Once all six planks were placed spanning across the stream, I was able to start to assemble them using both construction adhesive and galvanized three inch deck screws. If you are going to just use the bridge for foot traffic, two beams would be more than enough on a twelve foot span. I wanted to be able to take my lawn bridge inspections tractor and trailer across the bridge to the lake. With the additional center beam, the bridge easily handles the tractor, myself, the trailer loaded with camping or fishing supplies, chain saws and whatever else we needed that day. My bridge is six foot wide clear inside so there is plenty of room for all to cross.
Next, I added a 2 x 12 plank vertically at each end of the beams to connect them all together and also hold them apart in the six-foot, four inch width that I needed. Hand rail posts made of four by fours will reduce the width to an even six foot wide when installed. Before you start installing any decking, install two pieces of lumber, one by three inches wide or larger in an X pattern under the bottom of the beams. Crisscross them in the center by nailing the ends and the center point to the underside of the beams. These pieces will prevent the beams from "rolling over" as time passes and will hold them nice and straight vertically. These pieces must be pressure treated.
Next, apply your decking pieces. I recommend using two by six inch, PT lumber material as it's added strength will make your bridge last a good many years.
Again, I used three inch galvanized deck screws to fasten the decking boards to the beams. Leave a very small gap between the boards of about 1/4 inch or the thickness of a twelve penny bridge inspections nail shank. These gaps will allow water to quickly pass through the bridge and gives melting snow a place to drip as well.
This bridge is flat so water runoff has to be considered to avoid premature decay.
You may want to install one set of cross blocking at center line of the bridge before installing that last piece of decking for added protection against any bouncing of the bridge deck. Mine did not need it at all.
Handrails come next in the assembly. Although the bridge is short it does add added support for the older folks crossing over on the way to the lake. On the fourteen foot span I installed three vertical posts on each side of the bridge by notching the post bottoms a full twelve and one half inches high to allow the posts to half sit on the full 2 x 12. You will need to install some small blocking around the posts to support the decking pieces. Next I added another four by four for the handrail on each side all in one piece. A little pricey but after twenty plus years of use it is still in perfect condition. After fastening the handrails in place, I added one piece of cross bridging created out of 1 x 3 PT material and I inverted them into a V shape from post to post.
Mostly decorative but but it does say "Whoa", there is a three foot fall here.
Since I have a battery operated sander and five inch skill saw, I took the time to cut a slight bevel on the top of the handrail and then ran some sandpaper over it. This helps water shedding and makes the rail a little more comfortable to hold or lean on and stare at the steam below.
Before cleaning up, I applied a treatment to the ends of all my saw cuts to protect against the weather. Also try doing all your lumber cutting away from the water to avoid getting the PT sawdust chemicals in the water.
My bridge is now over twenty years old and other than some occasional bridge inspections up-keep at the bridge ends after the spring snow melt and heavy water runoff takes place to replace washed out stones or earth, this bridge will last forever.
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