A stream running through your property can be an idyllic garden feature, but crossing it and remaining dry might be difficult. Solution: Build a small foot bridge. It is not an overly difficult task and, provided you have just a small stream, should not require a major engineering endeavour.
Measure the width of the stream from high point to high point on the bank. Keep in mind that, because of soil conditions near a flowing body of water, you may need to put the entrance of your bridge a good way back from the bank. Assuming that you have at least 15 feet to cross, find a point that is free of large obstructions and on a straight part of the stream.
Select posts that will be heavy enough for the traffic you intend the bridge to handle. Old telephone poles or sleepers work well, but for a light-traffic bridge, 10 by 10 cm (4 by 4 inches) posts should work. Dig four holes, two on either side of the stream, about 40 to 45 cm (16 to 18 inches) deep at a point that is equidistant from the slope to the water and as far apart as the bridge will be wide. Centre the posts in the holes and fill with concrete. Be sure that the posts are tall enough to be above the maximum height of the bank.
Measure and cut your 5 by 30 cm (2 by 12 inches) sections for the length of the bridge. These will form the horizontal runners. If they are not long enough to reach across from starting point to ending point in one continuous piece, then you will need to cut an additional board to join them. This board should be no less than 1/8 longer than the gap between the pieces. Be sure that you try to fit the gap in the centre of the span.
Drill holes through the 5 by 30 cm (2 by 12 inches) boards where they will attach to the posts. There should be at least two bolts (four if you are using a wider post will add stability) holding each board to the post, and the boards should be aligned so that they are on their edges. Drill the holes at three inches from the edge of the board. Attach to the post using lag bolts.
Measure the distance from the base of the post to the top of the 5 by 30 cm (2 by 12 inches) sections that you just attached. Cut the 10 by 30 cm (4 by 12 inches) boards at a 45-degree angle on each end, pointing in the same direction and at the length you just measured. Drill and attach to the base of the posts and the horizontal 5 by 30 cm (2 by 12 inch) boards. This will provide added stability and help to ensure that your bridge will not collapse.
Measure the distance between the 10 by 30 cm (4 by 12 inches) boards. This is the maximum width of the bridge. Cut the 2 by 30 cm (1 by 12 inches) boards accordingly and affix them to the top of the 5 by 30 cm (2 by 12 inches) section. If your bridge is very wide, consider using 5 by 30 cm (2 by 12 inches) boards for these planking pieces for added strength.
If you would like your bridge to have a hand rail, simply attach vertical pieces of 5 by 10 cm boards (2 by 4 inches) from the horizontals of the bridge and either run a rope or lumber across the top. Remember to use more than one nail, screw or bolt to affix any piece. If you only use one fastener, you will have just created a hinge and the bridge will shift and shake.
Check all zoning and permit requirements before starting this project Remember to wear all necessary safety gear, including eye and ear protection. Select your wood according to your specific needs. If you are building a high-traffic bridge, increase the lumber size to help avoid the risk of bridge collapse.