Trail construction While many trails have arisen through common usage, quality trail design and construction is a complex process requiring certain sets of skills.
When a trail passes across a flat area that is not wet, often all that is required is to clear brush, tree limbs and undergrowth to produce a clear, walkable trail. When crossing streams, bridges may or may not be desirable, depending on the size of the stream and the depth of its banks. In wet areas, it may be necessary to create an elevated trailway with fill or by building a boardwalk[?]. One problem with boardwalks is that they require frequent maintenance and replacement - boards in poor condition are often slippery and hazardous.
A common mistake in establishing trails is to make them on slopes that are too steep for comfort and the environment. Such steep trails generally result in serious erosion, a wide swath of impacted area as walkers go to the sides to find better footing, and the inability of many hikers to walk the trail. An absolute limit for trail grades is a slope of one in six, and a more practical limit is a slope of one in eight. Trails that ascend steep slopes use switchbacks.
If a trail is being made to be accessible to off-road wheelchairs, the slope should be no more than one in ten. If a paved trail has to be accessible to all wheelchairs, the slope must be no more than one in twelve, with periodic level pulloffs.
The off-slope, or side-slope, of the trail also must be considered. This is the slope of the trail from side to side, and should never be more than one in twelve. Side-sloped trails are prone to gullying[?]. Ideally, the treadway of the trail should be almost, but not quite, level in cross-section.
Achieving the proper slope in hilly terrain usually requires the excavation of sidehill trail. This is trailway that is constructed by establishing a line of suitable slope across a hillside, then digging out by means of a mattock or similar tool to create the trail. This may be a full-bench trail, where the treadway is only on the firm ground surface after the overlying soil is removed and thrown to the side as waste, or a half-bench trail, where soil is removed and packed to the side so that the treadway is half on firm old ground and half on new packed fill. In problem areas, it may be necessary to establish the trail entirely on fill. In cases where filling is used, it's necessary to pack it firmly and to revisit the site periodically to add to the fill and repack it until fully stable.
An important and often-overlooked factor in trail construction is that of drainage. Where a trail is near the top of a hill or ridge, this is usually a minor issue, but when it is farther down it can become a very major issue. Trails, by their nature, tend to become drainage channels and eventually gullies if the drainage is not properly controlled.
In areas of heavy water flow along a trail, it may be necessary to create a ditch on the uphill side of the trail with drainage points across the trail. The cross-drainage may be accomplished by means of culverts, which must be cleared on a semi-annual basis, or by means of cross-channels, often created by placing logs or timbers across the trail in a downhill direction, called "thank-you-marms", "deadmen", or waterbars. Using timbers or rocks for this purpose also creates erosion barriers. Rock paving in the bottom of these channels and in the trailside ditches may help to maintain stability of these. Ideally, waterbars should be created, with or without ditching, at the most major points of water flow on or along the trail, and in conjunction, if possible, with existing drainage channels below the trail. Another important technique is to create coweeta dips, which are dips in the trail grade so that the hiker, ascending the trail, is hiking uphill, then slightly downhill for a few feet, then uphill again. These provide positive drainage points that are almost never clogged by debris.
For long-distance trails, or trails where there is any possibility of anyone taking a wrong turn, blazing or signage should be provided. This may be accomplished by using either paint on natural surfaces or by placing pre-made medallions. Generally speaking, every trail should have a distinctive blaze, of a particular color and shape. Horseshoe-shaped blazes are good for bridle trails (but be sure to have the "u" of the horseshoe opening to the top, or you'll offend some riders!). The Appalachian Trail is blazed with white rectangles. Blue is often used for side trails.
When using paint on trees, the preferred technique is to use a drawknife to smooth the outer bark of trees without penetrating to the inner bark (so as to not injure the tree), then using an oil-based paint to create the blaze. Stencils are often useful, and sash brushes are the preferred brush type for precise work. Oil-based paint seems to last longer than latex-based and seems to be more benign to the bark. Blazes may also be painted on obvious rock surfaces or on posts set into the ground (or on utility poles, fences, or other handy surfaces).
A very common and major error in building trails is assuming that once a trail is built, it needs no further work. Nothing could be further from the truth. Not only does a trail need annual clearing work to remove vegetation, fallen wood and other obstacles, but often needs minor or major regrading work from year to year, and often drainage improvements and erosion control, not to mention marking and signage.
See also the List of long distance footpaths.