A retaining wall holds back soil on a hillside, the edge of a lawn, along a driveway, around a garden or on a flower bed. Retaining walls may be any height, depending on what needs to be held back, but building one of 8 feet or more requires the services of an engineer in most locations. Build a retaining wall out of wood, old railroad ties, landscape timbers, poured concrete, concrete blocks, stones, bricks or even chunks of old concrete. There are common elements for building retaining walls, regardless of location or material.
Determine how high the wall needs to be. Set a stake vertically at the bottom of a hill or slope and measure with a tape measure from the ground to the top of the adjoining soil; add a couple of inches so the top of the wall is above the adjacent ground. Use stakes and a tape measure to figure the height for a garden or flower bed wall.
Pick a material for the retaining wall. Use what is easiest to get and looks satisfactory in the location. Select something that you are comfortable working with and fits the height; a very tall wall, 6 feet or more, requires sturdier materials. Consider construction and choose something you are familiar with -- poured concrete requires building forms and using stones, bricks or concrete blocks may require mortar.
Build a solid foundation, no matter what the material. Excavate an area to at least a foot below the ground level where the wall will start and a couple of feet behind it; shorter walls, less than 2 feet, require less excavation. Lay a course of compacted gravel in the trench where be wall will be; for taller walls, put a layer of landscape fabric under the gravel. Make the gravel a few inches wider than the first course of the wall and level.
Lay the first course of wall, with ties, timbers, blocks or concrete chunks. Use stakes and builder's twine to mark a straight line for the wall and use a level to ensure the material is level. Add other courses of the same material, overlapping seams where ties or blocks or other elements meet. For tie or timber walls of more than 4 feet, install a "deadman," a section of tie or timber perpendicular to the wall, every 4 feet. Secure ties or timbers with reinforcing bar or spikes driven into the layer below with a sledgehammer. Install bricks or blocks with mortar; simply stack concrete chunks, making sure they are solid.
Backfill the excavated area once you finish the wall. Put another layer of gravel at the bottom of a tall wall. For a garden or flower bed, simply fill the space with topsoil. Consider wood for garden or flower bed walls; set stakes every 2 feet and fasten 2-by-12-inch or similar boards on the insides to make a frame to hold the dirt. Use a similar technique for wooden retaining walls; set posts in concrete vertically, then nail boards on the inside.