Retaining walls are built to create level planting areas or to increase usable yard space. They are designed to resist the lateral movement of soil or other materials. They can be constructed as simple low wall units used to make raised planting beds or major walls that require special building techniques to resist thousands of pounds of pressure from the weight of soil, water and retained material.
Retaining walls can be made of pressure treated lumber, pressure treat timber, natural cut stone or rubble, formed concrete or concrete block.
Garden walls are decorative walls whose free-standing stability is dependent on their own weight. They are not used to retain earth or other material.
Garden walls are generally constructed as permanent fences to create patio and yard areas and to shield homes from street areas.
Garden walls can be made of decay resistant wood or they may be walls that are covered with plaster. They are sometimes made of concrete block of various textures and decorative surface treatments. Although a permit is not required for garden walls less than 6 feet in height, the excessive weight of a block wall must be addressed to prevent cracking or toppling of the walls in the event of an earthquake.
Construction of Retaining Walls:
Regardless of the construction materials utilized, retaining walls over four (4) feet in height from the bottom of the foundation footing to the top of the wall require structural engineering. Retaining walls less than four (4) feet in height may require engineering if weight from an adjacent structure or a steep slope is applying pressure against the wall.
Masonry & Concrete Retaining Walls:
Foundations for masonry retaining walls are made of steel reinforced concrete. A level base is necessary to provide a solid surface for the foundation. The correct footing size and reinforcing steel spacing is required to insure a stable and lasting wall. An information sheet available from the Building & Fire Safety Division provides engineered walls up to 5 foot 4 inches tall. The guide may be used by contractors or the do-it-yourself homeowner. A drain must be installed behind the wall to relieve hydraulic pressure. A building permit is required for concrete and masonry walls to insure correct installation of the reinforcing steel.
Cut Stone & Rock Retaining Walls:
Cut stone and rock retaining walls can be constructed on a foundation of coarse, compacted, gravel. The largest stones are installed at the bottom with a backward slant (batter) built into the wall. The wall can be constructed with or without mortar. The top row of rocks should be set in mortar to hold the rock in place to prevent dislodging. A behind-the-wall drain relieves the hydraulic pressure. A building permit is required for walls over 4 feet tall.
Garden Wall Construction
A garden wall is intended to be a decorative, permanent fence. Although a permit and inspection is not required, considerable care should be taken in how it is constructed. Construction specifications for block walls are available through the Building & Fire Safety Division.
Wooden Retaining Walls:
Timber Retaining Walls
Timber retaining walls supporting a level back fill are constructed of pressure treated lumber at least 5 x 6 inches in size. The pressure treated timber must be rated for direct burial. Care must be exercised when handling pressure treated lumber. Avoid inhaling the dust, and prevent sawdust contamination of water runoff as the chemicals are toxic to fish and birds. Pressure treated lumber should not be burned.
The timbers can be laid on a compacted gravel sub-base. Stagger each successive joint. Twelve (12) inch galvanized spikes driven every two (2) feet will maintain alignment. A drain must be installed behind the wall. Drill a 1 inch weep hole every 4 feet into the second layer of timbers. Vertical posts are installed every 3 feet if the wall is 2 feet high. The posts should be buried to a depth equal to the exposed height of the wall.
Lumber Retaining Walls
Wooden retaining walls supporting a level backfill can be constructed of 2 x 12 inch pressure treated lumber, 3 boards maximum in height, supported with pressure treated posts every 3 feet buried in concrete to a depth equal to the exposed height of the wall. Structural engineering is required for wooden walls over 3 feet in height.
Retaining Wall Drainage:
Water drainage behind retaining walls is accomplished by a French drain installed at the base of the wall. A 2 inch layer of coarse gravel is spread 12-18 inches wide behind the wall, the native soil below the gravel shall slope at a minimum 1/8 inch per foot to an outlet. A perforated drain pipe is laid over the gravel. A cleanout may be installed at the end to clean the drain if it becomes clogged. The pipe is covered with a minimum 5 inches of coarse gravel. The gravel is covered with a landscape fabric to keep the soil from washing into the gravel and then compacted soil is filled in behind the wall.
A Building Permit is Required For: