Landscape Retaining Walls

Page Outline:

I.    General Considerations
II.  Concrete Retaining Block Walls
III. Rock Walls
IV. Treated Timber Walls
V.   Concrete Walls
VI.  Brick Walls
VII. Planting to Hold a Slope

I.  General Considerations

Block wall with capstone

Most permitting agencies (i.e. County, City) require a permit to build a retaining wall over four feet high. Often these larger walls will need to be designed or approved by an engineer. This is a good idea, as there are considerable forces exerting pressure over time on higher walls.

Western Washington has an abundance of hills and slopes that, when well designed, allow for a much more interesting and attractive final landscape than a flat property (Imagine a flat Butchart Gardens – near Victoria, B.C.). Rodda and Sons Landscapes specializes in the installation of these rock, timber, poured concrete, brick or concrete retaining walls in Normandy Park, Burien, Des Moines, Bellevue, SeaTac and other Seattle areas.

II.  Concrete Retaining Block Walls

Low Stonewall block retaining wall with capstone

In recent years there has been a large increase in the number of manufactured concrete blocks that are engineered for retaining walls. Generally these walls are built on a base of compacted crushed gravel, often with drainage behind higher walls. The blocks come in many sizes, shapes and colors and from many sources (Mutual Materials, Home Depot, Lowe’s, etc.). The most common errors by homeowners are not using a compactor on the crushed gravel base, not using a level on the blocks, and not using engineering that is needed on higher walls.

III.  Rock Walls

Rockery rock walls, usually built using basalt or granite, are long lasting walls. Higher walls need larger rock. Our guidelines are: One-man rock (up to 300 pounds) can be used for walls under 2 feet high. Two-man (300-1000 pounds) can go to 4 feet high. Higher walls often need even larger rock. Drainage behind the rock wall needs to be considered. Experience in rocksetting is highly beneficial.

Interesting rock walls can also be constructed using a flat, dry-stacked flagstone or slate. Or the flagstone can be mortared together.

IV.  Treated Timber Walls

A well-built wall using good quality, new treated timbers should easily last 20 years. Timbers can be connected with spikes or long screws.   Our designers have the knowledge of structures to design a plan that will hold the wall up over time. ‘Deadmen’ are often buried into the hillside and connected to the wall for support. Treated timbers are easier to work with than large rock where tractor access is limited, and they take up less space, when space is tight.

In addition to a vertical timber wall, we have developed an open grid timber wall that can stabilize a slope at up to 45 degrees, or more, and has space for planting in the grid. The grid uses less materials that can lower costs.

Low walls can be made using vertical timbers or vertical round peeler poles.

Used RR ties have a considerable range of age and quality and are difficult to cut because of ground-in dirt. Also the creosote can come off on your shoes. So we tend to avoid used ties.

V.  Concrete Walls

Poured concrete is an excellent, long lasting material for retaining walls if well designed – if not, these walls do break or fall over in time. They are more expensive than the above walls due to the labor building forms, etc.

Using broken concrete pieces to build a low wall is an interesting way to recycle an old patio or sidewalk if the pieces are of uniform depth and size.  Care must be taken to provide a good, compacted, crushed gravel base and to make sure each piece is set level.

Broken concrete pieces dry stacked for a wall, concrete steps and landscaping

VI.  Brick Walls

Brick is a beautiful, classic, long lasting material that also needs to be well designed. Brick walls are more expensive than the above walls due to materials and labor. Very low walls (under one foot) can be just mortared brick. Medium height walls can be mortared double layers of brick. Higher walls can be done using brick as a facing on another structural wall, usually concrete.

VII.  Planting to Hold a Slope

Sometimes a retaining wall is not necessary. The soil may be stable on a slope and can be planted and mulched (eg bark) to stop erosion. Much erosion damage on a slope is caused by raindrops impacting against bare soil, loosening soil particles and allowing them to wash downhill. Bark mulch absorbs the impact of the raindrops and protects the soil structure. Mulch protection allows the soil surface to remain more porous, and allows water to soak into the soil much easier. Picture the loose soil under leaves in a forest versus the hard bare soil in a city lot. As plants grow, their foliage and roots help protect and stabilize the slope.

A second erosion problem on a slope is running water. It’s best to intercept the surface water and control its flow down the slope, such as in a pipe.

On larger slopes, hydroseeding (spraying mulch and grass seed in a water slurry) is often effective. The mulch (ground wood fiber) gives immediate protection and the grass grows fairly quickly.

Planting trees, shrubs and groundcovers on a slope and mulching can save a lot of expense over building walls and be very attractive.

Large rock outcroppings can be set into a slope with plantings for interest, but these rocks are added for their beauty and do not add much retaining value.

Contact Us for an Appointment!

- Janet C.