Important aspects you should consider when deciding on timber sleepers for a retaining wall.
There is quite a decision to make regarding sleeper retaining walls. They are likely to be in a prominent position, so you want to make sure your construction looks as good as possible. Timber sleepers are a popular choice for garden landscaping projects. They're traditional, natural-looking materials, best suited to retaining walls and many other outdoor applications (steps, decking, garden furniture, raised beds, etc). You can pick between hardwood retaining walls vs softwood retaining walls.
We invest a lot of time and money into the value of our properties and homes, so having the right choice is very important. This decision comes down to many different factors: cost, longevity, functionality and how well it fits in your surrounding landscape.
Softwood versus Hardwood Sleepers
There is a great deal of confusion about whether sleeper retaining walls should be hard or softwood. Essentially the difference is the wood from which they are sawn and treated for use sleepers. Softwood differs from hardwood in that it comes from gymnosperm trees (trees with unenclosed seeds such as pines, cypresses etc.), and hardwood comes from angiosperms (trees with enclosed seeds in fruits, such as oaks, apple).
The physical structure between softwood and hardwood varies, and it can be seen at a microscopic level and with the naked eye. The cellular structure of softwoods is simpler and more homogeneous than in hardwoods. It is characterized by tracheids with thin, flexible walls and small pore diameters that make them enormous conduits for water transport. The hardwood tends to be more massive and fibrous, with larger diameter pores or vessels. As a result, hardwoods are commonly referred to as porous woods and softwoods as nonporous woods.
Sleeper Retaining Wall Durability
Consequently, the differences in structure lead to diversity in physical properties. In general, hardwoods are denser and more resistant to fire, growing slower and heavier. Hardwood sleepers are solid, resistant to rotting and longer-lasting than softwood. The density of hardwood sleepers can vary greatly. From as low as 52 kg/m3 to as high as 85-90 kg/m 3. This is where you will see the outstanding quality of hardwood as it is a natural material. The heavier, the better!
Regarding outdoor use, the most critical property is timber's resistance to decay, and in general, hardwoods are far longer lasting when in contact with the ground than softwoods. Oak sleeper retaining walls, for example, are highly resistant to decay and have a lifespan of at least 50 years untreated. On the other hand, pine is less resistant to decay, with an expected lifetime of around 20-25 years when treated and maintained. Another factor worth considering is the treatment colour. Hardwood sleepers tend to retain the colour treatment, while softwood sleepers will fade to grey over time. Softwood sleepers are lighter in weight, smaller, and thus easy to handle, cut and drill.
Hardwood sleepers can be a challenge to deal with, especially when it comes to inserting nails. Hardwood requires predrilling pilot holes before screwing anything into it. Otherwise could crack or split the wood.
Sleeper Retaining Wall Cost
As with any project, there are a lot of factors that can affect the cost of installation. That said, the timber choice for a sleeper retaining wall is one of the first factors to be considered as it can be one of the main drivers of the overall job cost. Seeing the hardwood price tag for your sleeper retaining wall can sometimes make you think twice about the type of timber to choose. On average, hardwood sleeper retaining walls are approximately 20% more expensive than softwood structures. Softwood is less costly and easier to work with than hardwood; however, guaranteed durability and performance should be considered when making this decision.