Although raised beds can be built at any time, most gardeners find it convenient to build them in winter, as long as the soil is not too wet or frozen. Where winter waterlogging is a problem, build raised beds in late summer. When building raised beds, the following points need to be considered:
- Firstly, define how big your raised bed needs to be, and where you need it
- Walking or stepping on raised beds is best avoided, so go for widths of less than 1.5m to allow access from the sides
- Avoid long runs of beds so that people are not tempted to step on them to get to the other side
- Pathways should be wide enough to wheel a barrow or accommodate special needs such as wheelchairs; 30cm (1ft) is the minimum width for walking and 45cm (18in) the minimum width for wheelbarrows
- Consider the materials: timber is cheap, but even when treated is the least long-lasting; sleepers are long-lasting but costly, bulky and difficult to cut; masonry (for example, brick, stone or Paving slabs) is costly but permanent. Alternatives include recycled plastic ‘timber’ lookalikes
- Small scale projects might be accomplished using a ready-made kit
- If your garden lies within a conservation area, or is part of a listed building, local planning regulations may limit choice of materials. Additionally, if the bed is to be situated near a boundary or roadside, height limitations may be enforced. If in any doubt regarding these regulations contact your local planning authority.
Commonly used materials
Stone: Both natural stone and stone prepared for wall construction can be purchased. Skilled labour is required for construction and footings are nearly always required. Generally the most expensive material.
Brick: Strong and durable. Curves can easily be incorporated into the design. Skills are needed for construction, and footings are generally required. Engineering bricks are most suitable as they are weather resistant. Domestic bricks are much cheaper but porous and so much less durable.
Pre-cast units: Constructed from concrete or reconstituted stone, these materials are cheaper but less adaptable.
Timber: Very versatile. Pressure-treated (also called 'tanalised') wood is available. As a shorter-term alternative, untreated wood can be painted with a preservative. To prevent wood preservative leaching into the soil, line wood exposed to soil within the bed with black plastic sheeting. Untreated wood will have a shorter life than treated, although untreated hardwoods such as oak and western red cedar will still last many years. Gravel boards are generally sold only as pressure-treated timber.
Railway sleepers: It is no longer permitted to use railway sleepers impregnated with creosote in garden, due to the risk associated with frequent (daily) skin contact. If you already have raised beds made from old railway sleepers and have this level of contact, then protective clothing (gloves etc.) should be worn. For new beds, use sleepers treated with other preservatives, or untreated hardwood sleepers. Note that this material requires heavy lifting.
Paving slabs: Can be inserted on their side. At least 15cm (6in) of slab needs to be buried in the ground for stability, leaving only 45cm (18in) above soil level. As paving slabs often move over time, 30cm (1ft) -deep concrete haunchings can be laid for extra stability, and metal plates fixed at each vertical joint. Relatively inexpensive, but heavy to lift.