timber raised beds

construction note no.6002

subject:	timber raised beds
relating to design note nos:	2041: outdoor room; 3040: raised bed enclosures

As we have seen in the above design notes, raised beds introduce a vertical aspect and define the enclosed space as an outdoor room. They are also a great way of dealing with steep slopes, forming an excellent terracing system. Whilst any building material can be used to create such beds, timber is ideal for the purpose. It is quicker and cheaper to use but most importantly, it is visually a softer medium and acts as a foil to other hard landscape materials such as paving or steps. One can also experiment with the use of colour paints or pigments (see design note no 3022).

construction method:

Whilst there are many possible ways of constructing raised beds, an easy and effective method is as follows:

1. Use standard timber sizes such as 150x50mm (6x2") or 200x50mm (8x2") for the horizontal wall structure. The former has the advantage of working easily into step heights on terraced areas.
2. Use 75x75mm (3x3") vertical corner posts & 50x50mm (2x2") timber for internal framing.
3. Where a pergola structure is incorporated into the design, make posts part of the raised bed framework, especially at the corners. Size should be 100x100mm (4x4").
4. Use 150x25mm planking to form top capping rail, which is wide enough to sit upon.
5. Choose a timber that is renewable and preferably not imported - in the uk I use green oak for a high quality natural finish, douglas fir or western red cedar for a high quality softwood, all available from your local sawmill. If on a tight budget however, use sawn carcassing timber from your local timber merchant, although it will not last as long. Buy the best wood you can afford.
6. Attention to construction methods (i.e. joints), drainage and air circulation will ensure the longest possible life for your timber. Personally, I would not use tanalised timber as many such treatments contain arsenic and other noxious poisons (this may soon be made illegal by the EU). Pre-paint all surfaces and joints with a water-based wood stain as construction progresses.
7. Place all corner posts first, including any pergola posts. Soak the ends to be buried in wood preservative - but not the above ground part of the posts.
8. Mark out levels for the base row of horizontal planks, leaving a 25mm/1" drainage gap at the bottom.
9. Cut plank to length, allowing sufficient excess for butt joint (see fig. two) drill and secure with 75mm/3" zinc, brass or stainless steel screws. Use a spirit level to level planks.
10. Where several raised beds interlock, construct each horizontal layer before placing the next.
11. Place vertical internal framing at approximately 1.5m/5ft intervals to stop individual boards from bowing out. On a long unsupported run concrete in one or more vertical posts for additional rigidity.
12. When required height is reached (three planks gives 450mm/18", a comfortable sitting height) place a horizontal internal framework timber of 50x50mm/2x2" to support capping rail. Overhang capping rail by 25mm/1" each side of the timber frame. Cut corners to 45 degree angles and secure with 50mm/2" screws (countersunk if using a hard wood).
13. Give internal faces an extra coat of water-based fence paint or stain and allow to dry thoroughly - this can take several days.
14. There is some debate over the use of a plastic sheet secured to the inside of the raised bed to keep moisture off the timber. In practice this seems to trap and hold moisture against the timbers - so I stopped using it.
15. Dig over the soil in the bottom of the RB and if of heavy clay add grit or sand to hold open the structure. Add a 75-100mm(3-4") layer or shingle or stones as a drainage layer. Cover with a permeable membrane which will prevent the shingle from clogging with soil, yet allow water to drain through.
16. Place a good quality screened topsoil to RB in layers, firming as you go to minimise settlement. Pay special attention to the corners.
17. Work a good compost (not peat!) into the top 150mm layer of soil, raking smooth.
18. You're now ready to plant and mulch!

Other construction notes which relate to this subject:

choose a subject: 6001: log retaining walls 6004: vert. rly. sleeper retaining walls 6006: horz. rly. sleeper retaining walls