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A retaining wall supports a bank of earth on one side, and may be needed in a sloping garden, or as a base for a boundary fence where the neighbouring garden is at a higher level. It can be built of bricks or walling blocks in the usual way, but must be a full brick wall about 225mm thick, built with a strong bond (Flemish) and strong mortar (1 cement; 3 sand; ¼ lime). If the ground behind the wall either slopes upwards or seems unstable, or if you want to build a wall higher than 1m get the advice of a consulting engineer.

The strip footing should be set in a trench with the top of the concrete surface about 230mm below the level of the lower ground, so that the bottom of the front of the wall is a little below ground level.

To allow for drainage from the banked soil, make weeper holes through the wall. One way is to leave out the mortar in every other vertical joint on about the second course above the lower ground level. This is simplest to do between headers. Another way is to angle a plastic or clay pipe through the wall at 1m intervals.

MAKING MOVEMENT JOINTS

In a long run of wall, a movement joint is needed to allow for shrinkage or expansion of the materials. The joint is a narrow vertical gap about 10mm wide in the wall and coping, completely separating one length of wall from the next. Make movement joints at the intervals recommended (see Basic Bricklaying Introduction). Fill the gap with a strip of bituminised plastic foam and seal it on each side with a mastic ma­sonry filler suitable for outside use.

A RAISED PLANT BED

A hollow bed for planting is simply a wall that completely encloses a small area filled with soil. Build a full-brick wall 215mm thick (see Basic Bricklaying Introduction) either on a strip footing sur­rounding an area of soil, or on a completely solid surface, such as a patio.

Where there is an earth base to the bed, no drainage is needed in the wall. Where the surface is completely solid, make a series of 10mm drain holes round the base by leav­ing gaps through the wall on the first course. Choose a bond that includes headers, and leave out alternate mor­tar joints between them.

A useful height for a raised bed is 600mm. If it is any higher, the pressure of the soil as the weather causes it to expand or shrink could force the walls outwards. Do not make the bed wider than about 900mm. Not only will it take a lot of soil to fill it, but it will be difficult to reach plants in the center.