Garden Walling Blocks - Pierced Screen Wall Blocks

Open screen walls made from pierced blocks can provide an attractive feature to give partial shelter or as a background for plants. They can be used to screen a patio, to build a carport wall or a porch screen, or to hide an unsightly area such as a compost heap or the dustbin.

The blocks are made from dense concrete, often white or near white, although some makers offer them in grey or other colors. Designs are generally based on geometric patterns. Some are laid in groups of four to give a larger interlocking design. Many manufacturers also make co-ordinated blocks which are not pierced; these can be incorporated in the pattern for special effects or they can be used where full screening is required in the general run of the decorative pierced walling.

Designs vary between manufacturers and different brands may not be interchangeable. Precise sizes may vary too, but they are generally a nominal 300mm (12in) square (including an allowance of 10mm for joints) and build a wall approximately 90mm thick.

Pay attention to choosing the correct mortar mix for use with these blocks. White blocks should be laid either with a mortar made with white cement and silver sand to match, or else with a darker and deliberately contrasting mortar. The light grey color of plain mortar made with ordinary Portland cement does not offer sufficient contrast and tends merely to look dirty. Take care to keep the mortar off the face of the blocks when laying them, so as to avoid the risk of staining. Align and level the blocks carefully and finish the joints neatly, as the square grid pattern will accentuate any irregularities.

Screen wall blocks are not laid with a lapped bond, but are 'stack-bonded', one on top of the other with the mortared joints aligning throughout. Stack-bonding is not as strong as lapped bonding, so a wall that is more than a couple of blocks high needs supporting piers. These piers should usually be every 3m (10ft), but follow the manufacturer's instructions for high walls or exposed locations. For extra strength, reinforcing mesh can be bedded in the honzontal mortar joints.

Piers can be built of bricks, or reconstituted stone or concrete walling blocks, but perhaps the simplest method is to use tailor-made screen wall pilaster blocks: these are made to suit end, intermediate, corner and intersecting positions and have holes through the centre so that a steel rod can be threaded through, extending into the concrete foundation, to provide extra reinforcement for high walls; the hole is then filled with a weak concrete mix.

The tops of the pilasters are finished off with special caps, and copings are made to lay along the tops of the screen blocks. These strengthen the top of the wall, as the cappings each cover two or three blocks; they also help to shed rainwater, so minimizing staining of the wall surface.

It is essential that this walling be laid on a flat base, so if you have a sloping site you will have to level it up in steps by building a dwarf wall in solid blocks or bricks. Even on a flat site this is often preferable, as two or three courses of bricks or stone walling is more practical next to soil than pierced blocks.