Building Screen-block Walls

A screen-block wall should not span more than 3m without intermediate pillar supports. You cannot cut screen blocks nor the pilasters, so check the exact dimensions when planning. Blocks normally measure 300 x 300 x 90mm thick, and pilasters 195 X 195 x 200mm high including joints.

Foundations

For a low wall, foundations should consist of 100mm of hardcore topped by 100mm of concrete and be 300mm wide. Use a mix of 1 part cement to 5 parts ballast. Check with manufacturers for high walls. If using screen blocks on a sloping site it is necessary to lay a ‘plinth’ of bricks.

Stretch string lines through the site then dig the trench and lay the foundations. Although not strictly necessary on low walls of up to three courses, it is nevertheless well worthwhile reinforcing the pilaster blocks by setting an iron rod or angle-iron into the foundations. The pilaster blocks are lowered on to the reinforcement as the wall is built, so obviously the positioning of the angle-iron is vital. It must also be truly vertical and protrude from the ground to within a few centimetres of the top of the pilaster column. The mortar mix should be 1 part cement to 5 parts builder’s sand and must be pliable. If you want the mortar joints to blend in colour with the blocks use white cement and light-coloured sand. For a contrasting colour use ordinary grey cement. To fill the pilaster blocks use a fluid mix of 1 part cement to 3 parts sand.

Laying blocks

First lay a row of blocks and pilasters along the foundations to check positioning. The first pilaster can then be laid on mortar and checked both horizontally and vertically for level. It must also align with the string guide-line. Repeat the procedure with the other end pilaster and intermediate pilasters. If there will be more than three pilaster blocks in each column, pour in reinforcing mix after each is laid.

Spread mortar on the base and inside the pilaster groove, then position the first screen block in the groove. Check it for level and, if necessary, tap it down with the shaft of the club hammer. Screen blocks are quite fragile so, when tapping down to level, lay a piece of timber on the block to spread force of the light taps. Build up the wall checking as you go for both horizontal and vertical alignment. In addition to the horizontal layer of mortar on which a block is laid, trowel some on to one side before it is positioned, to serve as a vertical joint. Make this a little more sloppy to aid sticking.

When the required height is reached, add caps and coping, and point the joints.

And putting the moulding into it. The best (and most expensive), rigid glass-reinforced plastic types are almost self-supporting and there is no need to be too careful over their installation. However, the thinner moulded-plastic ponds can crack if they are not installed in a hole carefully packed round with sharp sand. To install this type, stand the moulding on a base of damp sand and pack round the sides with damp sand as it is filled with water. Take particular care to pack the sand under the shelves to ensure that these are well supported.

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