A Surveyor’s Guide to – garden walls

The Department for Communities and Local Government has produced the following advice on checking the safety of garden walls.

To check whether repairs are necessary, or if a wall needs rebuilding, garden and boundary walls should be inspected from time to time.  Garden walls are among the most common forms of masonry to collapse, and they are one of the commonest causes of death by falling masonry.  Your insurance may not cover you if the wall has been neglected.

Over the years, masonry walls may be affected by:

  • general deterioration and ageing;
  • an increase in wind load or driving rain, if a nearby wall is taken down;
  • the felling of nearby trees or planting new trees close to the wall;
  • changes leading to greater risk of damage from traffic;
  • alterations, including additions to walls or the removal of parts of the wall e.g. for a new gateway;
  • changes to the water table equilibrium.

Things to check

  1. Is the surface of the brickwork crumbling away? If the damage is restricted to a few bricks this may not be serious, but walls can be weakened by general crumbling across either face.
  2. Is the mortar pointing in good condition? If the hard surface layer can be picked out from the joint, or if the mortar can easily be scraped out, perhaps with a door key, then this is a good indication that the wall may need repointing.
  3. Is there a tree near the wall? As trees mature, there is a risk of the wall being damaged by roots, and from falling branches. Damaged sections may need to be re-built, perhaps with bridges incorporated to carry the wall over the roots.  Removal of mature trees can also lead to problems as soil accumulates more moisture and expands.  Dead tree roots can also collapse, undermining the wall’s foundations.
  4. Is the wall upright? Walls lean for a variety of reasons, sometimes because of failure below ground caused by tree roots, cracked drains, frost damage to the foundations or inadequate foundations. If your wall leans to an extent that it could present a danger e.g. more than 30mm (half brick wall), 70mm (single brick wall) or 100mm (brick and a half wall) you should seek expert advice, which may also involve checking the wall foundations.
  5. Is the wall thick enough for its height? The table below gives guidance on how high walls should be in different parts of the UK relative to their thickness. You should seek expert advice if your wall exceeds the recommended height, or where this guidance is inapplicable e.g walls incorporating piers, supporting heavy gates or retaining soil.
  6. Some climbing plants, such as ivy, can damage walls if growth is unchecked. Consider cutting them back and supporting regrowth with trellis, clear of the wall.
  7. Is the top of the wall firmly attached? Brick or concrete copings may be loose or the wall may have horizontal cracks in the brickwork a few courses down, possibly from frost damage. Loose or damaged masonry near the top of the wall will need to be rebuilt.
  8. Has the wall been damaged by traffic? Minor scratch marks or scoring of the surface may hide more significant cracks. Piers at vehicular entrances may have been dislodged by impact and be unsafe; in such cases, they should be rebuilt.
  9. Are there any cracks in the wall? Hairline cracks (0-2mm across) are common in walls and may not be an indication of serious problems. For wider cracks seek expert advice – some may indicate a need for partial or complete rebuilding.  Seek advice on any horizontal cracks which pass right through a wall or which are close to piers or gates.  Repointing cracks can lead to problems.  Never repoint without first establishing the cause of the cracking.

Safe heights for walls of different thicknesses (Government guidelines)

Zone 1 Zone 2 Zone 3 Zone 4
*Wall Thickness Max.  Height *Wall Thickness Max.  Height *Wall Thickness Max Height * Wall Thickness Max.  Height
½ Brick¹ 525mm ½ Brick ¹ 450mm ½ Brick ¹ 400mm ½ Brick ¹ 375mm
1 Brick ² 1450mm 1 Brick ² 1300mm 1 Brick ² 1175mm 1 Brick ² 1075mm
1½ Brick ³ 2400mm 1½ Brick ³ 2175mm 1½ Brick ³ 2000mm 1½ Brick ³ 1825mm
100mm Block 450mm 100mm Block 400mm 100mm Block 350mm 100mm Block 325mm
200mm Block 1050mm 200mm Block 925mm 200mm Block 850mm 200mm Block 775mm
300mm Block 2000mm 300mm Block 1825mm 300mm Block 1650mm 300mm Block 1525mm

¹ = 100mm; ² = 215mm; ³ = 323mm.

For more advice on all of these matters contact www.PropertySurveying.co.uk