Masonry Bee Holes in Structural walls by Harrow Chartered Surveyor

What (Masonry) Bee Causing Those Holes?

If you have an external wall that is beginning to resemble a block of Swiss Cheese with holes in it, you may have an infestation of Masonry Bees.

In a recent survey of a house in Hertfordshire, Chartered Surveyor Steve Cornish found just that, an external wall with many holes of between 5mm and 10mm in diameter and 15mm to 25mm deep. Another home owner said they had 70 to 80 such holes appear in one elevation.

Masonry (or “mortar”) bees are a solitary type of bee that do not nest in colonies but within individual holes in mortar joints and soft bricks. Solitary, female bees are attracted to sites where other females are present. In Britain, there are nearly 20 species, the most common being the Red Mason Bee. Masonry bees favour sunny, south-facing elevations as these enhance the germination of their eggs. Nests are established in spring or summer.

Recognised technical sources confirm that masonry bees do not necessarily cause serious damage to buildings. The bees excavate or enlarge holes, throwing out the spoil behind them. In extreme circumstances, the annual burrowing activities may cause an extensive system of galleries and this can disrupt the bearing capacity of masonry, which can fill with water that expands on freezing and cause further deterioration.

The Society for the Protection of Ancient Buildings (SPAB) recommendations on treatment include the following

a) Where a dense aggregation of bees occurs within the fabric of the building, the affected mortar joints should be cleared out to a depth of 15mm and re-pointed, preferably in late summer or autumn. The timing of this is fairly important because the female masonry bees commonly re-use their natal nest sites. This filling of the holes will break the cycle.

b) The new mortar should not be too strong for the bricks but hard enough to discourage the bees. Any galleries and burrows can be filled using a mortar gun with a wetter than usual mix to aid the flow.

c) SPAB note that chemical treatments alone are not always effective in the long term and may stain the masonry. It is also ecologically undesirable to use. Mortar-based insecticide is also sometimes added to mortar mixes, particularly when re-pointing in the spring rather than late summer or autumn and a risk exists of damage before the mortar hardens.

Woodward Chartered Surveyors have a surveyor in each of Oxford, High Wycombe, Chesham, Rickmansworth Northwood, Pinner as well as offices in Harrow on the Hill for West London and North West London and in the heart of London’s West End at Thayer Street, W1.