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Property Surveying Blog

Written by Bradley Mackenzie

As Party Wall Surveyors we regularly agree Party Wall Awards for rear or side extensions.

Within London and other well built up areas you will often find that the proposed extension will be built up to or alongside a neighbour’s extension or property.

This means that when the extension is complete, there will be two walls alongside each other with a very small gap between those two walls.

If appropriate weatherproofing isn’t installed at the time the latter extension is completed, this gap/strip of land can become a trap for leaves or pests and can eventually lead to property defect for both the building owner and adjoining owner.

The majority of properties built post 1930 will have some form of damp proof course installed. A Damp Proof Course (DPC) is a barrier built into the external walls of the property that sits 300 to 400mm above the external ground level.

The damp proof course is there to stop damp from rising up through the brickwork and then causing internal damage to the property.

In order for a damp proof course to work, the ground level must be kept clear to ensure that the distance between the barrier and the ground is always kept to a certain distance.

When there are two walls built alongside each other, this distance can quickly become compromised as leaves, detritus and even builder’s spoil builds up within the gap and eventually bridges the damp proof course.

In order to avoid such an issues, the common solution is to install a flashing or weatherproofing solution at roof level. This solution is usually in the form of a lead flashing which seals the highest point where the two walls meet one and other and importantly ensures the gap below remains a gap.

In these photographs you can see how these gaps can become a real issue for both respective property owners, in this instance the off cuts from an extractor vent has been disregarded into the gap. Rain water will fall onto the vent, bounce off the upper surface and penetrate the wall leading to penetrating damp.

If you are planning an extension, or your neighbour is planning on constructing an extension, make sure the weathering detail isn’t overlooked. The inclusion of a weatherproofing detail can avoid future issue for both your property and your neighbour’s.