Tuesday, 3 October 2017

Internal boxes with cement board

This project at Bob & Elaine Humphrey's house in Landbeach, Cambridge gave the opportunity to try something new - nest-boxes made out of cement board.

by Bill Murrells and Dick Newell

Most retrofit projects using the Cambridge System have had the nest box wholly inside the roof space. In this project it was preferred to have the boxes spanning the cavity, embedded in the inner leaf of a cavity wall. The boxes extend only a short distance into the roof space. Boxes this high up under eaves in a roof space are not going to have damp or cold spot problems.

Any commercial nest box would have needed a larger hole in the inner leaf and would need to be modified with an inspection door, so we went for the DIY approach. Although treated quality plywood in this situation is probably perfectly OK, we thought we would give a recognised building material a go.

We chose 12.5mm Knauf Aquapanel Exterior Cement Board, never having used it before. This is a high quality product, about twice the price of 12mm plywood. We bought a 90cm x 240cm sheet - sufficient for 12 internal boxes. We made 6 boxes for this project, 3 in the south gable and 3 in the north.

The material was cut using a small tungsten-tipped hand saw. This gave a good finish.

As the cement board could not easily take a screw without cracking, the whole thing was glued together (we used Hippo Pro 1). We constructed the back of the box, with a hinged door out of 12mm plywood. The hinges are 'piano' hinges

The boxes have an internal floor area 22.5cm x 22.5cm and headroom 15cm.

A hole was cut with a masonry drill and a handsaw through the inner-leaf slightly larger than the box cross-section: 25cm x 17.5cm.

The following pictures explain how it was done:

2 boxes complete. The entrance piece has been stained with tea. 
3 rectangular holes in the inner-leaf and 1 box in position. 

3 boxes mortared in. 
3 nicely positioned entrances under the eaves, facing south. Space is made for the entrances by trimming 56mm off the ends of adjacent bricks each side of a vertical bond. This is achieved by drilling a line of small holes with a masonry bit. A hammer and chisel is not used as this would risk cracking the brickwork.
A House Martin's nest, occupied in 2017 and 3 Swift entrances in the north gable