If you want to build an arch in a masonry wall, you will need a sturdy framework to appoint the plywood arch pieces. For this, construct an inner frame from 25mm square battens. Strengthen the side pieces first by cutting and nailing battens across their inside, one across the top, one at each side and two down the middle.

To finish the frame, cut and fix two cross bearers between the outside pieces. Make these shorter than the width of the soffit (the underside of the arch) so that the outside of the frame can be covered with plasterboard and a thin skim of plaster. If you are building an arch above an alcove and have only one outside piece, allow for this by cutting the cross pieces slightly longer (fig. 10 below).

Once you have nailed the outer skin to the cross bearers, fix the template to the inside of the soffit. Drill through the cross bearers and then screw the template to the brickwork on each side using wall plugs (fig. 11 below).

In a timber-framed wall, you may get away with a much simpler framework nailed direct to the wall framing members, with the plywood pieces nailed on afterwards.

To complete the frame, line the bottom of the template with a piece of 3mm flexible plywood. Cut this to size and then, starting from one end, push the board upwards; nail and glue it into place (fig. 12 below) using hardboard pins and PVA adhesive. If the board is not pliable enough to bend into shape, dampen it first with a little water.

10 Once you have constructed both frames, nail the sides together using short pieces of timber cut to the correct length

11 Drill holes for plugs in each side of the soffit and screw the frame into position across the top of the opening

12 The underside of the template is made from a piece of bent plywood or hardboard. Wet the material to make it pliable