The easiest way to form a plasterwork arch is to build a false arch, a template out of timber and fit this across the top of the opening. The template must be tailored carefully, with both outside faces slightly indented so they can be covered in plasterboard and then finished level with the existing wall.

Before you build the template check carefully that the headroom above the opening will not be restricted by the addition of an arch, especially important for a doorway in constant use. Once you are satisfied, remove the door and prise or cut away the frame and the door stop carefully so you can replace them afterwards. The template is made up of an inner frame of wooden battens, surrounded by an outer skin of plywood. The outer skin is fixed to both sides of the frame or one side only if you are constructing an arch in an alcove and can be cut into a number of shapes according to the design of the arch.

[edit] Semi-circular Arch

This is the most common type of arch and the easiest to mark and cut out. First measure across the opening directly below where you want the arch to sit and draw a line the same > length on a sheet of plywood. Mark its midpoint. With this as a center, and half the length of the line as a radius, draw a semi-circle above the line using a large pair of compasses or a pencil and string (figs 3 to 5). Complete the marking out by drawing a box shape around the semi­circle. Start from each end of the baseline and draw two vertical lines up from it. Continue these alongside the semi­circle and stop when you are about 50mm above the top of the curve at each side. Then connect the two with a horizontal line across the top; use a fretsaw to cut out the shape of the semi-circle and the box around it.

3 To construct the template, first measure the distance across the top of the opening at what will be its widest point

4 Then transfer this distance to the bottom of a piece of plywood. Measure and find the midpoint of this line and mark it on the board

5 Fix a piece of string to the midpoint and tie a pencil to the top end. Use this to draw a semicircular arc across the board

[edit] Segmental Arch

To reproduce the curve of an arch, fix a batten across the opening held in place with folding wedges. From the center of the batten stretch a piece of timber or string. By adjusting the position of the cross bearer, you can reproduce both semi circular and segmental arches

This arch is slightly more difficult to mark out but has the advantage of allowing more headroom.

Start as you did with the semi­circular arch by measuring across the opening and then draw a line the same size on the bottom of a sheet of plywood. Find the midpoint of this line and mark it clearly.

Then draw two lines running vertically up from each end towards the top of the sheet. If you extend your compass between these lines using the midpoint as a center, you will be able to draw a number of segmental arches, some almost as tight as a semi-circle, others with a very gently graded curve.

Once you have chosen the curve you want, draw it out and complete the shape by drawing a box around its top (see below). Then use a fretsaw to cut around the outside of the arch and the box above it.

[edit] Other Shapes

You can create a whole variety of shapes for plasterwork arches which are not part of a true circle.