The name describing each arch type comes from the basic geometric shape. In this case the top line, head, or extrados of the arch, is flat. Whilst the bottom, intrados or soffit, may be either flat or cambered. When the camber is present, the curve traditionally rises to the centre 1/8" per foot span, which is approximately 1% of the span. However, the camber may vary dramatically. The camber is not a structural feature; it acts to remove the optical illusion of the arch sagging due to the splayed skewbacks at either end.
This arch type arch is also known as a Jack arch or Skewback arch. The skewback is the face of the arch that meets the face work of the wall. The skewback sits at an angle to the horizontal level; this angle is the skewback angle. It can be described either as, an angle measured by protractor, or as a linear distance past the span. The mortar joint forming the skewback angle should match those joints of the arch itself.
The graphic shows an example of a Bulmer fabricated structural arch. The animated video demonstrates the simple construction process.
To specify a flat arch you will need the following information. When measuring period property it is useful to measure the work, using the system in which it was built. Typically before 1970 in the UK work was built using Imperial dimensions E.g. 4 courses per foot or in metric, 4 courses per 305mm:
- Brick type & colour
- A digital image of similar arches on the property
- Opening span
- Face height, (measured vertically at the jamb)
- Skew angle
- Joint width
- Number of voussoirs (complete tapered sections from Intrados up to Extrados counted across the arch).
- Bond pattern, (the horizontal joint pattern across the arch)
- Camber rise (measured at the centre from the lowest points of the arch)
- Soffit depth (from front to back)
- Brick finish (rubbed or sand faced)
We will also need to know whether the arch will have any special features such as a keystone or carved soffit. If you are not sure call us on 01787 269132 for advice or email your enquiry to firstname.lastname@example.org