Roof coverage - Imerys Roof Tiles

To use terms proper to geometry, the right angle is a line drawn perpendicular to another straight line. This straight line is a horizontal line defined as level in various ways.

This line is parallel to the gutter when the latter is recognised as being horizontal. This line can be defined as the line drawn between two points on the rafters with the help of a spirit level, well known to builders (A very reliable method especially when the gutter and the ridge are not level).
This line can be determined by taking a straight edge in wood or even better in aluminium on which is placed a spirit level. (slightly less precise when the roof is not perfectly level).

Generally the squareness of the perpendicular can be checked by measuring 60cm along the base line and 80cm on the vertical and the diagonal (Hypotenuse) should measure 1m.

■ Once the level has been drawn you trace parallel lines to this line which will allow you to line up the battens.
■ On the side of a batten draw a line x-y parallel to the top edge of the batten. Anywhere on this line mark a point A from which will be drawn the perpendicular. From this point mark two points B and C equidistant from A with a bradawl.
■ Using a batten with a nail at either end, make two arcs from point B and from point C.

To be certain that these two arcs intersect on the supports, it is recommended that you nail two pieces of plywood or metal at the place where you presume the arcs will intersect.The straight line joining these intersections should be marked with a builder’s chalk line. This is how the perpendicular is drawn and this represents the line of greatest fall.