Decide whether the door will open inward or outward and on what side it will be hinged. Refer to the hinge side when viewed from the outside. Check at the head, foot and sides for obstacles that may hinder the opening of the door.
Measure overall width and height (including frame and cill) and then measure diagonals to check that the aperture is square (A).
Existing frame/aperture may not be equal from top to bottom. Therefore, take 3 measurements, across top, middle and bottom. Deduct 10mm from the smallest measurement for the width.
Three measurements should also be taken of the height, on left, centre and right. Deduct 10mm from smallest measurement for the height. Be sure to measure to underside of existing cill, unless it is to remain, as a concrete cill might.
Measure diagonally from top left to bottom right and compare to measure of bottom left to top right. If difference between diagonal measurements is more than 5mm then the aperture is not square and frame size may need to be reduced to set the door plumb.
Width and Height measurements for the inside (B) are taken in the same way as those outside but without 10mm deductions. If the difference between outside and inside widths (C) is large then Add-ons may be needed to ensure neither side is fouled by plaster.
Add-ons are used to increase width of frame on hinge side or lock side when the reveal (difference between outside width and inside width) is considerable. If the inside height is much short than outside then an Add-on on top of the frame may also be needed.
A hinge-side plaster reveal of over 25mm would likely hinder the hinge (D), therefore an Add-on on that side should be used. A lock-side reveal of more than 40mm would require an Add-on on that side. Add-ons, which are available in 15mm, 25mm and 50mm thickness, are also useful to clear the door from other obstructions, such as skirting, dado rail and radiators.
