Method 4 - Aggregate Notional Area for Unprotected Areas

This method can be used for a building or compartment of any purpose group. A building or compartment should be so situated that the aggregate notional area of the unprotected areas in the side of the building or compartment does not exceed, in the case of – (a) a building or compartment of Purpose Group 1, 2, 3, 5 or 7(b) – 210 m2; or (b) a building or compartment of any other purpose group – 90 m2, when calculated from any vertical datum on its relevant boundary. The aggregate notional area is the aggregate of the areas of unprotected area in the side of the building or compartment, each such unprotected area being multiplied by the Factor given in Table 5.5 relative to the distance the unprotected area is from the vertical datum. A vertical datum is a vertical line of unlimited height at any point on the relevant boundary and the datum line is the line joining the vertical datum to the nearest point on the side of the building or compartment. When calculating the aggregate notional area no account should be taken of any unprotected area which is – (i) screened from the vertical datum by any part of an external wall which is not an unprotected area; (ii) outside a horizontal area centred on the vertical datum, having a radius of 50 m and extending 90° on either side of the datum line; (iii) facing away from the vertical datum; or (iv) making an angle of less than 10° with a line drawn from it to the vertical datum.