Effective Roof Area, (A)

• According to BS EN 12056-3, it is the surface area of roof where in with/ without wind driven rain is considered.

Without Wind driven rain

Where no allowance is made for wind, the Effective Roof Area (A) shall be calculated from the equation;

$A=L_R \times B_R$

where;

$A$ is the effective roof area, in square metres $(m^2)$;

$L_R$ is the length of roof to be drained, in metres $(m)$;

$B_R$ is the width of roof from gutter to ridge, in metres $(m)$.

With Wind driven rain

Where allowance is made for wind, the effective roof area shall be calculated as follows;

Effective impermeable roof area
Allowance to be made for the effect of wind	Effective impermeable roof area, $A m^2$
Wind driven rain, 26° to vertical	$A = L_R(B_R+\frac{H_R}{2})$
Rain perpendicular to roof (i.e. surface area of roof used)	$A = L_R \sdot T_R$
NOTES: $L_R$ is the length of roof to be drained, in metres $(m)$; $B_R$ is the plan width of roof from gutter to ridge, in metre $(m)$; $H_R$ is the height of roof from gutter to ridge, in metres $(m)$; $T_R$ is the distance from gutter to ridge measured along the roof, in metres $(m)$ $A$ is the effective impermeable roof area, in square metres $(m^2)$. In areas where wind is taken into account in rainfall calculations, where rain driven against a wall by the wind can run down onto the roof or into a gutter, 50% of the area of the wall shall be added to the effective area of the roof. Error creating thumbnail: Unable to save thumbnail to destination References BS EN 12056-3:2000 Gravity drainage systems inside buildings