Standards Library · 03
BRE Digest 489
Array wind uplift
Guidance published by the Building Research Establishment covering wind loads on roof-mounted photovoltaic and solar thermal systems. It exists because the Eurocode does not provide pressure coefficients for array aerodynamics.
What it is
BRE Digest 489 provides net pressure coefficients for PV arrays on flat, low-pitched and high-pitched roofs, at a range of tilt angles and positions within the array.
The coefficients account for the aerodynamic interaction between the array surface and the roof plane, including pressure on the upper panel face and suction on the underside of the panel and the racking. The net coefficient is the structurally meaningful one.
Why it matters for rooftop solar
BS EN 1991-1-4 predates widespread rooftop PV. Without Digest 489 there is no accepted UK basis for the array's own aerodynamics, only the bare roof's.
Position within the array matters. Edge and corner modules see higher suction than modules in the field, which is why a single uplift figure applied uniformly across a layout is not an assessment.
Digest 489 is used with BS EN 1991-1-4, not instead of it: the Eurocode and its National Annex establish the site peak velocity pressure, and Digest 489 supplies the coefficients applied to it.