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. Revised 2014, replacing the 2004 first edition, and written on the Eurocode. It exists because no British or European Standard gives specific guidance on wind loads for these systems.
What it is
BRE Digest 489 provides net pressure coefficients for arrays on flat, low-pitched and high-pitched roofs, at a range of tilt angles and positions within the array. The 2014 revision defines ten generic solar system types covering integrated, above-roof and free-standing installations, and applies to solar thermal as well as PV.
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.
The Digest is written on BS EN 1991-1-4 and the UK National Annex, and gives a simplified route to peak velocity pressure using five UK wind zones banded by basic wind speed: Zone 1 below 22 m/s, Zone 2 at 22 to 24, Zone 3 at 24 to 26, Zone 4 at 26 to 28, Zone 5 above 28, with the Channel Islands taken as 24 m/s. Corrections apply for site altitude above 100 m and for hill slope.
For roof-mounted solar it gives a safety factor γf of 1.35, derived from a partial load factor for wind action of 1.5 multiplied by a consequence class factor (CC1) of 0.9, as recommended in MCS 012.
Why it matters for rooftop solar
🚨 The 15% rule is the part most often missed, and it is Building Regulations rather than a voluntary scheme. The Digest states that where a retrofit solar system increases the applied roof loading by more than 15%, Approved Document A of the Building Regulations 2010 requires that, for building works carried out in England and in Wales in some circumstances, the structural integrity of the roof structure and the supporting structure should be assessed. Scotland and Northern Ireland have their own equivalents. This applies regardless of MCS scope, so it reaches the commercial work above 50 kWp that MIS 3002 V6.0 does not.
The Digest goes further than the threshold: it says the roof should be assessed even where the loading increases by less than 15%. And where the roof cannot support the additional load, strengthening is required, which is classed as a material alteration under building regulations.
It also names the failure mode a simple weight check misses: a change in load distribution can change the behaviour of the trusses, causing members designed for tension to go into compression, and vice versa. Solar installations also generate point loads at every fixing. Neither shows up in a spare-capacity calculation.
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.