The short answer
It depends on the structure and the covering, and only a structural check can say. Many modern steel-framed sheds and concrete-deck buildings can carry solar panels, but older roofs, lightly framed roofs and roofs with brittle sheets may not. Two questions decide it: can the frame take the panel weight, fixings and wind uplift, and is the roof covering sound enough to last as long as the array? If the roof will need replacing soon, do that first.
The two questions that matter
The first question is structural. Panels add dead load, which is the weight of the panels, rails and ballast or clamps. They also change how wind acts on the roof, because an array can catch the wind and transfer uplift into the fixings. A roof built in the past was designed for the loads of its day, and the original drawings, if they exist, may show spare capacity or none. A structural engineer, working from the drawings or an inspection of the purlins, rafters, trusses or deck, gives a safe figure. We do not judge that by eye, and nobody else should.
The second question is the covering. Panels are expected to stay on a roof for a long time. If the roof beneath them wears out first, the array has to be taken off to repair the roof, which is slow and costly. So the covering needs a realistic remaining life, and where it is short, the sensible order is to renew the roof and then fit the panels. Our guide to repair, overlay or replace helps you judge the position of your own roof.
How the panels are held on
The fixing method depends on the roof type. On metal sheeting with a standing seam, clamps grip the seam, so no holes are drilled through the sheet, which is why the seam roof is so popular for solar. Our page on what a standing seam roof is explains the profile. On trapezoidal sheets, brackets are fixed through the sheet into the purlins, and each hole is sealed. On a flat roof, frames can be ballasted, which means weighted down without penetrations, though the ballast adds load, or mechanically fixed through the membrane with sealed details.
Each method carries risks. Penetrations create leak points, so the details need proper flashings and a roofer who understands the membrane. Ballast can hold water, block drainage or move in wind, so outlets must stay clear. On a green or planted roof, the layers add load of their own, as discussed on our page about green roofs on commercial buildings.
Roof types and what they mean for solar
Concrete-deck roofs with a membrane tend to have the most spare capacity, and ballasted frames are common on them. Steel portal-frame sheds vary: a modern building designed with a solar allowance may be fine, while an older one built close to its limit may not be. Roofs of fibre-cement or asbestos cement sheets are a special case, as the sheets are brittle and must not be drilled or walked on, and we cover them on our page about asbestos cement roofs. Timber-framed roofs on older shops and offices need a close look at the rafters and any past alterations.
Roof age and shape matter as much as the type. Valleys, parapets and plant areas limit where panels can sit, and shade from taller neighbours affects the layout. Aim to put the array on the best-supported, most sheltered part of the roof.
Other things to settle before ordering
Check the warranty. Many membrane makers require their own approved detail for fixings and may void cover if work is done another way. Tell the roof installer and the solar installer about each other, so the two warranties do not clash. Ask who will make good the roof if a bracket leaks.
Plan access and safety. Panels are walked past, cleaned and maintained, and routes must be clear of fragile rooflights, with edge protection on the perimeter. Our page on roof edge protection sets out what is needed. Leave space for maintenance walkways and for the drainage. Note too that a new array may need approval. Ask your building control body and your council's planning team what they expect, because rules vary by site and building type.
The roofing check that comes first
Before any array is quoted, a roofer can survey the covering and the drainage, so you know whether it is fit to carry panels. We look for ponding, soft spots, open laps, corroded sheets and wet insulation, and photograph what we find. If the roof is sound, we tell you; if it needs work first, we tell you that, with a fixed written quote for the repair or renewal. We do not install the panels or supply the structural figure, and we say so plainly.
If your building needs a new roof soon, it can make sense to do both together, since a single scaffold and a coordinated programme save cost. Our free survey is the start. Business roofing in general is covered on commercial roofing in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
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