A flat roof is often the better solar site, because you choose the angle and the orientation rather than accepting the ones the builder gave you. The trade is that nothing holds the array down by gravity alone on a flat surface, so the system is either ballasted, bonded or mechanically fixed, and each of those is a different conversation with the roof.
Installing solar panels on a flat roof: three ways to hold an array down
Ballasted systems sit in trays or frames weighted with pavers or blocks, touching the membrane through protective mats and penetrating nothing. Bonded systems are adhered to the membrane using a manufacturer approved method. Mechanically fixed systems go through the membrane into the deck and are flashed. Ballast is the most common on commercial roofs because it protects the waterproofing warranty; it is also the heaviest, and that weight is the reason a structural check is not optional.
Angle and spacing are design decisions, not defaults
On a flat roof the mounting frame sets the tilt, and a shallower tilt lets rows sit closer together without shading each other, which fits more capacity on the same area at slightly lower output per panel. Steeper rows produce more each and need more space between them. East west facing systems, where panels face in two directions at a shallow angle, spread generation across the day rather than peaking at noon, which suits a building with a long working day. Wind uplift rises with tilt, and so does the ballast needed.
Can solar panels be installed on a flat roof without voiding the warranty?
Most flat roof coverings are warranted by the manufacturer or the installing contractor, and putting anything on, through or stuck to them can affect that warranty. Settle it before design: whose warranty is it, what does it permit, and does the covering's manufacturer approve the mounting system and the protection mats. On an older roof there is a second question, which is remaining life, because an array on a covering with a few years left is a job that has to be undone.
What a solar panels flat roof quote should state
The mounting system by name and whether it is ballasted, bonded or fixed. The total imposed load in kilograms per square metre and the point loads. The structural statement confirming the roof takes it. The tilt angle, row spacing and the modelled generation at that tilt. The membrane manufacturer's position on the fixing method. Access and how the ballast gets up there. Without those, the quote is describing panels rather than an installation.
Questions people ask about solar panels on flat roof
Do flat roof solar panels need to penetrate the roof?
Not necessarily. Ballasted systems hold the array down with weight and penetrate nothing, which is why they are common on commercial membranes. The trade off is added load, which has to be checked against the structure.
What angle should flat roof panels be set at?
It is a design choice between output per panel and how many panels fit. Steeper tilts produce more per panel and need wider spacing and more ballast; shallower tilts fit more capacity into the same area. An east west layout spreads generation across the day.
Will it invalidate the roof warranty?
It can, which is why the covering manufacturer's position on the mounting method should be established in writing before the design is fixed rather than after the array is up.
How much does a flat roof array weigh?
Enough that it always needs checking. Ballast is usually the dominant weight rather than the panels, and the figure should be given as a distributed load and as point loads so an engineer can assess it.