Roof plant waterproofing
Roof-mounted plant and AC units: protecting the membrane
The short answer
Plant stays on a commercial roof without ruining the membrane when it sits on its own raised supports, every pipe and cable passes through a properly formed and sealed detail, and engineers walk a protected route instead of the bare surface. Leaks around plant tend to start at holes cut after the roof was finished, so the waterproofing needs planning before the unit is lifted into place.
Where air-conditioning condensers, extract fans and other plant damage flat roofs, and the details that keep the covering watertight around them.
Why plant is where flat roofs start to leak
A flat roof with nothing on it is simple. Add a condenser, an extract fan or a run of ducting and the roof gains weight, holes and visitors. Each of those three problems can end up as a leak.
The weight sits on feet or frames that press into the covering. The holes are the pipes, cables and refrigerant lines that run from the unit into the building. The visitors are the service engineers who climb up every few months and walk across whatever lies between the access hatch and the machine, often carrying tools and spare parts.
Plant is often fitted years after the roof, by a different trade, with nobody asked to look at the waterproofing. A tidy installation can still leave a puncture hidden behind a bracket. That is why we ask who fitted the equipment, when, and whether the covering was repaired afterwards, before we tell you anything about the roof itself.
Supports that do not pierce the covering
The cleanest way to carry a unit is on a raised frame or set of plinths that stand on the structure or on load-spreading pads, with the membrane dressed neatly around the base. A bracket bolted straight through the covering creates a hole that has to be sealed every time it moves.
Where a fixing must go through, the usual approach is to form a raised collar or upstand round it, lap the membrane up the sides and finish the top so water runs away from the opening. The top of the detail is the part that gets forgotten, and it is the part that fails first.
Weight deserves a check too. A heavy unit changes what the deck carries, and the structure has to be shown to cope before any plant goes up. Whether the structure can take it is an engineer's call, so for anything substantial we suggest a structural engineer confirms it before the plant goes up. Ask for that opinion in writing and keep it with the roof records.
Pipes, cables and the holes they need
Refrigerant pipes, condensate drains, electrical conduit and ducting all need a route through or over the roof. The weakest option is a hole sealed only with mastic, which shrinks and splits as the roof moves with the temperature, usually within a few seasons of the unit being fitted.
A better detail uses a purpose-made collar or boot that suits the membrane, with a flashing (the folded strip that keeps water away from a joint) lapped over it, and a clamp at the top. Where several pipes travel together, one larger upstand with a cover often works better than a string of small seals.
Condensate is a quiet cause of trouble. A drain that discharges onto the membrane keeps one patch wet for months, stains the surface and speeds up wear. Pipe it to an outlet or gutter, or run it to a purpose-made tray, so the water never has a reason to sit on the surface. Our commercial roof maintenance guide covers keeping outlets and details clear.
Walkways, edges and safe access
Foot traffic does more harm than weather on a roof that is serviced regularly. Marked walkway pads from the roof hatch to each unit give engineers a safe path and spare the covering. Without them, a dropped spanner or a dragged toolbox can cut through the surface in a second.
Access has a safety side as well. Any roof that people walk on needs a plan for the edges, and some coverings and rooflights are fragile enough to fall through. Read our guides to roof edge protection and fragile roof safety before anyone is sent up to service equipment.
If your engineers visit often, say so at the survey. It changes the covering we recommend, because a tougher surface or a protected route costs less than repairing the same patch again and again. Tell us what is on the roof today and what may be added in the next few years, so details can be left ready.
Planning the work with the plant installer
The tidiest results come when the roofer and the plant installer agree the order of work. The roofer prepares the supports and penetration details, the installer sets the equipment and connects the pipes, and the roofer returns to close up and test the finished details.
Photographs matter on both sides. A record of the roof before the unit goes on, and again afterwards, settles any later argument about where damage came from. Agree in writing, before the work starts, who pays to make good if a membrane is cut.
On a roof carrying plant, our survey covers every support and penetration. We photograph the weak points, tell you which jobs are urgent and which can wait, and then put a single total in writing. See our commercial roofing page for the other flat-roof work we carry out.
Related questions
Plant on the roof
Can air-conditioning units stand directly on a flat roof?
They can, once the roof has been assessed, but not straight onto the bare covering. A raised frame or pads spread the load and protect the surface. A structural engineer should confirm the deck can take the weight, and your building control body can say whether the work needs approval.
What if the installer has already damaged the membrane?
Photograph the damage and tell the installer in writing at once. A roofer can survey it, cut out the damaged area and patch it, then form a proper detail round the fixing. Who pays depends on your contract with the installer, so settle that before repairs begin.