Twelve things you can see on the roof without touching anything

Water sitting on a roof after it has had time to drain is a roof problem, not a unit problem — a blocked drain, a sagging deck, or a curb that has been reset badly. It matters to the plant because standing water rots the curb, the flashing and eventually the base of the unit from underneath, and because a unit sitting in water is a unit nobody wants to service.

It is also the single most common thing that quietly destroys rooftop equipment years early.

The curb is the hole in your roof that the unit sits over. When the flashing or the sealant round it fails, water goes into the building at that penetration — usually appearing on a ceiling somewhere that looks unrelated to the roof.

We can lift and reset a unit so the work can be done properly, but the repair itself belongs to whoever is responsible for the roof, and doing it in the wrong order means paying twice.

Rain, snow and wind-driven dust go straight into the electrical section and the blower compartment. A panel that has been off since the last visit usually means somebody was interrupted, and it will not have been recorded anywhere.

It is also a live electrical enclosure standing open on a roof that other trades walk across. This one is worth a call the same day.

A blocked condenser coil cannot reject heat, so head pressure rises, capacity falls and the compressor runs hot for every hour the unit runs. It shows up on your electricity bill long before it shows up as a fault, and it is the most preventable cause of compressor failure there is.

A coil that looks clean from above can be packed solid on the inside face, so this one is worth a closer look even when it seems fine.

Black rubber dust on the deck is a belt wearing out fast, usually because of tension or pulley alignment rather than the belt itself. Squealing on start-up is the same story.

When the belt goes you lose all airflow. The unit may keep running and heating or cooling nothing at all, and on a heating unit that can trip it out on temperature instead.

The economiser brings in outside air when outside air is useful. Stuck open in January it is heating the street. Stuck shut in April it is running the compressor on a day when free cooling was available.

Nothing about it is audible or visible from inside the building, which is why a failed economiser can run for years. The actuator, the linkage and the controls are all ordinary repairs.

Ice on an evaporator or at the base of a unit in summer means the coil is running below freezing, and that is either low airflow or low charge. Low airflow is usually a filter, a blocked coil or a belt. Low charge means a leak, because refrigerant is not consumed.

Switch the cooling off and leave the fan running if you can do that from the thermostat or the panel indoors, then call. Running a compressor with a frozen coil is how they get liquid back and fail.

Condensate should leave the unit by its drain, not over the side. Running onto the membrane it stains, grows things and eventually undermines the roof; overflowing the pan inside the unit it goes through the curb and comes out on a ceiling.

The cause is almost always a blocked drain, a dry or missing trap, or a pan that has corroded. All three are small jobs and all three get much larger if they are left through a season.

This is an exposed electrical enclosure on a surface other people walk on, in the weather. It is not something to reach into, close, or tidy up.

It is also not our repair. The disconnect, the conduit and the supply to it belong to a licensed electrician, and that is who should be called. We will work with them and we will not touch it in the meantime.

Pipe resting directly on a roof abrades the membrane underneath it every time the roof moves with temperature, and it corrodes from the underside where nobody looks. Collapsed or rotted sleepers put strain on joints that were never meant to carry it.

Re-supporting the pipe is our work; the membrane where it has been sitting is the roofer’s. It is worth doing both in one visit.

A new mechanical noise is almost always a bearing, a fan wheel out of balance, or a mounting that has worked loose. A hum you can feel through the structure rather than hear is usually a mount or an isolator that has perished.

None of it is urgent on the day and all of it is cheaper before the thing it is attached to fails. It also tends to be the first complaint from whoever works under that part of the ceiling.

Not a fault, but it is the reason everything else on this page takes twice as long. When nobody can say which unit serves which part of the building, every call-out starts with somebody walking round switching things off to find out.

Tagging and recording the units is part of the first visit on a maintenance agreement, and it is the cheapest thing on this list by a distance.