The short answer
Flat roof trims are the edge profiles that finish the waterproofing. A drip trim carries water off the edge into the gutter, an upstand trim finishes the covering where it turns up a wall, and a fascia trim covers the board edge. They are made of aluminium, galvanised or coated steel, or uPVC. They fail through poor fixing, movement, corrosion and a lack of sealing.
The trims you will see on a flat roof
Most flat roofs have two or three kinds. A drip trim, sometimes called a drip edge, sits at the lower edge and has a small kicked lip so that water falls clear of the fascia instead of running back under the covering. It usually discharges over a gutter.
An upstand is the part where the covering climbs a wall, a kerb or a parapet. Along the head of that vertical section, a wall trim, often with a flashing above, holds the covering and stops water getting behind it. Where a roof meets another roof or the side of a house, a flashing, which is a strip of metal that bridges the joint, takes the place of the trim.
A fascia trim is a deeper profile that covers the front edge of the fascia board as well as shedding water. It protects timber that would otherwise rot.
Aluminium, steel and plastic
Aluminium is light, does not rust and is the usual choice for a domestic flat roof. Coated or galvanised steel is stiffer and stronger, and suits long straight edges, but if the coating is scratched it can corrode from the cut edge. Plastic trims, usually uPVC, cost little and are easy to cut, but they become brittle over time and can distort with heat.
Colour and finish are worth a thought. A powder-coated aluminium trim can match the fascia and gutters, and keeps its look for years. A bare mill-finish strip is cheaper but dulls and stains. On a front edge that people see, like a bay window roof or an entrance canopy, the appearance of the trim is part of how the house looks from the street. Match it to the colour of the fascia and gutters where you can, and ask for the finish to be named in the quote, so nothing arrives that you did not expect.
Some systems come from the roofing manufacturer to suit the membrane. Rubber sheet, for example, is often finished with trims designed for it. See our guide to EPDM rubber roofing for how the edges are detailed. The trim should match the system, and should not be an afterthought bought separately.
How a trim lets water in
Trims usually fail at the fixings and the joints. Screws that are too few, too far apart or driven through the surface without sealing give water a path. Joints between lengths need a proper lap or a joint cover. A bead of sealant alone does not last on a surface that moves in sun and frost.
Movement is the second cause. Metal expands and contracts, and a trim fixed with no allowance for it will work loose. Third is corrosion, from a trim that was cut and left unprotected or from two different metals touching. Finally, a trim bent out of shape by a ladder or a falling branch leaves a gap that no sealant cures.
Sealant has its place, but as a finishing touch over a well-lapped joint and not as the only barrier. A trim whose joints were simply smeared with mastic will usually show gaps again within a couple of seasons. Bear in mind that the trim only protects what is behind it. A deck edge that has rotted will not hold a new trim in place, and the fascia board behind it needs checking at the same time, because a new strip of metal fixed to soft wood will be loose again within a year.
Trims and ponding at the edge
Water collects at the lowest edge of a flat roof, so the trim there works hardest. If the roof has too little fall, water backs up against the drip trim and presses on the joint where the covering meets the metal. Where the trim is set above the surface and acts like a small dam, pools form behind it. We look at this in our page on water pooling on a flat roof.
A sedum or green roof needs an edge trim that holds the planting back from the edge, plus a clear drainage path. The detail is covered in sedum green roofs.
Walk-on roofs and heavy use
Where the roof is used as a balcony or terrace, trims have to take knocks as well as water. Door thresholds need careful finishing so that water cannot run back through the door. Trims at the edge of a decked area should sit clear of the decking, with a gap for drainage. See our page on a walk-on flat roof.
Warm and cold roofs differ in how the edge is built, because a warm roof has insulation above the deck and a thicker build-up at the edge. Our guide to warm and cold flat roofs explains why that affects the depth of trim you need.
Repair, replace or re-seal
A loose or open trim can sometimes be re-fixed, with new sealed fixings and a fresh joint. A bent, corroded or brittle one should be replaced. Re-sealing alone is a stopgap, and it is the usual reason a leak returns after a short repair.
Where the covering is also tired, new trims on an old roof are poor value, since the edge will outlast the field of the roof. At survey we photograph the trims, fixings and joints along with the rest, and the pictures arrive with the written quote. Our advice is to spend only what the fault needs. The flat roofing page lists the work we do, and a copy of our 10-year workmanship guarantee is in writing.