The short answer
Trims and upstands are the edge and junction details of a flat roof. A trim is a shaped strip of metal or plastic that ends the covering at an edge and throws water into the gutter. An upstand is a short vertical turn-up of the covering at the point where the roof runs into a wall, a kerb or a pipe.
Look up at a flat roof and your eye goes to the flat part. Roofers look at the edges and the junctions, because that is where water gets in.
Drip trims and edge details
Where a flat roof stops at the front or side, the covering has to stop somewhere, and water has to leave cleanly. A drip trim does both jobs. It is a profile, often aluminium or coated steel, fixed along the edge of the deck. The covering is dressed over it and bonded down, and the trim projects beyond the fascia, the board that closes off the roof edge, so water drops clear and does not creep back underneath.
Why does an edge trim matter so much?
An edge with no trim, or one set too low, lets water track back along the underside of the deck. It soaks the fascia, stains the wall below and rots the timber from the outside in. Trims also hold the edge of the covering down against wind. A felt or rubber sheet that is simply folded over a sharp board lifts first at the corners, in exactly the places where the wind pulls hardest.
A good trim sits on a firm, straight, dry timber, with joints between lengths that overlap and are sealed. A bent, sprung or rusting trim is a sign that water has already been getting behind it, and that matters when you weigh a repair against a new roof.
Upstands at walls, pipes and chimneys
An upstand is where the waterproof layer turns up a vertical surface instead of stopping at it. The turned-up part must rise high enough that rain bouncing off the roof or standing in a puddle cannot reach the top edge.
What is an upstand in practice?
Think of the roof as a tray. The covering is the base, and the upstands are the sides of the tray. Where a roof butts against a house wall, the covering runs up the brickwork, and a flashing over the top, usually lead, seals the upper edge. The same idea applies around a soil pipe, a vent, a chimney or a hatch. Each one needs the covering carried up its sides, with a proper finish at the top.
Approved Document L has something to say here: it asks for an upstand built on site to have a maximum U-value of 0.35 W/(m²·K), the figure for how fast heat passes through. In plain terms, the upstand has to be insulated, or heat leaks through it and condensation can form on the inside.
A separate answer on sealing a flat roof to a house wall goes through that junction step by step.
Kerbs round rooflights and hatches
A rooflight set into a flat roof sits on a kerb, a raised timber or insulated upstand frame. The kerb lifts the glazing above any water on the roof, and the covering is dressed up its outside face and finished under the rooflight's own cover flashing.
Why do leaks start at a rooflight kerb?
Four corners, four joints, and a lot of movement. A kerb that is too low, a corner where the covering has been folded and not welded or bonded, or a rooflight fixed after the covering was laid, are all common causes. Sealant smeared round the base can hide the problem for a season, but it does not cure it. The covering has to run properly up the kerb.
A rooflight also has its own thermal figure. Approved Document L gives 2.2 for a rooflight, whereas a roof window takes the window figure, so check the product data before you order. If you are replacing a roof, a kerb is far easier to rebuild now than to retrofit later.
A leak that appears next to a rooflight is rarely the rooflight itself. It is nearly always the junction around it.
Why leaks start at the details
The flat middle of a roof is a single, continuous sheet or a bonded layer. The edges and junctions are where one material meets another, and where movement concentrates. Timber, brick and metal expand and shrink at different rates, so the joint between them is stressed every day.
How can I spot a failing trim or upstand from the ground?
Look for a stained or damp fascia, a trim that has bowed away from the edge, a gap where the covering meets a wall, or a dark tide-mark running down the brickwork below a flat roof. A gutter that is slow to empty can also point to a blocked outlet, or to an edge holding water back.
Fixing these details is often a small job on a roof that is otherwise fine, which is why we look at every junction in the survey. We take photos of each one, and if they need work you see them. A fall too shallow to clear water is a separate problem, covered under flat roof minimum fall, and the whole subject sits within our flat roofing work.
More on flat roof details
Can a failed trim be replaced without a new roof?
Sometimes. If the covering beneath is sound and flexible enough to be lifted and re-bonded, the old trim comes off, any rotten timber is cut out, and a new profile is fitted and sealed. If the covering cracks as it is lifted, the repair grows into a larger one, which the survey will show.
How high should an upstand be?
The exact height depends on the system and the manufacturer's data sheet, so check that sheet rather than rely on a rule of thumb. What matters is that the top edge sits above any water that can build up on the roof, and that a flashing or cover seals it from the weather.
The full picture: Flat roofing in Taunton · More short answers