The short answer
A centre-pivot sash turns on a bar through its middle, so the top half swings out and the bottom half swings in, and the glass can be turned over for cleaning. A top-hung window hinges along the head and swings out from the bottom edge. Centre-pivot is the common choice, and top-hung suits low sills and rooms where headroom matters.
How each sash moves
A centre-pivot sash rotates around a horizontal axis near its middle. You pull the handle at the top edge, the lower half comes into the room and the upper half swings out over the roof, and a full turn brings the outer pane inside. It opens wide at the head, and the hardware is simple, which makes it a natural place to start.
A top-hung sash is hinged along the top edge, so lifting the bottom edge swings the whole sash outward and upward, like a lid. Nothing moves into the room, and the opening is largest at the sill end, where people reach. Because the sash opens outward it stays clear of curtains, blinds and furniture on the inside.
Each has variants. Some centre-pivot windows have the handle at the bottom edge for reach, and some top-hung units have a restricted opening position for ventilation in the same way. Makers publish the pitch range each model suits, and the data sheet is the thing to check for your slope. We confirm the pitch of your roof before any model is offered, because a sash that suits a steep slope may not work on a shallow one.
Headroom and reach
Headroom is the first practical difference. When a centre-pivot sash is open, its lower half sits inside the room at head height, and a person standing below it may find it in the way. Under a sloping ceiling with a low sill, that can be an irritation or a bump on the forehead.
A top-hung sash is clear of the room when open, so the space below it stays open, which suits a window over a desk, a bed or a stair landing. The handle on a centre-pivot sits at the head, which can be a stretch on a high window, and a pole or an electric opener helps. Our answer on the electric roof window explains that option.
View and light follow from the same geometry. Top-hung windows offer a broad opening at the sill, so you can lean out for the view, while centre-pivot windows give a taller opening. For a loft room with a knee wall, we measure the sill height against the floor before choosing, so the opening lands where people actually stand.
Escape and fire safety
In a loft conversion, a roof window may be the way out in a fire. Approved Document B gives escape windows three minimums: a clear opening of 0.33 square metres, 450 mm of height and 450 mm of width, with the bottom of the opening no higher than 1100 mm from the floor. The guidance relies on such windows only for storeys that are within 4.5 metres of the ground.
What matters is the clear opening when the sash is open, not the size of the frame. A centre-pivot sash splits the opening into two gaps either side of the bar, and the clear area can be smaller than people expect. A top-hung sash opens in one piece, though a restrictor or a rail may reduce the gap.
Some makers sell a version of each type with a specific escape rating in its data sheet. We read the clear-opening figure for the model you are shown and compare it with the sill height you plan, then tell you plainly if a model fails. Our answer on the escape roof window deals with that choice for a loft conversion.
Cleaning and day-to-day use
Cleaning is where centre-pivot earns its place. Turn the sash through its half-circle and the outer pane faces you from inside, so a cloth can reach it with no ladder and no roof access. That is a real safety gain on a high or steep slope, where reaching the glass from outside means a roof ladder or a scaffold tower.
The outer pane of a top-hung window faces away and out of reach, and cleaning it generally means leaning through the opening or using a long-handled tool. In a house where the roof is steep, that leaves moss, dust and bird droppings unremoved. Our answer on replacing a roof window from inside shows how much can be done without climbing out.
Choose by the room. A centre-pivot suits most pitched loft rooms, and a top-hung suits spots where headroom or a low sill decides. Our roof windows and skylights page covers both, and the survey measures the slope, the sill height and the clear opening before a model is picked. If you are torn between them, a visit with the room in front of us settles it faster than a brochure does.
