The short answer
A Velux window that will not close is usually held open by something simple: a stiff or damaged hinge, a handle or catch that has come loose, a sash that has warped or dropped in its frame, or a fault in an electric or solar motor. Check the gap for debris and look at the control first, then see whether the sash sits square. Do not force it.
The usual reasons a roof window stays open, what you can safely check from inside, and when it needs a roofer.
Check the simple things first
Before you suspect a fault, look for what is physically in the way. Leaves, grit, a stray blind cord, a bird's nest or a fallen twig can lodge in the gap between the sash and the frame, and so can a flap of seal that has folded out of its groove. With the window open, shine a torch round the whole edge and clear anything you find with a soft brush. Do not use a screwdriver to poke at it.
Next, make sure nothing is tangled with the window. Blinds and their cords can snag on the sash as it swings, and a blind that has slipped out of its rail will stop the window shutting. Curtains and shelves placed under the window can also catch the sash when it comes down. Move them and try again.
Then check how you are closing it. Some windows tilt and others pivot, with a bar or a handle on the top or bottom edge. If the window has been tipped into a cleaning position, the catch will not engage until the sash is turned back. Try closing it slowly and note where it sticks, because that spot points at the cause. Our answer on how to clean a Velux window explains the cleaning position.
Hinges, handles and the catch
If the gap is clear, the next suspects are the moving parts. Hinges carry the full weight of the sash, and over time they can stiffen, wear or work loose. A sash that swings with a grinding feel, or that drops on one side when you let go, suggests a tired hinge. Light lubricant on the moving surfaces can ease a stiff one, but only use a product that suits plastics and rubber, and keep it off the seals.
The handle and catch are the next link. A handle that spins without gripping, or a catch that no longer reaches its keep, will leave the sash standing proud. Look for loose screws and a plate that has shifted. Tightening a fixing can cure it, but stripped holes mean a part needs replacing. Many handles, catches and keeps are sold as spares for the window's make, and a roofer can match one from the data plate, which our answer on Velux window sizes shows how to read.
Do not force a window shut. Pushing hard on a sash that is jammed can bend the frame, crack the glass or snap a hinge. If it needs more than a gentle push, stop and ask for help.
A sash that has warped or dropped
Timber and plastic both move. A wooden sash can swell in a damp spell, shrink in a dry one, or twist slightly if moisture has got into the wood. A frame that is no longer square, or a sash that has dropped on its hinges, will rub on one corner and refuse to meet the seal. Look along the edges for a line where paint is worn away, a dark smear on the seal, or an uneven gap that is wide at one corner and narrow at the other.
Slight drops can be adjusted. A roofer can realign the sash on its hinges, plane or ease a rubbing edge, and reseal the finish so that moisture stays out. Where the wood is soft with rot or the frame is out of shape, that will not last, and replacing the sash or the window is the sound fix. Water getting in can be the cause of the warping, so a window that sticks and also drips should be looked at together. Our answer on why a Velux window leaks covers that side.
A building settling or a roof that has moved can also pull a frame out of square. Photos of the corners and gaps help.
Electric and solar controls
Electric and solar windows open and close by motor, and they add a few more things to check. First, find out whether the window is dead or just slow. A wall switch or remote that has lost power, a flat battery in a handset, or a trip on the circuit that feeds the unit will all stop it. A rain sensor can also hold the window shut, or force it shut, as it was designed to do, so check whether it is wet or blocked.
Solar windows rely on a battery charged by a panel. If the panel is shaded, dirty or damaged, the battery runs down, and the window may stop part-way. Wipe the panel and give it daylight. Our answer on solar powered Velux windows describes how they work. A window that hums but does not move, or stops with a click, points at the motor or the chain it drives, and that needs an electrician or a specialist, not a roofer alone.
Do not open the casing or work on the wiring yourself. If a window is stuck open and rain is due, close it by hand if the maker's release allows, or cover the opening from inside and call for help. We can make the opening safe, then fit a new motor or sash, quoting a single total once we have seen it. See also roof windows and skylights.
More on this in Velux Roof Windows & Skylights in Crawley
Two more questions on stuck windows
Is it safe to leave a window stuck open overnight?
Only if rain and security are not a worry. An open roof window lets in weather and can be a way in. If you cannot close it, cover the opening from inside with a board or a heavy sheet, keep the room dry and arrange a visit. Never climb out onto the roof to shut it.
Can a sagging sash be adjusted instead of replaced?
Often a small drop can be set right by realigning the sash on its hinges or easing a tight edge. If the wood is rotten, the frame is twisted or parts are no longer made, adjusting will not last. A survey shows which applies, and we say plainly when a repair beats a new window.