The short answer
A roof window is generally harder to break into than a ground-floor window, because it sits on a sloping roof that is difficult to reach. Security still depends on the lock, the glass and the position it is left in. Check that it has a proper lock, ask whether the glazing is laminated, and close it fully when you are out. A window left on its ventilation setting is not locked in the same way.
What makes a roof window secure, which parts to check, and how the ventilation setting changes the picture for an empty house.
Where a roof window sits
Height is the first layer of protection. A window set into a pitched roof is above the reach of a casual passer-by, there is usually no ledge to stand on, and a climber would need a ladder or a scaffold to get near it. On a bungalow, an extension roof or a garage that adjoins the house, that changes. A low roof can be stepped onto from a wall, a bin store or a flat roof next door, and the window then behaves more like any other first-floor window.
Look at the approach as an intruder would. Is there a drainpipe, a fence or a flat roof below the window? Is it hidden from the street and your neighbours? A roof window in a loft conversion may also be the means of escape, and then it must be easy to open from the inside by anyone in the room.
Approved Document B sets sizes for escape windows: a minimum area of 0.33 m², at least 450mm high and 450mm wide, with the bottom of the openable area no more than 1100mm above the floor. A good lock must work with that, not against it. Our roof windows and skylights page covers the wider work.
Locks and handles
Look at how the sash is held. Most modern roof windows have a handle with a locking point, and many can be fitted with a key lock or a restrictor that limits how far the sash opens. Ask the maker's data sheet or the installer what the specific model offers, instead of assuming, as the options differ between brands, ages and sizes. A lock that nobody uses protects nothing, so choose one that is easy for the household to operate.
Old windows deserve a look. Handles loosen over time, catches wear, and a seal that has shrunk can leave the sash a little proud. If the window rattles in wind, or closes with a gap you can see, it is worth having a roofer check it. Our answer on a Velux window that will not close covers the usual causes.
If you are replacing a window, say that security matters when you ask for a price, and ask for the lock to be named in the written quote. We share photos of your own roof at the survey, and the written total names the window we propose, so you can compare it with the data sheet before agreeing.
Glass and glazing
Glazing is the part people ask about first. Laminated glass has a plastic interlayer that holds the pane together when it cracks, so a broken pane stays in the frame instead of falling out. Toughened glass breaks into small pieces instead. Which one is fitted depends on the model, the age and the glazing specification, so ask for it by name and check the data sheet for the exact pane build.
For a roof window the inner pane matters most, since an intruder or a falling object reaches the outer one first. Laminated glass on the inner side helps if the outer pane is damaged, and it is also what you would want over a bed or a stairwell. If you are unsure what an existing window has, look at the edge label stamped on the glass, or ask a roofer to read it.
Replacement glass can be fitted into an existing frame in some cases. See our answer on Velux replacement glass for how that works. Part L sets a lower heat-loss limit for new windows, and the glazing type affects both warmth and noise, so it is worth deciding on all three together.
Ventilation positions
Many roof windows can be left slightly open for fresh air, using a ventilation bar or a vent flap in the frame. These settings keep out rain and most insects, but they hold the sash a small distance from the frame, so the window is not locked in the same way as when fully closed. Treat that setting as a daytime or home-only choice, and shut the window completely when the house is empty or you go to bed.
A vent flap on the frame lets air in without opening the sash at all. It is usually the better option for a bedroom or a loft room, because the glazing stays shut and locked while the room still breathes. Some windows also have a trickle vent or a bar that can be added later. Ask which your model supports.
Ventilation helps the room as well, and our answer on a roof window in a bathroom shows why it matters in a damp space. Before you open any window, check the weather, as rain in the gap is the cause of many a stained ceiling. If a window has stopped sealing when closed, tell a roofer.
More on this in Velux Roof Windows & Skylights in Crawley
Two more security questions
Do roof windows need bars or grilles?
Not as a rule, but it depends on the roof. A window over a low roof, a garage or a flat roof that anyone can climb onto is the one to consider. Any bars or grilles must still allow quick escape from a bedroom, so ask your building control body before you fit them.
Can I add a lock to an existing roof window?
Often you can, but the fitting depends on the make and age of the window. Some have a factory option for a key lock or restrictor, and others need a different handle. Ask a roofer to look at the frame, say which model you have, and put the fitting in the written price.