How to babyproof a sliding glass door: a track lock, a second lock up high, and an alarm if there is water outside

Three hazards, one door

The route out. A slider is the biggest, easiest door in most houses: no handle to turn, no latch to grip, just a push. On the other side is a yard, which may contain a pool, a driveway, a hot tub or a gate to the street. For a child between one and four this is the hazard that matters, because drowning is the leading cause of death in that age group and the door is usually how they get there.

The pinch. A sliding door closes with the full weight of a glazed panel behind it, and the gap at the leading edge closes to nothing. The injuries here are crushed and amputated fingertips, the same mechanism as the hinge side of a hinged door covered on best door pinch guards.

The glass. A clean full-height pane is invisible to a toddler running at it. Modern patio doors are tempered, which means they break into blunt fragments rather than shards, but the impact itself still hurts and older doors may not be tempered at all.

The track lock, and where to put it

A track lock is a small device that clamps or mounts to the frame and physically blocks the sliding panel from traveling. It is the primary fix, because it works on the door rather than on the latch a child can reach.

Two placement decisions. First, decide how far you want the door to open: fully shut is simplest, but a four-inch gap for air is a reasonable setting on an upper floor as long as nothing a child can climb sits beside it. Second, pick a lock with an on-off function rather than one you peel off, because any device an adult has to remove to use the door gets removed permanently within a fortnight. The whole family of track-blocking devices is the same one used on sliding windows, which is why the comparison on window guards vs window stops is worth reading alongside this.

A security bar in the bottom track is the cheap version and it works, with one caveat: a bar is only as good as the habit of putting it back, and it does nothing about the door being opened the length of the bar.

The second lock, up high

The single most effective thing you can do to a sliding door is add a lock your child cannot reach. A flip latch, hook or pin lock mounted near the top of the fixed frame is beyond a toddler's reach even with a chair, costs very little, and does not depend on anyone remembering to re-engage a removable device.

Fit it so an adult can release it in one movement, and square it with your fire escape plan before you drill: a patio door is an exit, and every lock you add is a second of delay on the way out. If the slider is your secondary escape route, use a lock that releases by hand without a key.

When the door needs an alarm

Any door that leads to a pool should have one, and the reasoning is in the drowning data: a child has to leave the house before they can reach the water, so the door is the earliest point at which you can be told. A UL 2017 listed door alarm sounds a few seconds after the door opens unless an adult presses the bypass, and the bypass belongs at the top of the frame where it is not a toy.

An alarm is a warning rather than a barrier and it is never the whole answer for a pool - the fence is. The layers are on how to childproof a pool, and the alarms themselves are ranked on best pool alarms.

What to buy

The gap that takes fingertips

Watch a toddler use a sliding door and you will see the hazard immediately: they push the panel with a flat hand near the leading edge, and their other hand is in the jamb. A door that rolls well is a door that closes fast.

There is no good off-the-shelf pinch guard for a slider the way there is for a hinged door, so this one is mostly habit and setup. Keep the door either fully closed or locked at a set opening, so there is no rolling panel to play with. Slow the door down if yours is on worn rollers that run away at the end of the travel. And teach the handle, not the glass: a child who is shown to push the handle keeps their fingers out of the jamb.

The glass itself

Two cheap jobs. Put decals, frosted film or a strip of tape at child height on the pane so it reads as a surface rather than an opening - adults benefit from this too. And check whether the door is tempered: modern patio glazing carries a small etched mark in a corner, and if yours has none, treat it as ordinary glass and consider safety film, especially in a house with a running three-year-old.

While you are there, deal with the vertical blind cords beside the slider, which hang full-length and form exactly the loop the CPSC warns about. Cut, cleat or replace them; blind cord safety has the detail, and the anchors are on best blind cord anchors and cleats. Last, the screen: a flyscreen holds no weight at all, so a closed screen in an open slider is an open door. The same rule applies at every window, which is the point of how to childproof windows.