How do I fix a window box so it cannot fall?

Window boxes fall in two ways: they slide off a sloping sill when the compost swells or someone knocks the window open, and they overturn in wind when the plants have grown top-heavy. Both are prevented by restraint at each end rather than by weight or friction, and a wet window box is heavy enough that the difference matters.

Safety

What you'll need

• Window box with drainage holes, true
• Sill brackets or wall cradle brackets, true
• Masonry drill, plugs and outdoor-rated screws
• Restraining wire or strap and eyes
• Spirit level

Step by step

1. Understand the sill.Every external sill slopes outward and usually has a drip groove underneath. A box sitting on it is on a slope, and the whole load is trying to slide. Sills are also often not structural — a tiled or timber sill can break under a concentrated point load.
2. Choose the fixing method for the situation.Sill brackets that hook over and clamp under the sill are the least invasive and suit ground-floor windows. Wall-fixed cradle brackets take the load into the masonry below the sill and are far stronger. A wall-mounted trough hanger is the option where there is no usable sill at all.
3. Fix the brackets into masonry, not into the sill or the render.Drill into the brick or block below the sill, allow for render thickness in the screw length, use the correct plug size, and use outdoor-rated stainless or coated screws.
4. Level the box, or set it very slightly back.Use packers under the front edge so the box sits level or tips a fraction towards the wall. A box tipping forward drips down the wall, stains the render, and encourages the compost to migrate over the front edge.
5. Restrain both ends.A stainless wire, a strap or a metal restraint from each end of the box back to an eye fixed in the reveal or the wall. This is what stops a box being lifted or knocked sideways, and it is what turns a resting box into a fixed one.
6. Make sure the window still opens.Check the window operation with the box in place, especially outward-opening casements and any window that is a designated fire escape. A box that blocks an escape window is a serious problem, not an inconvenience.
7. Deal with the water.The box must drain, and the drainage must not run down the wall or onto whoever is below. A drip tray beneath, or a box with the holes set back from the front edge, keeps the render clean and the pavement dry.
8. Check it annually, loaded.Each spring, before replanting, put weight on the box and check for any movement in the brackets, corrosion on the screws, and softness in the box itself if it is timber.

Tips

• Above the ground floor, and anywhere over a pavement or a shared path, some leases and building insurance policies restrict window boxes entirely. In a flat, check the lease before you fix anything to the external wall.
• Plastic and fibreglass boxes are a fraction of the weight of terracotta or concrete for the same volume, which meaningfully reduces the load on every fixing.
• Do not use a box longer than the sill. Overhanging ends have no support and are what people catch with a ladder or a window pole.

Common mistakes

• Resting the box on the sill and trusting its weight — The sill slopes, the box is on a wet surface, and wind loads a planted box laterally. Weight is what makes the fall dangerous, not what prevents it.
• Fixing the brackets into the sill itself — Sills are frequently not structural, and drilling into a concrete, stone or tiled sill can crack it and create a water path into the wall.
• Fixing to a uPVC window frame — The frame is not a structural fixing point, the screws pull out of the thin section, and you have just made a permanent hole in a weathersealed unit.

Questions people ask