The intent of D3D29 is to limit the risk of a person (especially a young child) falling through an openable window. Where the floor level below an openable window is less than 2 m there are no specific requirements. For an openable window in a bedroom of a Class 2 or 3 building or Class 4 part of a building or in a Class 9b early childhood centre 2 m or more above the surface beneath, openable windows are required to restrict the passage of a 125 mm sphere using any one of the following design solutions:

• The window be designed such that any opening does not allow a 125 mm sphere to pass through (e.g. louvres).
• The window be fitted with a fixed or dynamic device that is capable of restricting the window opening so it does not allow a 125 mm sphere to pass through and is difficult for a young child to operate. The restricting device must be capable of resisting a 250 N force when directed against the window such as a casement window or in attempting to push a sliding window open. An internal screen with similar parameters may be installed.
• The window be fitted with an internal or external screen that does not allow a 125 mm sphere to pass through and which must resist a horizontal outward force of 250 N.

If the openable part of a window is at least 1.7 m above the floor, no further protection is required.

D3D29(2)(b)(iii) relates to a screen or window restricting device protecting an openable window in a bedroom of a Class 2 or 3 building or Class 4 part of a building or in a Class 9b early childhood centre. The screen or opening restricting device may be installed in a manner that allows it to be removed, unlocked or overridden in the event of a fire or other emergency to allow safe egress. In these situations the unlocking device must be child resistant.

Child resistance could be achieved through the use of a tool, key or two hands.

There are a number of hardware options available. Short chain winders and barrier screens will allow windows to comply with this requirement. Sliding window locks may lock a sash so a 125 mm sphere cannot pass through. Where provision is made to fully open the window beyond 125 mm then the child resistant release mechanism is required in addition to the device resisting a 250 N force as required by D3D29(2)(b)(ii).

D3D29(3) in addition prescribes that an 865 mm barrier (sill) would be required. A wall beneath an openable window can be considered as the barrier if the criteria in D3D29(4) are met.

D3D29(3)(b) relates to the height of a barrier under an openable window that is not in a bedroom in a Class 2 or 3 building or a Class 4 part of a building.

The term 'window' is not italicised in D3D29 and as such, is not restricted to the definition of 'window' in the NCC. The reason for this is to also capture windows that may let in air but not light, e.g. metal louvres. A metal louvre or openable panel would not fit in the NCC definition of window but is subject to the window barrier provisions.