Frequently Asked Questions

Check whether the outer window will open with the proposed secondary window fixed within the reveal. If not, a wall mounted timber sub frame will be required and the cill may have to be suitably modified.

Side hung casements: If the external windows open in pairs check clearance at the centre line since horizontal sliding units could restrict opening and double side-hung casements may then have to be considered.

The offset dimension for a Series 10 sliding unit is 26mm and a cavity of approximately 200mm is required for clearance. The Series 80 sliding unit has a minimum offset of 63mm and needs a cavity of about 350mm for clearance.

When the reveal depth is insufficient the frame can be face fixed (link to Minimal reveal) and offset relative to the central line of the external window. This will allow one window to open.

Vertical pivots: If cill clearance is the only problem consider the use of a horizontal sliding unit with an 18mm high slimline bottom track. Allowing for fixing tolerances this should accommodate clearances down to 25mm.

Horizontal pivots: If a horizontal sliding secondary window is specified the sashes may have to be removed to allow cleaning. Side-hung units can, in certain circumstances, allow full access to the pivoting window. Multiple sets of horizontally pivoting windows may present clearance problems at the mullion points.

Transoms: When a secondary transom has to be introduced it should be positioned to permit clearance for the external window or allow it to open in sufficiently for the required access.