Approved Document F outlines ventilation and air quality standards for all buildings and provides guidelines to prevent condensation. For existing windows with background ventilators, replacements should also have them, ensuring they’re not smaller and are controllable. For windows without background ventilators, replacements should consider the building’s increased airtightness. Ventilation should either match or improve upon the original, with specific equivalent area requirements for different rooms. If meeting these requirements isn’t feasible, aim for as close to the specified values as possible.

A cill, located beneath your door frame, mainly functions to divert water away, preventing it from entering the house. It’s important to recognize that the size mentioned refers to the total projection from the back of the bi-fold frame, not the entire projection from the frame’s front.

You can find detailed cill technical drawings below, which provide further clarification on this matter.

It’s generally advised to have a cill for most doors to assist with drainage, even if the objective is to create a low walkthrough effect. For instance, external decking or paving can be extended right up to the cill’s edge.