Windows and Glazing: When to Install Them During a Build

Installed too early, windows get damaged; too late, interior work stalls. The right timing, preparing the opening and building the installation joint.

Windows play two different roles during construction. On the one hand they are part of making the building weathertight: until they are in, interior plastering, screeding and services work depend on the weather. On the other hand a window is a delicate product: glass scratches, frames get damaged and mechanisms are spoiled by dust and mortar. Finding the right time to install means balancing these two demands.

When windows are measured

Windows are usually made to order and take time to manufacture. That is why they are measured right after the openings are finished, at the end of the frame and masonry stage. Ordering in advance from the drawing dimensions is risky, because the actual opening may differ by a few centimetres.

When measuring, the width and height of the opening are taken at several points and the diagonals are checked. A gap between window and opening is left for the installation joint, as the design and the window system require. It also needs to be known in advance how the opening will be finished inside and out: if external insulation is planned, the window's position within the wall thickness is chosen accordingly.

Weathertight stage and the best time to install

Windows are usually installed after the roof is covered and the masonry is complete, but before the interior wet trades. This protects the building from rain and wind and allows interior work to continue in winter.

Sometimes, as winter approaches, a temporary solution is used to close the building: openings are covered with sheeting, and the final windows are installed after heavy work is done. This approach helps protect large and expensive glazing systems from construction damage.

For large glazing, installation timing is also tied to equipment. Heavy glass units need a crane or special lifting gear, and scaffolding and site layout must be organised for it.

Preparing the opening

A window must be installed in a properly prepared opening. The reveals should be sound and clear of dust and debris. Sharp projections and voids undermine the tightness of the installation joint.

The bottom of the opening needs special attention. Water collects there, and the sill area is the most leak-prone point. The external sill is fitted with a fall to shed water and projects beyond the facade so that water does not run down the wall.

The installation joint: three layers

The junction between window and wall is as important as the window itself. A correct installation joint usually has three functional layers:

• the outer layer keeps out rain but lets moisture from inside escape;
• the middle layer is the insulating filler, often expanding foam;
• the inner layer stops vapour from the room entering the joint.

If any layer is omitted, moisture collects in the joint, the foam deteriorates, and mould and a cold strip appear around the window. Leaving foam exposed is also unacceptable, as sunlight quickly degrades it.

The window frame is fixed to the wall with the fixings specified in the design. Foam is not a fixing; it only fills the gap.

During installation the window is carefully checked for level and plumb. A deviation of a few millimetres can make an opening sash move on its own, or leave the seal compressed on one side and loose on the other. In large glazing, correct setting blocks under the glass units in the frame are essential to prevent the glass from cracking.

Protection and sequence with later work

Installed windows are covered with protective film during interior and exterior work. However, the manufacturer's film becomes hard to remove if left too long in the sun, so its removal is planned.

Plastering, facade work and finishing of the reveals follow the windows. The edges are masked with tape so mortar does not get onto frames and glass. Window mechanisms are adjusted after the works are finished, because construction dust and settlement of the building can affect them.

Checks at handover

When windows are accepted, the opening modes, seal seating, scratches and damage on glass units, whether the installation joint is covered on both sides, and the fall of the external sill are checked. Looking around a window on a rainy day is the most reliable way to see whether it leaks.

Zaferoğlu İnşaat plans the construction of private houses and villas stage by stage, from design to handover. You are welcome to discuss the sequence of glazing and window work for your house with us.