The Self Build Self Help Site window design
Aluminium window set in a mahogony frame

Photo of aluminium window

(Click on photo for a larger view)

External or Internal Beading

One critical decision to be made will depend on the prevailing weather conditions. uPVC windows can be either externally or internally beaded - relating to whether the beading that retains the window glass is clipped in from the outside or the inside. Normally the beading will be internal, so that it cannot be removed from the outside for security reasons. The problem with this is that, where there is driving rain, water may be driven through weak areas in the rubber seal - often at the corners. Sometimes the drain holes in the bottom of the profile do not then drain the water adequately and water will then easily leak through the internal beading. Chiseliing out the drain holes to remove any burr, and making them bigger, does not always cure this problem, where there is extreme driving rain.

The cure is to have externally beaded windows from the outset. If driving rain gets past the external bead, it does not get through to the inside - there is no joint of an internal bead to get past. To prevent the window glass being removed from the outside, the glass is held to the frame by double sided sticky tape. If you have ever tried to get window glass off such tape, you will know that external beaded windows do not pose any break-in threat!

You can see the external bead in the left hand photo on the preceeding page.

Your window supplier should be able to give further advice on these options.

Window Sills

Windows, wood or uPVC, usually come with their own sills, but it is usual to support the window and sill on another, larger section sill. Concrete sills are a favourite but their are many other types including stone, a course of bricks and overlapping plain concrete roof tiles.

The author's preference is for black synthetic slate - see photo below. These typically come in 3m lengths which can lead to significant wastage (a window 1.8m wide will need a sill of approx 1.9m, leaving 1.1m - too short for any windows of 1.2m.)

Photo of window with slate sill

(Click on pictures for a larger view)

Photo of window with concrete sill


These are more expensive than concrete. Concrete can, however be painted in any colour required.

The above photo on the left shows the window on the synthetic slate sill without the uPVC sill (the window shown in the photo on the right includes the uPVC sill). This does mean a bit of work to get the fit right. The following diagram shows the normal arrangement where the window opening has been built to accomodate window and both sills. This leaves sufficient window frame showing above the internal window board.

Diagram of sill arrangement

(Click on diagram for a larger view)

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