Installation of a plastic window sill on PVC windows

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Today we will describe all the technological aspects of fixing and sealing plastic window sills for various installation options. When completing the installation process of PVC windows, the same close attention is required as when fastening the frame, and when forming foam seams.

Installing a plastic window sill

Requirements for the preparation of the opening

Although the installation of the window sill is carried out at the final stage, the possibility of correct installation depends on timely preparation. There is no universal recipe, because the material and structure of the walls can be completely different, moreover, the window sill itself can have different purposes. However, there are a number of nuances that should be considered in advance..

The most important requirement is the quality of the preparation of the lower edge of the opening. Ideally, preparatory plastering is carried out, due to which a flat horizontal plane is formed under the frame, which serves as a base for the subsequent alignment of the window and a support for the window sill. It is important to remember that the window sill never rests directly on the prepared plane. The presence of a warm seam helps to avoid freezing and the formation of condensate caused by it in the lower junction.

Installing a plastic window sill

We’ll have to work hard on the side edges of the opening. The classical installation method involves cutting the ends of the window sill into the slopes, but the depth of this very cut is practically not regulated by anything. Construction practice shows that the optimal depth of overlapping a window sill with a slope is 30–50 mm. However, if the slope is made with plastic panels, the window sill should protrude 15–20 mm beyond the framing decorative profile. Thus, on the side faces, it is necessary to cut two niches at the level of the upper edge of the mounting groove.

Installing a plastic window sill

Correct installation of the window sill is impossible without following the correct installation sequence. Please note that first the frame is fixed and the external waterproofing is glued, then the glass unit is installed in place and foam seams are formed. At this stage, the lower plane of the opening must be leveled, undercuts are made in the corners, all surfaces are dust-free and primed, an internal sealing tape is glued. Only after meeting these requirements is it possible to install a window sill. Slope finishing is carried out at the final stage.

Window sill position and function options

Installation of the window sill is performed with some specific differences depending on the purpose of this structural element. There are three most common options:

  1. The sill simply forms the lower abutment knot.
  2. The window sill will be used to install flowerpots and other items.
  3. The window sill is designed for an operating load of over 30 kg / running. m.

Installing a plastic window sill

In almost all cases described, the window sill also overlaps the heating device located under the window. If there is none, the window sill has almost zero overhang, that is, it protrudes above the wall plane exclusively by the thickness of the visor. Otherwise, the window sill should completely cover the heating device with an overlap of about 10-15 mm.

Installing a plastic window sill

If the window sill protrudes above the wall plane by more than 2/3 of its own width, its installation is carried out in the same way as for increased operational loads. This method consists in placing at least 2 support brackets under the window sill plate. on rm. m. For fixing these brackets on the rear side of the window sill, longitudinal dovetail grooves are provided. The reverse side of the brackets is attached to the wall with plastic dowels, while they are hidden in undercuts 20-30 mm deep, which are subsequently walled up. Fastening of the window sill brackets to the wall is carried out at the final stage of installation after landing on the mounting foam. In frame buildings, for the purpose of strengthening, the lower edge of the opening can be formed by a wider board, which is supported from below by slopes, but the thickness of the board should not exceed the protrusion of the visor above the back plane of the window sill.

Also, a window sill can have one or both open ends, for example, when decorating parapets of panoramic windows or balcony blocks. In this case, it is highly desirable that the open corner is held by at least one bracket, which will prevent the window sill from blowing upwards. To mask the cut edge, you will need an end cap for the entire depth of the window sill.

Marking and cropping

Before proceeding with the fastening of the window sill, it must be “driven into size” in accordance with the size of the opening and frame, as well as taking into account the desired projection size. The window sill is trimmed on three sides – two lateral and one rear facing inside the installation groove. Trimming is most conveniently done with an electric jigsaw, and in its absence – with a hand hacksaw with a fine tooth.

Installing a plastic window sill

First you need to trim the back. If the window is set correctly and the width of the slopes at the bottom on both sides is the same, you can skip this step. Otherwise, longitudinal trimming helps to level the installation flaws and to ensure that on both sides the window sill protrudes above the wall plane by an equal amount. Please note that the plastic window sill has a cellular structure, which can complicate trimming if a vertical lintel falls into the cutting line. In such cases, it is recommended to slightly increase the ledge of the window sill..

Installing a plastic window sill

The ends are trimmed in such a way that the total length of the sill is equal to the sum of the length of the visible part of the frame, sunrises and side ledges. To facilitate installation, the rear corners can be cut obliquely so that at the edges of the window sill goes into the body of the slope at least 20 mm and about 30-50 mm in the places of the side protrusions. When trimming is completed, decorative plugs need to be glued to the ends of the windowsill, shortening them if necessary. Before installing on the rear edge of the window sill, you need to make two marks, the distance between which is equal to the width of the visible part of the frame, while the sides should remain equal symmetrical indents. Thanks to such markings, you can easily align the window sill horizontally..

Installing a window sill

Before fixing the window sill, you need to select the support pads, which are usually used as sets of mounting wedges glued together to avoid displacement. The linings are placed on the lower plane of the opening with an indent of 25–30 mm from the leading edge. The thickness of the lining is selected in such a way that the window sill has a slight slope towards the room of the order of 1-1.5 °. The installation step of the linings should not be less than 60 cm for window sills with a purely decorative function and not less than 40 cm with increased operational loads..

Installing a plastic window sill

When everything is ready, the window sill is installed on the lining and is pushed in with the rear edge until it stops in the installation groove. You can slightly widen the gap in this assembly and fill it with plastic sealant, removing excess after the sill is crushed into the groove. Next, you need to pick up a wide board and place it on top of the window sill so that the longitudinal axis is located strictly along the installation line of the lining. After that, an oppression of about 15–20 kg per linear meter is installed on the board. In this case, the load must be placed with a slight offset towards you, ensuring that the back of the window sill is tightly pressed against the edge of the mounting groove without any gap.

Installing a plastic window sill

When positioning is completed, foam is injected into the gap between the window sill and the opening. Previously, it will not be superfluous to irrigate the cavity with water from a spray bottle, which will contribute to more active polymerization. First, the deepest part of the gap is foamed, which is filled to 100%, so that when expanding, the foam presses the board tightly against the frame. After that, you need to wait until the filler has completely expanded and partially polymerized, and then check the correct installation and the absence of shifts. If everything is in order, the foam will fill the entire remaining space by about 50-70% of the volume, taking into account further active expansion. It is necessary to fill the gap in such a way that, after increasing in volume, the foam fills the space between the linings, but does not protrude beyond them, thus forming a groove with a depth of about 30 mm for further embedding.

Sealing gaps

To preserve the integral look of the finish, the gaps under the window sill, notches in the slopes and the attachment points of the brackets must be repaired, bringing the adjacent sections of the wall into a common plane. Polyurethane foam is poorly suited for this, in monolithic buildings it is better to use ordinary cement mortar, in frame buildings – to fill the voids with fragments of expanded polystyrene or mineral wool, and then carefully “bypass” the window sill with finishing material.

Installing a plastic window sill

It is better to fill in the gaps at a stage when preparations for interior decoration have not yet been carried out. In this way, the overlap of the filling with a common preparatory and decorative layer is achieved, which will reduce the likelihood of cracks and irregularities. Particular attention should be paid to the gap under the windowsill. Although it is almost never seen, there is almost always a thin gap between the plastic and the body of the wall. In this place, it is recommended to select a shallow groove of about 5 mm with a thin scraper, and then fill it with a thin strip of plastic sealant.

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