Appendix
Structure of a window sill
Window sills with HPL on wood substrates are suitable for use in wet rooms, depending on the model. Suitable window sills made from HPL-Compact, com- pact forming elements or HPL composite elements made of HPL and low-swelling substrates are available for wet rooms.
Window sills with HPL are usually composite elements. The excellent surface properties are determined by the coating. The static and building physics proper- ties, on the other hand, are primarily determined by the substrate (e.g. chipboard or plywood) and the bonding of the individual components.
The rear edge can be sealed with an edge finishing material. As the window sills
are available by the metre and are cut individually to the required lengths, the lateral edges also have to be sealed. Depending on the application and the expected exposure, a tension-absorbing balancing has to be applied to the underside of the window sill.
with edge banding 1 with inset edging tape 2
a Balancing | b Substrate | c HPL | d Edge banding | e Inset edging tape
As window sills are structural elements, it is absolutely necessary for preventing any structural damage to the building to observe certain principles during planning, element machining and window sill installation.
Mechanical stress
The window sill has to be sufficiently supported depending on the load case. It should always be inserted under the window frame, as otherwise tilt stability will not be ensured. If it is not possible to insert the window sill under the frame, a U-profile or L-profile has to be attached to the frame or to the side of the inner face.
It is recommended to rest the window sill full-surface in a bed of mortar, on the brickwork or – for renovations – on the old window sill. If this is not possible, it has to be fastened to brackets, consoles or battens. The defined support span has to be observed depending on the load bearing capacity of the material. The following diagrams provide orientation for selecting the spacing of the fastenings with different loads. Generally, a support span of 600 – 800 mm is sufficient for a 20 mm thick window sill. On 20 mm thick material, the projection of the window sill to the front should not exceed 100 mm, referring to the last point of support.
Moisture
To prevent moisture damage, the following always applies for installation: “more sealing inside than outside”. This means that no moisture can condensate at the dew point on sensitive points such as the narrow surface on the rear. A moisture proof installation is achieved by inserting a suitable vapour barrier and with targeted permanently elastic sealing in the joint areas towards the interior.
Thermal insulation
Cold bridges should always be avoided. Among other things, this minimises the risk of condensation while preventing unnecessary energy losses. While timber substrates or HPL-Compact already have a low thermal conductivity, thermal insulation is still recommended especially in the rear edge area, which is especially at risk. This can be achieved by inserting foam sealing tape, PUR foam or PS foam. A combination of insulation and vapour barrier is ideal.
a Silicone seal | b Round cord | c Vapour barrier | d Support block / spacers | e Adhesive/expanding foam | f Joint sealing / silicone
Temperature and humidity fluctuations
Heating and cooling lead to expansions and contraction on virtually all materials. For this reason, window sills – especially in longer lengths – have to be installed with expansion joints. Temperature fluctuations result in length changes Δl [mm] at temperature difference ΔT [K], a material-specific, thermal length expansion coefficient α [K -¹] and an installation length l [mm], calculated with the following formula:
Δl = ΔT × l × α.
The following diagram can be used to easily determine the expansion joint, taking into account the thermal length expansion:
1 Expansion joint at 60 K temperature fluctuation 2 Expansion joint at 40 K temperature fluctuation
x Overall window sill length in mm | y Expansion joint in mm
Compared to the length changes caused by temperature fluctuations, the influence of humidity on timber materials is more important. For a chipboard bonded with melamine resin or urea resin, the swelling coefficient at sheet level is between 0.002 and 0.008 % for each 1 % of relative humidity change. The swelling coefficient of HPL is in the same range. The following diagram shows length changes as a function of the change in relative humidity. For the dimension of the expansion joints, they always have to be taken from the first diagram (fig. 63) and added to the expansion values which are caused by the temperature fluctuations.
In contrast to temperature fluctuations, humidity fluctuations only cause slow length changes in the window sill. This means that it is not short-term room climate changes which cause significant length changes, but rather the length change of the window sill with regard to the seasonal climate changes has to be taken into account. A difference of the relative humidity of 80 % is quite realistic for this.
1 Expansion joint at 80% relative humidity
2 Expansion joint at 60% relative humidity
3 Expansion joint at 40% relative humidity
x Overall window sill length in mm | y Expansion joint in mm
Calculation example:
The following diagram is used to determine the maximum support spacing of 20 mm thick, self-supporting window sill elements with a constant point load of 1000 N, depending on the structure (with and without crosspiece) and window sill depth (20 cm and 30 cm). This is based on a maximum permissible bending between two support points of approx. 1 / 100 support width. A modulus of elasticity of 4000 N / mm2 and a sheet density of approx. 650 kg/m³ are assumed as material characteristics for chipboard with HPL (not tightly clamped on both sides). HPL-Compact window sills have a significantly greater strength at the same thickness. For a tightly clamped structure, the maximum permissible support spans increase.
The swelling coefficient at sheet level of 0.008 % for every 1 % of humidity change was used as a starting parameter.
| Example calculation for expansion joint dimensions: | ||
|---|---|---|
| Window sill length: 2000 mm Maximum window sill temperature in summer: Minimum window sill temperature in winter: | +50 °C – 10 °C | |
| Maximum temperature difference | 60 K | |
| Expansion coefficient taken from Fig. 63 | 2.4 mm | |
| Maximum relative humidity | 70% | |
| Minimum relative humidity | 30% | |
| Maximum humidity difference | 40% | |
| Expansion coefficient taken from Fig. 64 | 6.5 mm | |
| Maximum possible expansion (‘worst case’) | 8.9 mm | |
For an expansion joint size of 6 mm at both window sill ends (overall expansion joint = 12 mm), it is ensured for this case that the window sill has sufficient room to move and that the sealing material can be compressed.
The expansion joint should be placed towards the inner faces of the window opening (also with gliding material for elements set in plaster). For divided elements, the joint can also be placed at the separating point (the colour of the sealing material has to be adapted, or a connecting bar can be used).
Machining and shaping window sills
The composite system of window sill with HPL allows conventional wood tools to be used. Any complex solution can be implemented resulting from requirements due to the geometry of the window inner face, ventilation screens, power sockets, cable ducts or for design reasons. This combination of simply mechanical machining and the variety of design options offer a high added value. Machining on site with the option of adapting elements to the local requirements is a particular advantage. Many other materials – especially those based on inorganic raw materials such as artificial stone, marble or granite – do not have these additional benefits. Visually appealing results can be achieved with the combination of window sill elements with other materials (edges, end strips, support bars, etc.).
Corners and joints
Corner joints and elements joints have to be sealed. They must not be weakened by cutouts or notches. All cut edges generally have to be protected against possible influence of moisture. The elements are attached using mechanical fastening and bonding. For hygiene reasons, we recommend sealing all open edges of the substrate.
On postformed window sills, corner joints can be achieved with mitre cuts or template routing. Suitable plastic or metal cover profiles are used for butt joints. Edges have to be cleanly routed and the two elements have to be joined tightly. An exact, level transition from one board surface to the other is achieved by using springs or short springs. Short springs with or without board connectors are suitable for connections (2 for every 30 cm of window sill depth). To achieve flush surfaces, the HPL surface is selected as a reference surface for producing the grooves for loose tongues or short tongues. The tongues should have a tight fit. The sealing compound is applied directly into the board joint and also acts as an adhesive. When tightening the board connector nuts, it has to be ensured that the two window sill surfaces are aligned at the same level and that the sealing compound is emitted on all sides. Excess sealing compound has to be removed immediately. We recommend pressing the joint together horizontally (e. g. by blocking against the wall) until the sealing compound has cured.
Appropriately shaped profiles are suitable for covering a board joint. While they save accurate machining to a certain extent, they do interrupt the level, easy-toclean HPL surface. We recommend applying sealing compound – which then also acts as an adhesive – to all cut edges before attaching (screw fixing) the metal profile.
Connecting joints
Before sealing towards the window sill and to the wall, it has to be ensured that the window sill – especially for larger, self-supporting sections – is sufficiently supported, as otherwise the sealing joints could be destroyed under load. Smooth surfaces have to be degreased with suitable cleaning agents, just as the window sills, and coated with a primer. Compatibility of the cleaning agents with the materials to be cleaned has to be ensured. Porous surfaces have to be coated with a film-forming primer. For pretreatment with primers, the instructions from the sealing compound manufacturer have to be followed closely. To prevent contamination of the element surfaces with sealing compound and to achieve an evenly wide joint pattern, we recommend masking the joint edges with tape before applying the sealing compound. It is important that the sealing compound sufficiently overlaps onto the surface of the window sill to prevent trapped moisture from entering into the rear edge. Three-flank adhesion has to be avoided. When installing the window sill, it also has to be ensured that it absolutely does not slope towards the window. This type of installation would also create excess stored moisture. At this point, we would like to point out again that all cut edges of the window sill element have to be sealed against moisture ingress from the brickwork. Smoothing the sealing compound can produce visually appealing joints. If a plastic or wood connecting profile is used for visual reasons, the connection between the window sill and the window frame and the wall also have to be sealed. When fastening wall connection strips to window sills with nails or screws, there is a risk of subsequent moisture ingress.
Holes and cutouts
Cutouts, holes or access openings which expose the chipboard substrate have to be carefully sealed. Due to the expected movement in the window sill itself or from installed pipes, these have to be centred so that a minimum spacing of 2 – 3 mm is ensured in all locations. This is to prevent condensation from reaching the chipboard. As a rule, all parts to be attached to the window sill should be bonded if possible. If fastening is only possible by means of screws, the holes have to be drilled so that the HPL has an at least 2 mm larger diameter than the diameter of the screw (ideally approx. 4 mm larger). This is necessary to avoid tensions in the material. Due to risk of moisture damage from exposing the wood substrate, sufficient sealing always has to be ensured.
Cutting and sizing
For determining the length it has to be noted that the length is reduced by the required width of the expansion joints and the dimension of any rails attached to the wall. Inversely, the length increases if the window sill is set in plaster at the inner faces.
Installing a window sill with screwed-on brackets
The bracket is fastened to the sill with wall anchors, and then the window sill from underneath.
a Silicone seal | b Round cord | c Vapour barrier | d Support block / spacers | e Adhesive / expanding foam | f Joint sealing / silicone
Bonding window sills
After cutting the correct size, the underside of the window sill has to be cleaned, degreased and primed. As an alternative, adhesive cleaner can be used which combines all these work steps. A lint-free cloth should be used for this. The surface to be bonded to has to be solid, clean and dry. The bonding described here is achieved with permanently elastic adhesives. The adhesive thickness and adhesive width differ depending on the adhesive system used. The applied quantity and the application type (beads or full-surface) are also specific to the adhesive. If required, spacers have to be used directly next to the adhesive beads to ensure that the minimum thickness of the adhesive layer is achieved and that the window sill can be aligned with weights or by clamping. The clamping time depends on the adhesive system used. To bridge larger distances, wooden strips with suitable thickness are adhered to the building sill and then the window sill is adhered to this. If an old window sill has sturdy fastenings and a smooth, intact surface, a new window sill can be adhered to it directly using construction adhesive. Tilt stability has to be ensured in particular, e.g. using profiles screwed onto the window frame. 2-component PUR adhesives can be used for bonding, for example. To achieve best possible adhesion, it is always recommended to sand the adhesion surfaces, remove any dust and then degrease them.
