Window Boards
At some stage during the first fix process, once the building has been suitably weathered in from above, the external windows and doors of the building will be installed in order to seal the remaining openings. In modern site carpentry, these external components are typically installed by specialist companies, regardless of the material that they are made from. In the past, wooden sash windows, wooden openers, and wooden doors would have been made in the workshop and fitted on site by the carpenters. Whilst it’s still possible, the flow of modern construction is better suited to window companies manufacturing and installing their own windows and doors - a cost-effective method for both carpenters and window fitters alike. Once the windows are fitted, window boards can be installed in order to finish the top of the exposed masonry.
What is a window board?
A window board is a traditional decorative timber component that finishes the surface of the masonry on the interior of a window opening. In the past, before the introduction of cavity closures, windows boards were installed to cap the top of the exposed cavity masonry on the inside of the window opening. These window boards provided a functional, hardwearing, and aesthetic solution to sealing the cavity. These boards were formed from solid timber and were wired in place to prevent lifting - with the walls and window reveals then being plastered to hold the window boards captive.
In modern times, cavity closures are required to be installed to seal the top of the exposed cavity masonry. Window boards are still installed on top of the masonry, though they are more commonly machined from MDF. Specialist manufacturers offer ranges of machined hardwood window boards to suit specific interior aesthetics. These window boards are secured in place through the use of modern adhesives, and are often dry-lined into place once secured.
But why install a window board; why not plaster the bottom of the opening like the side reveals and underside of the lintel? Functionally, a window board provides a more durable surface than a plastered surface. It’s common for picture frames, vases, and other assorted ornaments to be placed on window sills - a timber window board is a more suitable surface for these items to sit. Additionally, a painted or varnished timber window board can be easily wiped clean during house cleaning duties. Aesthetically, the window board, with its typical rounded projection past the face of the internal masonry and the horns that extend past the reveals provide a balanced appearance to the finished opening. It’s not uncommon for modern designs to implement plastered window sills, though these areas mark easily and are relatively delicate. After all, where else do we typically see upwards facing horizontal plastered surfaces within a residential property?
What are the criteria for the installation of a window board?
Window boards must be installed level in both directions to maintain a professional finish. The window boards must be fixed securely in place with no humps or divots along its length. The back edge of the window board must make full contact with the window frame to prevent cracks appearing in the future due to movement and shrinkage. An appropriate style of horn should be moulded onto each end of the window board, typically projecting past the window opening.
How are window boards installed?
Traditionally, before the creation of modern adhesives, window boards were fixed in place with a combination of wire and plaster. The underside of the window board once cut would have wires nailed to it, with these wires being pulled down the face of the masonry and being wrapped tightly around nails fixed into the wall. Once the wires were tight and the window board was held in place, the wires were plastered in, with the plaster also coming down the reveals into the top of the window board. This system of fixing can still commonly be seen during demolition and renovation procedures. Modern window boards are commonly foamed into place. The implementation of this fixing foam has the additional benefit of sealing any unwanted drafts that may be present in the opening or the cavity. Cutting these window boards to fit them in the first fix applications does not require incredible precision, as the modern dry-lining applied to the reveals will cover a large portion of any gaps that are left. As such, we can give ourselves a small tolerance in order to ease the installation and prevent the window board from being too tight.
To mark out the correct length for the window board, first measure the structural opening of the window. The dot and dab will cover approximately 25-30mm on each side of the opening, and so we need to take this into consideration when marking out the horns on the window board. A typical projection past the finished reveal is around 50mm, and so we should start by marking this distance in from the end of the length. This line displays the finished corner of the reveal. In order to give ourselves appropriate coverage, we can measure backwards from this around 20mm. This line will be our cutting location to create the horn. From this point we can measure out the width of the window opening, with another 5-10mm allowance for the window board to fit in easily. From this point, another 50mm inset mark should be made. This mark denotes the total length of the window board and should be cut from the whole length here. At this point we’re left with a piece the correct length, and now we need to cut the horns.
To mark out the rest of the horn, measure the distance from the window frame to the inside face of the internal skin. 5-10mm can be added to this length in order to ensure that the horns do not prevent the back of the window board from the meeting the window frame. By marking this on the window board we can see the material waste that must be removed. In cutting this we have the overall shape of the window board. Now all we need to do is round over the ends of the horn with a plane and sandpaper to match the radius on the front of the window board. Alternatively, we could round over the ends before removing the waste in order to give ourselves more stability with the plane - perfect for beginners learning how to use the tools. With the horns rounded over we’re ready to fit the window board. Place the board on the masonry and centre it in the opening. Use packers where required to elevate the board to the correct height where the rebate fits over the sill piece of the frame. The board should also be checked for level in both directions, as well as square with the frame. With adequate packers supporting the board in the correct position, we can wedge the board in place with battens as shown in the diagram or weigh it down with blocks. We can then use fixer foam to fix the window board in place. Expanding foam may cause humps in the finished product
Window boards are sold in a variety of different widths, to accommodate varying thicknesses of masonry. In order to select the correct width, a measurement can be taken from the back of the window to the front of the masonry reveal. An additional 30 mm is added here for the thickness of the dry lining, as well as around 25-50mm for the overhang of the board over the plaster line. This width should fall close to the standard size of a manufactured window board. If not, the next size up may have to be ripped down to the desired width.