Window Boards/ Sills

If you are new to the world of window installation or are just looking for a little extra advice in picking out the perfect window sill for your home, take a look through our handy guidance section below and get yourself up to speed on everything you need to consider.  

A fully supported window frame should incorporate both an internal window board and an external window sill. But what do they both do and what’s the real difference? 

The main purpose of an external window sill is to ensure your window and home are fully protected from the elements. With most typical window installations, you will generally find that a part of the façade is left exposed to weathering. A sill is installed at an angle over this exposed section to guide any water and moisture away from the window and building, preventing internal leakages.  

Internal window boards offer a range of benefits too. Firstly, they help to protect the interior of your home and the structural integrity of your walls from any water that may seep through. Window boards also act as a base for the window, preventing cracking and damage to the structure should shaking or disturbances occur. Aesthetics wise, window boards are also great for creating a clean finish internally whilst even offering up decorative shelving space for ornamental objects or plants under direct sunlight.  

With your perfect material chosen, its now time to consider the sizing and dimensions of your window sill/board.  

Window sills are installed below the window frames and architraves so it is suggested that window sills should be around 3 up to 15cm longer than these. Generally speaking, the average thickness of a standard wall in the UK is between 25 and 36cm. It is typically recommended that your window frames should sit halfway between the interior and exterior of the walls, so a 150mm window sill for example, may give you around a 30mm overhang with a surface area of 120mm.  

The depth of a window board depends on how deep your window reveal is. You need to ensure the board covers the reveal and overhangs the front edge. Try and aim for an overhang of around 30mm. There is no one thickness that is compulsory for window boards, however you should make sure that whichever thickness you choose, that it can definitely fit into the window reveal space.

The length of your window board is dependant on the width of your window reveal. Take the time to measure this space carefully using your tape measure – optimum accuracy will ensure the best fit.