How to choose the right cill depth for period-style windows 

Getting the cill right might not be the first thing you think about when specifying new windows, but it can make or break the finished look, particularly on a period or heritage-style property. Whether you’re working with a recessed masonry reveal, replacing original timber windows in a conservation area, or fitting flush casement windows on a new build with traditional styling, your window cill depth options will affect both the visual result and how well the installation performs long term. 

This guide is designed to help homeowners and installers make a confident, informed decision before finalising their specifications. As experts in heritage uPVC window design, at Timberlook we’re well-placed to provide all the information you need. 

Why cill depth matters more than most people realise

The cill sits at the base of the window frame and plays a crucial role in drainage and weather protection. A correctly sized cill directs rainwater away from the facade and the base of the window, reducing the risk of moisture ingress and long-term damage to the surrounding masonry. 

Get the overhang wrong and you’re left with a cill that either pools water against the wall or looks visually out of proportion with the window above it. For period-style windows in particular, where aesthetic is everything, the depth of the window sill and its relationship to the wall thickness and reveal depth will be immediately obvious to anyone who knows what traditional windows should look like. 

Understanding window cill depth options: a practical breakdown

At Timberlook we offer four main cill profiles, each suited to different installation scenarios. 

85mm stub cill

The stub cill is a compact option, often used where a property has existing natural stone window cills or brick cills that are being retained. It works with the existing stone or masonry rather than replacing it, and is designed primarily to direct moisture away from the frame. If your property has original stone surrounds that you want to preserve, this is typically the right choice. 

150mm standard cill

This is the industry standard and the most commonly specified option. It protrudes 80mm from the front of the frame, which gives a clean overhang over the drip edge without dominating the facade.  

It suits most standard installations where the frame sits relatively flush with the exterior wall, and it works well across a wide range of property styles. Low maintenance and available in a choice of colours, it covers the majority of residential window installations. 

180mm cill

Where window frames are recessed into thicker walls, or set back within a deep reveal, a longer cill projection is needed to maintain a proper overhang and ensure rainwater clears the masonry below. The 180mm cill protrudes 110mm from the face of the frame and is the largest option in our Timberlook cill range.  

Radlington 165mm heritage cill

For those looking for the most authentic heritage cill option possible, the Radlington cill is the design most specific to period-style installations.  

While standard cills are 30mm deep, the Radlington profile is 55mm deep, projecting 95mm from the face of the frame. That extra depth gives it a distinctly traditional appearance that mirrors the proportions of original Victorian and Edwardian window components.  

For anyone fitting Timberlook windows in a conservation area or on a heritage property, the Radlington cill is often the detail that ties the whole installation together visually. 

How do I know which cill is right for my property?

When deciding on the best cill for your property, our team at Timberlook is always happy to help if you have any questions. As a general rule, we’d recommend that:  

• Shallow reveals (frame close to flush with the exterior) work well with the 150mm cill 

• Deeper reveals or thicker walls typically require the 180mm cill 

• Where stone or brick cills are being retained, the stub cill is usually the practical solution 

• Period properties where visual authenticity matters should consider the Radlington profile 

If you are unsure, one of our Timberlook installers will take a final measurement on site and advise accordingly. It’s worth noting that getting the window cill depth options agreed at the quote stage avoids delays during installation. 

Timberlook: heritage design done right

At Timberlook, our flush sash windows and flush doors have been engineered from the ground up to faithfully reproduce the appearance of 19th-century timber frames, without any of the maintenance, rot, or cost that comes with real wood.  

• Timberweld technology eliminates the diagonal weld marks found on standard uPVC windows, replacing them with clean, sharp corners that mirror traditional joinery 

• Authentic mortise and tenon joints replicate the structural detailing of original timber sash and casement windows 

• Concealed trickle vents maintain the clean sightlines of heritage window designs without visible modern additions breaking up the frame 

• A wide range of woodgrain and dual-colour finishes including light oak and anthracite grey, allowing the windows to suit everything from rural cottages to urban Victorian terraces 

• Approved for use in conservation areas, with installations that have satisfied planning officers across the UK 

Frequently asked questions

What is the standard window cill depth for uPVC windows?

The industry standard is the 150mm cill, which protrudes 80mm from the front of the frame. This suits most residential installations where the frame sits close to the exterior wall face. For deeper reveals or period properties, a wider or deeper profile may be more appropriate. 

Do I need a different cill depth if my walls are thicker than average?

Yes. Thicker walls typically mean a deeper reveal, which requires a longer cill to maintain adequate overhang and drainage. The 180mm cill, projecting 110mm from the frame, is designed for exactly this scenario. 

Can Timberlook windows be used in conservation areas?

Yes. Timberlook’s flush casement and sash windows have been used in conservation areas and designed to meet Article 4 conservation area guidelines. Their authentic detailing, including the Timberweld joint and concealed vents, makes them suitable for heritage and period properties. 

What is the difference between a cill and a window board?

The exterior cill manages drainage and weather protection on the outside of the window. The window board, or stool, is the interior sill. Both need to be correctly sized for a functional and visually consistent installation. 

Does cill colour have to match the window frame?

Not necessarily. Timberlook offers a wide range of colours and woodgrain finishes, and dual-colour options allow for different internal and external finishes. Many homeowners choose to match the cill to the frame for a clean result, but contrasting colours can also work well depending on the property style. 

Find a Timberlook installer in your local area: contact us today

If you’re at the stage of finalising specifications and want to make sure every detail, including your window cill depth options, is right for your property, the Timberlook team is on hand to help. 

To find a Timberlook installer in your local area, get in touch via the online contact form, or call the team directly on 01253 888 222.