Choosing a cill

The cill sits under the window frame and throws rainwater clear of the wall below. Choosing a cill is a quick decision in the designer, but picking the wrong depth can leave water running down your brickwork or a cill that sticks out too far.

This guide explains the cill options we offer, how to measure for the right size and how a cill affects your overall window height.

What a window cill does

Rain runs down the glass and frame and collects at the bottom. A cill carries that water outwards and drops it off a drip edge, away from the face of the wall.

Without a proper cill, water can soak into the brickwork or mortar under the window. Over time that can lead to damp patches, staining and frost damage.

The cill also works with the window’s drainage. Water that gets into the frame drains out at the bottom, onto the cill. Our window drainage guide explains how.

Choosing a cill: the options

No cill

Choose this if you are keeping an existing stone, concrete or tiled cill and the new frame will sit on it.

Check the existing cill first. It should be sound, slope away from the window and have no cracks or loose sections.

Stub or 85mm

A short cill that projects slightly. Used where the window sits on an existing cill but you want a neat drip edge.

150mm

The standard replacement cill for most brick walls.

180mm and 220mm

For deeper walls and windows set further back in the opening.

Cill option	Typical use
No cill	Keeping a sound existing stone, concrete or tiled cill
Stub or 85mm	Sitting on an existing cill, adding a neat drip edge
150mm	Standard replacement for most brick walls
180mm	Deeper walls or windows set back in the opening
220mm	The deepest reveals

How to measure for a cill

Measure from the face of the frame position to the outside face of the wall and choose the cill that overhangs it.

The aim is for the front edge of the cill to project past the brickwork, so drips fall clear of the wall rather than running down it. If you are between two sizes, the deeper cill is usually the safer choice.

Note where the frame will sit before you measure. A window set further back in the opening needs a deeper cill than one set close to the outside face.

How cills are sized

Cills are made to the width of your window plus a small overhang each side. The height in the designer is the frame only; the cill is added on top.

This matters when you measure. Enter the frame height without the cill. If you add a cill in the designer, it is fitted below the frame and its height is added to the overall size. Our guide on how to measure windows walks through the full process.

Cills for bays and doors

Bay windows need cills that follow the angles of the bay. Our bay and bow windows guide covers how they are measured.

Doors do not use window cills. They sit on a threshold instead, which is covered in our door thresholds guide.

Common mistakes

• Including the cill in the frame height and then adding a cill in the designer, so the window ends up too tall.
• Choosing a cill that is too shallow, so water drips onto the brickwork instead of clearing it.
• Choosing “No cill” over a damaged existing cill. Repair or replace it first.
• Forgetting to seal the cill. The joint between cill and wall should be sealed to keep water out.

Quick answers

Which cill size do most people need?

The 150mm cill is the standard replacement for most brick walls. Measure your wall first to be sure.

Is the cill included in the window height?

No. The height in the designer is the frame only. The cill is fitted below the frame and its height is added on top.

Can I keep my stone cill?

Yes. Choose “No cill” if the new frame will sit on a sound existing stone, concrete or tiled cill.

How wide will the cill be?

Cills are made to the width of your window plus a small overhang each side.

Design and price your window →