Window sills

Types of exterior sill

Section through a sill

1. window frame with a sill groove; the sill is slid under the frame
2. exterior sill falling outwards
3. drip nose projecting beyond the finished facade
4. bedding under the sill (wedge or insulating sub-sill, depending on the system)
5. external wall insulation with render
6. masonry below the window

Drawing no.

KV-06

Title

Exterior window sill, section and plan of end

Scale

diagram, dimensions in mm

Revision

14 October 2026

How the depth is worked out

ZOFI fasády describes a method that our window sill calculator also follows:

1. measure the depth of the outer reveal (špaleta) on the left and on the right (they may differ),
2. add about 10 mm for the part that slides under the window frame,
3. add 30–40 mm for the projection of the drip nose,
4. round up to the next depth the manufacturer supplies.

ZOFI’s example: a 160 mm reveal + 10 mm + 30 mm = 200 mm, so 210 mm is ordered.

Length and end finishing

For a sill with end caps fitted on the render, ZOFI recommends deducting 5 mm from the measured length between the reveals for the caps and leaving 2 mm for expansion. For a sill set into the render it is the other way round – the sill is lengthened, with at least 17.5 mm set in at each end. Sills longer than 5 m are split into two pieces joined with an expansion joint.

When the sill goes in

On a new build or when external wall insulation is added, the exterior sill is fitted after the insulation but before the final render – that is when the position of the finished facade is known and the end caps can be worked in properly. Measuring before the insulation goes on is the most common reason for short sills.

Typical defects

Cost

Prices are indicative, October 2026, materials not included:

We do not give a materials price: in October 2026 we were unable to verify current window sill manufacturers’ price lists directly at source. The cost of materials depends mainly on the type (pressed / extruded), depth, colour, end caps and the number of pieces.

• Exterior window sillsfitting step by step
• How to measure a window sillon a new build and when replacing
• Choosing an exterior window silltypes compared for different situations
• Window sill calculatordepth and cutting length

Questions and answers

How far should an exterior window sill project beyond the facade?

ZOFI fasády gives a nose projection of 30–40 mm beyond the finished facade. To the reveal depth you also need to add about 10 mm for the part that slides under the window frame.

Is a pressed or an extruded aluminium sill better?

An extruded sill is thicker and stiffer, takes loads better and sags less at greater depths. A pressed sill is cheaper and is made in many depths and shapes. On an ordinary window, both work well if fitted correctly.