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Building components · Openings & Frames

Window Sill

Explains the sill as a water-management element whose whole function depends on shape and continuity.

Form:Profile or trimRoles:Weather controlDrainageTermination

Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.

Overview

What a sill is

A window collects water. Rain running down the glass arrives at the bottom of the opening in a concentrated stream, and the sill is what receives it and takes it away from the wall.

It does that by shape: it slopes outward so water runs off, it projects beyond the wall face, and it carries a drip on the underside so the water falls clear instead of tracking back. Remove any of the three and the sill delivers water to the wall it was installed to protect.

Boundaries

How it differs from nearby elements

The distinctions that keep this entry from being a restatement of its neighbours.

  • It is not a window board. The board is the internal shelf at the bottom of the opening; the sill is external and manages water. `internal-window-board` is a separate published material entry.
  • It is not a threshold. A threshold is at a door, is walked over, and has to reconcile weathering with level access.
  • It is not stone copings and cills as a product. That material entry covers the procured stone unit; the sill is the position and the function.

Terminology

Common names and aliases

These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.

  • cill
  • window cill
  • external sill
  • sub-sill

Dependencies

What it depends on

What has to be right elsewhere for this element to do its job.

  • Its outward fall, since a level or back-falling sill directs water toward the wall.
  • Its projection past the wall face, which determines where water lands.
  • The drip on its underside, without which water tracks back to the wall regardless of projection.
  • Its ends, since water reaching the end of a sill without a stop runs sideways into the reveal.

Considerations

Topics that genuinely apply here

Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.

Moisture

Sills concentrate the water from a whole window into one line, so a defective sill wets a specific band of wall repeatedly.

Durability

The band of wall beneath a failing sill deteriorates faster than the wall around it, and frost damage there is common.

Movement

A sill spans the opening beneath the frame, and joints in a long sill have to accommodate movement without opening a water path.

Buildability

Stop ends at the sill's ends are frequently omitted, and the consequence appears in the reveal rather than beneath the sill.

Materials

Commonly formed from

Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.

Materials LibraryStone Copings and Cills →Stone sill units are a published material entry; the sill is the position they occupy.Materials LibraryPrecast concrete panels →Precast concrete sills are widely encountered, cast with fall and drip.Materials LibraryClay facing brick →Brick-on-edge sills are common in traditional construction and depend heavily on jointing.Materials LibraryAluminium →Pressed metal sub-sills are used beneath frames, particularly in framed and clad construction.Materials LibraryHardwood timber →Timber sills are met in traditional joinery, forming part of the window itself.Materials LibraryCast Stone →Cast stone sills are encountered where the appearance of stone is wanted at lower cost.

Assemblies

Building systems this element is met in

The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.

Edge and terminationDrainage planeWindow Opening Interface →Receives water from the glazing and directs it clear of the wall beneath.Edge and terminationTwin-Leaf Cavity Wall →Closes the base of the opening and is a position where a tray beneath is commonly required.Drainage planeCavity Trays →A sill in a cavity wall commonly has a tray beneath it collecting what passes the sill itself.Edge and terminationRainscreen Facade →Openings in a drained facade require a sill arrangement that returns water to the drained plane or sheds it clear.

Junctions

Interfaces this element takes part in

Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.

Shed or drainSealSupport

Sill to Frame

Where a frame meets the sill, at the point a window's whole collected water arrives.

Raises: Moisture · Weathering · Movement · Buildability

Where it is met

Application contexts

Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.

  • Opening

Clarifications

Commonly misunderstood

  • That the sill is part of the window. In many walls it is a separate element of the construction, and replacing a window does not replace it.
  • That staining beneath a window means the window leaks. Far more often the sill is delivering water back to the wall rather than clear of it.

Conversations

Questions for qualified professionals

Questions worth asking someone with the project in front of them. This page cannot answer them.

  • Does the sill fall outward, and does it project past the wall face?
  • Is there a drip on the underside, and is it clear of paint or render?
  • How are the ends of the sill stopped so water does not run into the reveals?
  • What is the condition of the wall directly beneath, and is it consistent with the sill working?

Go deeper

Related Build Design Hub guides

Planning guidance behind the decisions this element involves.

Preparation

Related planning checklists

Owner-side preparation before the conversation where this element comes up.

What this entry does not do

  • No fall, projection, drip dimension or jointing arrangement is stated here.
  • Sill arrangements are part of the opening design and are specified rather than improvised.

Openings & Frame Elements

The frames, heads, jambs, sills, thresholds and closers that form and finish an opening through a wall.

Browse all openings & frames entries →