Building components · Terminations
Flashing
Explains flashing as the element that works by overlapping from above, which is why the direction of lap governs everything.
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 flashing is
A flashing covers a junction between two parts of a building so that water running down one is directed across or over the other rather than into the gap between them. It works by being lapped from above: whatever is higher overlaps whatever is lower.
That single principle governs flashing more than material choice does. A flashing lapped the wrong way delivers water into the junction 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 an upstand. An upstand is the waterproofing turned up; a flashing is the piece that covers and closes the top of it from above.
- It is not a cavity tray. A tray sits inside a cavity and directs water out; a flashing is on the outside covering a junction.
- It is not a sealant. A sealant fills a gap; a flashing sheds water past one, and a junction relying on sealant alone has no second line.
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.
- flashings
- cover flashing
- apron flashing
- soaker
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The direction of lap, which is the whole mechanism.
- How its upper edge is secured and closed so water cannot get behind it.
- How it accommodates movement between the two elements it bridges.
- Continuity around corners and at the ends of runs, which is where flashings most commonly fail.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Flashing works by geometry — laps in the direction of flow — rather than by sealing, which is why it continues to work as materials age.
A flashing bridges two elements that move differently, and how it accommodates that is part of its arrangement.
A flashing is exposed on the outside of the building and is generally the shortest-lived part of the junction it protects.
Flashings are commonly at high, awkward positions, so failures persist for a long time before being noticed.
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.
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.
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.
Upstand to Flashing Termination
Where waterproofing turned up a vertical face stops, and where a flashing above closes that stop.
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.
- Building Envelope
- Flat Roof
- Opening
- Pitched Roof
Clarifications
Commonly misunderstood
- That sealant can substitute for a flashing. Sealant is a single line with a finite life; a flashing sheds water by shape and does not depend on adhesion.
- That any lapped piece is a flashing. Lapped the wrong way it is a funnel into the junction.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Which way do the laps run at this junction, and is that the direction of water flow?
- How is the upper edge of the flashing secured and closed?
- How does the flashing accommodate movement between the two elements?
- How are corners and the ends of runs formed?
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 material, gauge, lap dimension, fixing or cover is stated here.
- Flashing arrangements are specified by the covering or waterproofing system and by qualified designers.
Terminations & Weathering
The elements where a plane stops, turns up, sheds water or is deliberately interrupted — parapets, upstands, flashings, drips, weeps and movement joints.
Browse all terminations entries →