- Roof flashing explained in plain English
- Where roof flashing goes on a house
- Types of roof flashing and what each piece does
- What roof flashing is made of
- Signs your roof flashing may be failing
- Why roof flashing repairs are a job for professionals
- Learning more about roofs and home upkeep
- Frequently asked questions
Roof flashing is thin, waterproof material, often lead, aluminium or another metal, fitted where a roof meets something else, such as a chimney, wall, valley or skylight. It stops rainwater getting into those joints by sending it back onto the roof. Failing flashing often shows up as damp patches indoors, and repairs are a job for a roofer.
If a roofer's ever mentioned your flashing, or you've spotted a damp patch on the ceiling near the chimney, you've probably wondered what flashing is and why it matters so much. It's one of those parts of a house you rarely see and almost never think about, right up until water finds its way in.
Here's how I'd explain it in plain English, based on the Wikipedia entry on flashing, the Health and Safety Executive's roof work guidance and This Old House's guide to common roof problems. One thing up front: this is a guide to understanding flashing, not a DIY repair guide. Roof work is work at height, and it's a job for professionals.
Roof flashing explained in plain English
Flashing is thin pieces of material that won't let water through, fitted at the joints and edges of a roof to stop rain getting into the building. A roof's tiles or slates do a good job across the open slope. The weak spots are where the roof meets something else: a chimney, a wall, a window or another roof slope. Flashing bridges those gaps and sends water back onto the roof and away from the joint.
Wikipedia notes that flashing also helps reduce the damp that can lead to mould indoors, which is why a small fault up top can show up as a bigger problem inside.
Where roof flashing goes on a house
- Chimneys. One of the most common places, with several pieces working together around the stack.
- Abutments. Where a roof meets a wall, such as an extension built against the house.
- Valleys. Where two roof slopes meet and rainwater runs together into one channel.
- Skylights and roof windows. Flashing seals around the frame to stop leaks.
- Vent pipes and other penetrations. Anything that pokes through the roof needs sealing round it.
- Roof edges. Drip edges help shed water clear of the building.
Types of roof flashing and what each piece does
Flashing pieces are usually named by where they sit or what shape they are. Wikipedia lists these among the main types:
| Flashing type | Where it sits | What it does |
|---|---|---|
| Step flashing (soakers) | Along a roof-wall line | Overlapping pieces laid in steps so water's shed down the slope |
| Counter or cap flashing | Built into a chimney or wall | Covers the top of the base flashing below it |
| Apron flashing | The lower side of a chimney | Directs water from the front of the stack back onto the roof |
| Valley flashing | Where two slopes meet | Lines the channel that carries concentrated runoff |
| Kickout flashing | The bottom of a roof-wall junction | Angled to push water away from the wall |
What roof flashing is made of
According to Wikipedia, metal flashing materials include lead, aluminium, copper, stainless steel and zinc alloy, along with coated metals such as galvanised steel. Non-metal options include plastic, rubber and flexible products such as rubberised asphalt and butyl rubber.
Lead has a long history here. Wikipedia notes that a book by Loudon described installing lead flashing by 1832, and lead's still widely used in the UK. Wikipedia also mentions newer options, such as soft zinc, promoted as a recyclable alternative to lead.
Materials can't always be mixed freely. Wikipedia explains that copper or lead in contact with, or above, aluminium, zinc or coated steel can cause early corrosion, which is one more reason the choice and fitting are best left to a roofer.
Signs your roof flashing may be failing
This Old House says flashing can crack, corrode or come loose, creating an easy entry point for water. You can look for clues without going anywhere near the roof:
- Water stains on ceilings or walls, especially near a chimney or skylight.
- Damp spots in the loft.
- Damaged or missing flashing around chimneys and vents, visible from the ground.
- A stain that keeps getting bigger after rain.
- Gaps around vents, skylights or the chimney, which This Old House says can let water in even when they're small.
Wikipedia adds that badly fitted flashing can direct water into a building rather than away from it, so a leak can appear soon after other roof work too.
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Why roof flashing repairs are a job for professionals
It's tempting to pop up a ladder and have a look, but the Health and Safety Executive is blunt about the risks. Its roof work guidance says the main causes of death and injury are falls from roof edges or openings, through fragile roofs and through fragile rooflights, and that many deaths each year involve smaller builders working on the roofs of homes.
- The HSE says all roofs should be treated as fragile until a competent person has confirmed they're not.
- It expects edge protection and scaffolding on sloping roofs, with protection at the eaves, including the rear of terraced houses.
- Ladders and roof ladders are only for short jobs measured in minutes, and even then they need to be secured.
For a homeowner, the safe approach is to spot the signs from inside or from the ground, then call a roofer. A good one will explain what's wrong and how they'll access the roof safely.
Learning more about roofs and home upkeep
Not every job at home needs a professional. Smaller fixes, like fixing a squeaky door, are perfectly safe to tackle yourself with a little oil and a screwdriver.
Roofers and builders who write can find roofing blogs accepting contributor articles in my directory.
Frequently asked questions
What is the purpose of roof flashing?
It stops rainwater getting in at the joints of a roof, such as around chimneys, along walls, in valleys and around skylights, by directing water back onto the roof and away from the gap.
What is lead flashing?
It's flashing made from lead sheet, a traditional material in the UK. Wikipedia notes lead flashing was being described in building books by 1832 and it's still widely used, alongside aluminium, copper, zinc and other materials.
How do I know if my flashing is leaking?
Look for water stains on ceilings or walls near the chimney or a skylight, damp spots in the loft, and damaged or missing flashing you can see from the ground. A stain that grows after rain is a strong clue.
Can I repair roof flashing myself?
It's safest not to. The Health and Safety Executive says falls from roofs are a leading cause of death and injury in construction and that every roof should be treated as fragile until a competent person says otherwise. Call a roofer.
What is step flashing?
It's a series of overlapping pieces laid in steps along the line where a roof meets a wall or chimney, so each piece sheds water onto the next and back down the slope.
