How to Seal Roof Flashing – DIY Guide
To seal roof flashing, clean the joint thoroughly, remove all old sealant, prime the metal if required, then apply a compatible flashing sealant or lap sealant in a continuous bead. Press firmly and smooth off. A well-sealed joint prevents water ingress at vulnerable roof junctions.
Remove All Old Sealant and Debris
Use a putty knife or scraper to cut away every trace of old, cracked, or peeling sealant from the joint. Work carefully to avoid lifting the flashing itself. Follow up with a wire brush to remove rust, oxidation, and loose mortar from masonry joints. Any remaining debris will stop the new sealant bonding properly and the joint will fail again within months.
Clean and Degrease the Surface
Wipe down the entire area with a clean rag dampened with mineral spirits or a solvent cleaner suitable for the flashing material (lead, aluminium, or zinc). Remove dust, moss residue, and any greasy film. Allow to dry completely — typically 15–20 minutes — before applying anything. On lead flashing that has oxidised heavily, lightly scuff with fine wire wool before cleaning.
Apply Primer Where Required
Check the sealant manufacturer’s instructions — many modern flashing sealants require a compatible primer on lead, aluminium, or porous masonry to achieve full adhesion. Brush a thin, even coat of primer onto both surfaces being joined, covering at least 25 mm either side of the joint. Allow the primer to tack off fully before applying sealant; rushing this step is one of the most common reasons joints fail prematurely. For chimney flashing specifically, see our detailed guide on fixing a roof leak around a chimney.
Apply Sealant and Bed the Flashing
Load the caulking gun and apply a continuous, unbroken bead of flashing sealant into the joint, working from one end to the other without lifting the nozzle. For step and cover flashings, work the sealant firmly under any lifted edges before pressing them back down. Where you are applying self-adhesive flashing tape instead, peel back 150 mm of backing at a time, press firmly into position, and use a roller or the back of a trowel to eliminate air pockets. Overlap tape joins by at least 75 mm.
Smooth, Check, and Allow to Cure
Use a dampened spatula or gloved finger to tool the sealant bead into a neat, slightly concave profile — this sheds water rather than pooling it. Inspect the full length of the repair for any gaps or thin spots and fill immediately. Do not expose the joint to rain for the full cure time stated on the sealant packaging, typically 24–48 hours. Check the repair again after the first heavy rainfall and re-dress any areas that have not fully bonded.
Frequently Asked Questions
What is the best sealant to use on roof flashing?
For most domestic lead or aluminium flashing, a high-modulus, non-acid-cure silicone sealant or a dedicated bitumen-based lap sealant is the most reliable choice. Self-adhesive butyl flashing tape works well for step flashings and flat-to-pitched junctions where a wider coverage is needed.
Can I seal roof flashing myself or do I need a roofer?
Resealing existing flashing is a manageable DIY task provided you can access the roof safely with the correct equipment. If the flashing itself is corroded, cracked, or has lifted away from the masonry chase, it may need re-dressing or replacing — that level of work is best left to a roofing contractor.
How long does flashing sealant last?
A correctly applied, primed flashing sealant should last 10–15 years on a typical UK roof. Cheap sealants applied without primer or onto damp surfaces may fail within 2–3 years. Inspect flashings as part of your annual autumn roof check, particularly after severe wind or frost.
Why does my roof still leak after I sealed the flashing?
How do I seal flashing around a chimney?
Chimney flashing consists of stepped side flashings, back gutter flashing, and a cover (soaker) flashing — each joint needs to be cleaned, primed, and sealed separately. Pay particular attention to the mortar pointing that holds the flashing into the masonry chase, as this is typically the first point to deteriorate. You can also repoint the chimney at the same time to address any wider masonry deterioration.
When working with lead flashing, use an oxidising patination oil on any freshly dressed lead before applying sealant — it prevents the natural salts in rainwater reacting with the lead and breaking down the sealant bond at the edges. This is standard practice on heritage and conservation roofs but works on any lead flashing job.
Sources
- HSE — Working at Height Regulations 2005: safe use of ladders and roofwork — hse.gov.uk
- Which? — Roof repairs: what you can do yourself and when to call a roofer — which.co.uk
- NICEIC / Competent Roofer — Guidance on safe roof access and repair — hse.gov.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
