Can I Seal a Small Gap Around a Chimney Flashing Myself?

For small, superficial gaps where the sealant or mortar has simply dried out and cracked, a confident DIYer can often reseal chimney flashing as a maintenance task. However, if the flashing itself has lifted, corroded, or pulled away from the chimney, or if your roof is steep or high, you should call a qualified roofer rather than risk injury or making the problem worse.

Understanding Chimney Flashing

Chimney flashing is the weatherproofing system where your chimney meets the roof surface. It typically consists of two layers:

• Step flashing – individual metal pieces woven into the roofing material along the sides of the chimney.
• Counter flashing (cap flashing) – metal strips embedded or sealed into the chimney mortar joints that lap over the step flashing.

A saddle or cricket may also be fitted behind larger chimneys to divert water. The joint between counter flashing and the chimney is usually sealed with a flexible sealant or pointed with mortar. This is the area most likely to develop small gaps over time due to thermal movement and weathering — and the most practical area for a DIY repair.

Assessing the Gap First

Before you reach for a tube of sealant, take a close look (safely from a ladder at eave height, or using binoculars from the ground) and ask:

• Is the gap hairline or superficial — just cracked sealant or mortar?
• Is the flashing still firmly in place, or has it lifted, buckled, or separated from the roof?
• Is there rust, corrosion, or holes in the metal?
• Are there water stains on your ceiling inside, suggesting an active leak?

If the flashing itself has moved or is damaged, sealant alone won't fix the underlying problem. A licensed roofer should inspect and refit the flashing properly to comply with NZ Building Code clause E2 (External Moisture).

What You'll Need

For sealant, look for products specifically labelled for roofing or flashing use. Sika and Ardex both offer suitable flexible sealants available through NZ trade and hardware suppliers. You'll find a range of options at Bunnings or Mitre 10.

Step-by-Step: Resealing a Chimney Flashing Gap

1. Work safely

Roof work carries real fall risk. If your roof pitch is greater than about 25 degrees, or the chimney is more than one storey up, seriously consider hiring a professional. If you do go up, wear rubber-soled shoes, use a properly footed ladder, and consider a safety harness anchored to a ridge bracket. Review WorkSafe NZ guidance on working at height before you start.

2. Clean the joint thoroughly

Use a wire brush and scraper to remove all old sealant, loose mortar, moss, and debris. The joint must be clean and dry before you apply anything new. Wipe down with methylated spirits and allow to dry completely.

3. Apply backing rod if needed

For gaps wider than about 6 mm, press a foam backer rod into the gap first. This gives the sealant something to bond to and prevents it from sinking too deep, which can cause it to crack.

4. Apply sealant neatly

Cut the nozzle at a 45-degree angle and apply a smooth, continuous bead of sealant into the joint. Tool it with a wet finger or spatula to press it firmly into the gap and create a neat, slightly concave finish. Avoid applying in direct hot sun or if rain is forecast within a few hours — check the product's data sheet for cure times.

5. Check your work

Once cured, inspect the seal visually. Run a hose over the area (with someone watching inside) to confirm the repair is watertight before you consider the job done.

NZ Building Code and Consent Considerations

Simply resealing an existing flashing joint is generally maintenance, not building work, and won't require a building consent. However, if you're replacing or refitting flashing, this may be restricted building work depending on the scope. Always check with your local council if you're unsure. The MBIE Building Performance website has clear guidance on what requires consent and what falls under Schedule 1 exemptions.

When to Call a Professional

Call a qualified roofer if:

• The flashing has lifted, separated, or is visibly damaged
• You can see rust, holes, or corrosion in the metal
• There is active water ingress or ceiling staining inside
• Your roof is steep, high, or made of fragile material (like old terracotta tiles or brittle fibrolite)
• You're not confident working at height
• The chimney mortar itself is crumbling and needs repointing beyond the flashing joint

A professional roofer will ensure the repair meets NZ Building Code E2 requirements for weathertightness. You can find qualified local roofers on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Building Code Clause E2 (External Moisture)
• WorkSafe NZ – Working at Height
• BRANZ – Weathertightness Guidance
• Sika NZ – Sealants and Adhesives
• Licensed Building Practitioners – Find an LBP
• Bunnings NZ – Roofing Sealants