Expanding foam might seem like a quick fix, but it's generally not suitable for sealing gaps between brickwork and roof flashing. While it's tempting to grab a can from your local hardware store, using the wrong product here can lead to water ingress, structural damage, and potential issues with the NZ Building Code. Read on to understand why, and what the correct approach looks like.
Why Expanding Foam Is the Wrong Choice Here
Expanding foam (polyurethane foam) is a useful product in many situations — insulating wall cavities, filling gaps around pipes — but it has real limitations when used at the junction between brickwork and roof flashing:
- It is not waterproof. Standard expanding foam is moisture-resistant at best, but open-cell foam will absorb water over time, making leaks worse rather than better.
- It degrades in UV light. Any foam left exposed to sunlight will break down within months, crumbling and opening the gap again.
- It doesn't bond well to masonry and metal simultaneously. The thermal movement between brick and metal flashing causes the foam to crack and lose adhesion.
- It can trap moisture. Rather than directing water away, foam can hold water against the brickwork and flashing, accelerating corrosion and mortar decay.
- It may void warranties. Using non-approved materials at weatherproofing junctions can affect manufacturer warranties on roofing products.
In short, expanding foam is a temporary cosmetic fix that is likely to make the underlying problem worse.
What the NZ Building Code Requires
The junction between brickwork and roof flashing is a critical weatherproofing detail. Under the NZ Building Code, specifically Clause E2 (External Moisture), buildings must be constructed to prevent water from entering. Flashing details at masonry junctions are specifically addressed in acceptable solutions and must be durable and properly sealed.
BRANZ publishes detailed guidance on flashing and weathertightness that is widely used by NZ tradespeople. Any repair at this junction should meet or exceed those standards.
The Right Materials for Sealing Brickwork-to-Flashing Gaps
The correct approach depends on the size of the gap and the type of flashing involved. Here are the standard trade solutions:
Mortar Pointing
For small gaps where the flashing is embedded into the brickwork (a stepped or cover flashing tucked into a mortar joint), the correct fix is to rake out the old mortar and repoint with a suitable mortar mix. This is the most durable long-term solution for masonry junctions.
Flashing Sealant / Polyurethane Sealant
For gaps between the flashing edge and the brick face, a high-quality polyurethane or modified silicone sealant rated for exterior masonry and metal is the right product. Look for products from reputable suppliers — Sika offers a range of sealants suitable for this application. Key properties to look for:
- UV stable
- Paintable (if needed)
- Rated for masonry-to-metal bonding
- Flexible enough to handle thermal movement
Lead or Aluminium Flashing Tape
In some situations, self-adhesive flashing tape can be used as part of a repair, particularly on low-slope roofs. This should be used in conjunction with sealant, not as a standalone fix.
Re-flashing
If the gap exists because the flashing itself has lifted, corroded, or was never correctly installed, the proper fix is to re-flash the junction entirely. This is the most common situation on older NZ homes, particularly those with clay brick veneer and corrugated iron or Colorsteel roofing.
Step-by-Step: Basic Sealant Repair (DIY-Suitable)
If the gap is small (under 10 mm) and the flashing is otherwise sound, a homeowner can attempt a sealant repair:
- Clean the area — remove all old sealant, loose mortar, moss, and debris. A stiff brush and a scraper work well.
- Allow to dry completely — do not apply sealant to damp surfaces.
- Apply masking tape either side of the joint for a neat finish.
- Apply a backer rod (foam rod) into deeper gaps before sealanting, to control sealant depth.
- Apply sealant in a continuous bead, pressing firmly into the joint.
- Tool the joint with a wet finger or jointing tool for a smooth, slightly concave finish.
- Remove masking tape before the sealant skins over.
Always follow the manufacturer's instructions for cure times before exposing the repair to rain.
A Note on Checking for Consent
Simple sealant repairs and repointing are generally maintenance tasks that don't require a building consent. However, if the flashing needs to be replaced or significantly altered, check with your local council — some re-roofing or cladding work may require consent or must be carried out by a Licensed Building Practitioner (LBP). When in doubt, always confirm with your council before starting work.
When to Call a Professional
You should call a qualified bricklayer or roofing tradesperson if:
- The gap is large, recurring, or the flashing has lifted away from the brickwork
- There are signs of water staining, damp patches, or mould inside the building near the junction
- The mortar around the flashing is crumbling or the brickwork is damaged
- The flashing itself is corroded, cracked, or incorrectly installed
- You are unsure of the correct repair method for your specific roof and wall type
A weathertightness failure at a brick-to-flashing junction can cause serious and expensive damage if left unaddressed. Getting it right the first time is always cheaper than remediation later. You can find qualified local bricklayers and roofers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
