Yes, using a foam backer rod before applying silicone in a shower is not just acceptable — it's considered best practice by waterproofing professionals. It controls the depth of your sealant joint, prevents the silicone from bonding to three surfaces (which causes premature failure), and ultimately gives you a more durable, flexible seal that stands up to the wet conditions of a NZ bathroom.
What Is a Foam Backer Rod?
A foam backer rod (sometimes called a backing rod or joint filler rod) is a flexible, cylindrical foam strip — usually made from closed-cell polyethylene foam — that is pushed into a joint or gap before sealant is applied. It acts as a backing material that:
- Controls the depth of the silicone bead so you don't waste product filling a deep void
- Creates the ideal 2:1 width-to-depth ratio for a flexible sealant joint
- Prevents three-sided adhesion, which restricts the silicone's ability to flex and leads to cracking
- Provides a firm surface to tool the silicone against for a neat, concave finish
In a shower, movement joints (such as where the wall tiles meet the shower tray, or internal corners) are constantly exposed to thermal expansion, structural movement, and water. Getting the joint geometry right is critical.
Why the Width-to-Depth Ratio Matters
Silicone sealant performs best when it can stretch and compress freely. The industry-standard guideline is a width-to-depth ratio of approximately 2:1 — so a 10 mm wide joint should be roughly 5 mm deep. Without a backer rod in a deep gap, you might end up with silicone that is thicker than it is wide, which severely limits flexibility and causes early failure.
For shower joints, a depth of 6–10 mm is generally ideal. Use the backer rod to set this depth before you apply any silicone.
Choosing the Right Backer Rod Size
Backer rods come in various diameters — commonly 6 mm, 10 mm, 12 mm, 19 mm, and larger. The key rule is to choose a rod that is slightly larger (around 25–30%) than the width of your joint so it compresses snugly and stays in place without adhesive.
| Joint Width | Recommended Rod Diameter |
|---|---|
| 6–8 mm | 10 mm |
| 10–12 mm | 12–15 mm |
| 15–20 mm | 19–25 mm |
You can find foam backer rods at Bunnings, Mitre 10, and PlaceMakers, usually sold by the metre or in rolls.
Step-by-Step: Using a Backer Rod in a Shower Joint
1. Prepare the joint
Remove all old silicone, mould, and debris. The joint must be clean, dry, and free of dust or soap residue. Use a silicone remover tool and wipe down with methylated spirits. Allow to dry fully — this is especially important in NZ's humid bathroom environments.
2. Select and insert the backer rod
Cut the backer rod to length with scissors or a knife. Press it firmly into the joint using a blunt tool (a flat-head screwdriver works well) so it sits at the correct depth — leaving enough room for your silicone layer above it. Do not puncture or tear the rod, as this can affect its performance.
3. Apply masking tape
Run painter's tape along both edges of the joint for clean, straight lines. This is especially worthwhile in a tiled shower where a neat finish matters.
4. Apply the silicone
Use a sanitary-grade, mould-resistant silicone rated for wet areas. Products from Sika are widely used by NZ waterproofing trades. Apply in a single, steady pass and tool immediately with a wet finger or a capping tool to create a smooth, slightly concave profile.
5. Remove tape and cure
Peel the masking tape while the silicone is still wet. Allow the silicone to cure fully — typically 24–48 hours — before exposing the joint to water. Check the manufacturer's datasheet for exact cure times.
Common Mistakes to Avoid
- Using open-cell foam — this can absorb moisture and harbour mould. Always use closed-cell polyethylene backer rod in wet areas.
- Applying silicone over damp surfaces — adhesion will fail. The joint must be completely dry.
- Skipping the backer rod in deep joints — excess silicone depth wastes product and reduces flexibility.
- Three-sided adhesion — if silicone bonds to the back of the joint as well as both sides, it cannot flex properly and will crack.
NZ Building Code Considerations
Shower waterproofing in New Zealand must comply with NZ Building Code clause E3 (Internal Moisture) — note that E3 covers internal moisture more broadly, with wet area construction details also addressed through acceptable solutions and NZS standards; confirm current requirements with MBIE Building Performance or your local council. Movement joints in wet areas — including the junction between the shower floor and walls — are a critical part of a compliant waterproofing system. BRANZ and MBIE Building Performance both provide guidance on wet area construction standards. If your shower is part of a renovation involving structural changes or new waterproofing membranes, check with your local council whether a building consent is required.
When to Call a Professional
Re-sealing an existing shower joint is generally straightforward DIY work. However, you should engage a qualified waterproofer or tiler if:
- There is evidence of water damage, rot, or mould behind the tiles — this may indicate membrane failure requiring full remediation
- The shower is leaking into a floor cavity or adjacent room
- You are installing a new shower or retiling, which may require a compliant waterproofing membrane system
- You are unsure whether your existing waterproofing meets current NZ Building Code requirements
A licensed waterproofer or tiler can assess the full system and ensure the work is done to code. You can find qualified local waterproofing tradespeople on tradedirectory.co.nz.
Site Safe
SiteWise
Licensed Building Practitioner (LBP)
