Yes, in many cases you can waterproof a leaking garage floor yourself — but success depends heavily on identifying why it's leaking in the first place. Surface moisture, minor cracks, and damp concrete are all manageable DIY jobs. However, if water is entering through structural cracks, rising damp from below, or failed drainage, you may need a licensed waterproofer or builder to sort the root cause first.
Why Is Your Garage Floor Leaking?
Before reaching for a waterproofing product, spend time diagnosing the problem. Common causes include:
- Condensation — warm, humid air hitting a cold concrete slab (very common in NZ coastal and northern regions)
- Rising damp — groundwater wicking up through the slab due to no or failed damp-proof membrane
- Surface water ingress — rainwater running in under the garage door or through cracks
- Cracked concrete — settlement or shrinkage cracks allowing water through
- Failed drainage — blocked or inadequate floor drains
A simple test: tape a piece of plastic sheeting flat to the floor and seal the edges with tape. Leave it for 24–48 hours. If moisture appears under the plastic, you have rising damp. If it appears on top, it's condensation.
What the NZ Building Code Says
Garage floors in New Zealand must comply with NZ Building Code Clause E3 (Internal Moisture) and Clause E1 (Surface Water). If your garage is attached to the house, moisture entering the garage can affect the habitable spaces and may also be relevant to Healthy Homes standards for rental properties. For guidance on what the Building Code requires, MBIE Building Performance is the authoritative source.
Most waterproofing work on an existing garage floor is exempt from building consent under Schedule 1 of the Building Act — but always confirm this with your local council before starting, especially if you're making changes to drainage.
Preparing the Surface — The Most Important Step
Waterproofing products will fail quickly if applied to a poorly prepared surface. Good prep is 80% of the job.
- Clean thoroughly — remove oil, grease, paint, and dirt. A degreaser and stiff brush work well; a water blaster is even better.
- Repair cracks — fill cracks wider than 1–2 mm with a flexible polyurethane or epoxy crack filler. Sika and Ardex both offer suitable crack repair products available in NZ.
- Allow to dry — concrete must be dry before applying most waterproofing membranes. Check the manufacturer's minimum moisture content requirements.
- Grind or etch the surface — for penetrating sealers or epoxy coatings, the concrete surface needs to be open and porous. Acid etching or mechanical grinding achieves this.
Choosing the Right Waterproofing Product
There are several product types suited to garage floors:
| Product Type | Best For | DIY Friendly? |
|---|---|---|
| Penetrating concrete sealer | Mild damp, condensation | Yes |
| Epoxy floor coating | Surface water, light rising damp | Yes, with care |
| Polyurethane membrane | Cracks, moderate water ingress | Moderate |
| Cementitious waterproofing | Rising damp, wet areas | Yes |
| Crystalline waterproofing | Rising damp from below | Moderate |
Ardex and Mapei offer cementitious and membrane systems widely used in NZ. For epoxy coatings and sealers, check the ranges at Bunnings or Mitre 10. Always read the product data sheet and follow the manufacturer's instructions exactly — coverage rates, recoat times, and temperature requirements all matter.
Step-by-Step: Applying a Basic Waterproofing Sealer
- Prepare the surface as described above.
- Apply a primer coat if required by the product (many penetrating sealers don't need one).
- Apply the first coat using a roller or brush, working it into the surface.
- Allow to cure fully before applying a second coat — don't rush this.
- Allow full cure time (often 24–72 hours) before driving or walking on the surface.
Ventilate well during application — many waterproofing products contain solvents. Check the safety data sheet and wear appropriate PPE including gloves and a respirator if required. WorkSafe NZ has guidance on working safely with chemical products.
Dealing With Drainage
If rainwater is running into your garage, waterproofing the floor alone won't fix the problem — you need to address the source. Options include:
- Installing or improving a channel drain at the garage entrance
- Improving the fall (slope) of the floor toward the drain
- Fitting a garage door threshold seal
Note: any work connecting to the stormwater or wastewater drainage system is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed drainlayer. Check licensing at the Plumbers Gasfitters & Drainlayers Board.
When to Call a Professional
Call a licensed waterproofer or builder if:
- Water is entering through structural cracks or movement joints
- You have significant rising damp that a surface treatment won't solve
- The slab itself is deteriorating or spalling
- Water is also affecting adjoining walls or the house structure
- You're unsure of the cause after your diagnosis
A professional waterproofer will have access to commercial-grade products and application equipment, and can provide a warranty on their work. You can find qualified local waterproofers on tradedirectory.co.nz.
For technical guidance on moisture in buildings, BRANZ is an excellent NZ-specific resource.
Site Safe
SiteWise
Licensed Building Practitioner (LBP)
