Attaching a deck frame to a concrete block foundation wall is a common approach in New Zealand, particularly on homes with raised concrete block (also called concrete masonry) perimeter walls. Done correctly, it creates a strong, durable connection — but it does require careful planning, the right fixings, and in most cases, a building consent from your local council.
Understand the Consent Requirements First
Before you pick up a drill, check whether your deck needs a building consent. In New Zealand, decks that are more than 1.5 metres above the ground generally require consent. Even lower decks may require consent depending on your council's rules and the specifics of your site. Schedule 1 of the Building Act 2004 lists some exemptions, but these have conditions — always confirm with your local council before starting work. You can find general guidance at MBIE Building Performance.
If your deck is attached to the house structure, it may also need to be designed to meet NZS 3604:2011 (Timber Framed Buildings) or be engineer-designed if it falls outside the scope of that standard.
What Is a Ledger Board?
A ledger board is the horizontal timber that is bolted directly to the foundation or house wall and carries one end of your deck joists. It effectively transfers the deck load into the wall structure. For a concrete block wall, the ledger is typically 100 × 50 mm or 150 × 50 mm H3.2 treated timber (check your specific span and load requirements).
Use H3.2 treated timber at minimum for any timber in contact with or close to concrete and masonry, as concrete can wick moisture. Check with your timber supplier or PlaceMakers for the correct treatment class for your application.
Choosing the Right Fixings for Concrete Block
This is the most critical part. Concrete block (hollow or solid) behaves differently from poured concrete or timber framing, and not all fixings are suitable.
Suitable fixing options include:
- Chemset (epoxy resin) anchors — these are generally the preferred method for hollow or solid concrete block. A two-part epoxy adhesive is injected into a drilled hole, and a threaded rod or bolt is set into it. Brands like Sika offer anchor systems suitable for masonry. Always follow the manufacturer's load ratings and embedment depths.
- Mechanical expansion anchors — suitable for solid concrete block only. Not recommended for hollow block as they can crack the block face.
- Through-bolts — if the block wall is accessible from the inside, M12 or M16 galvanised bolts passing completely through the block with a large washer and nut on the inside can be very effective.
Avoid standard masonry screws (like Tapcons) for primary structural ledger connections — they are not rated for the shear and withdrawal loads a deck ledger experiences.
All fixings must be hot-dipped galvanised or stainless steel (Grade 316 in coastal areas) to prevent corrosion. Spacing and embedment depth must meet the fixing manufacturer's specifications and, if consented, your engineer's or designer's requirements.
Step-by-Step: Attaching the Ledger Board
- Mark the ledger position — account for decking thickness and any step-down from the interior floor level (typically 150–200 mm below the interior floor to keep water out of the house).
- Check the wall — inspect the concrete block for cracks, spalling, or signs of moisture. Do not attach to a damaged wall without remediation.
- Apply a flashing or moisture barrier — before fixing the ledger, install a flexible flashing tape or metal flashing behind and over the top of the ledger to direct water away from the wall junction. This is critical for compliance with NZ Building Code Clause E2 (External Moisture).
- Drill fixing holes — use a hammer drill with a masonry bit sized to the anchor manufacturer's specification. For epoxy anchors in hollow block, you may need a screen/mesh sleeve to contain the resin.
- Install anchors — inject epoxy per instructions, insert threaded rod, and allow full cure time before loading (typically 24–48 hours depending on temperature).
- Fix the ledger — bolt the ledger board to the anchors using large washers to spread the load. Do not overtighten and crack the block face.
- Seal and flash — seal around bolt penetrations with a compatible sealant and ensure flashing directs water clear of the wall.
Joist Hangers and the Rest of the Frame
Once the ledger is secure, joist hangers (galvanised or stainless steel) are used to connect your deck joists to the ledger. The opposite end of the joists bears on a bearer and post system founded on concrete pads or piles. Ensure post sizes, bearer spans, and joist spans all comply with NZS 3604 or your engineer's design.
Weatherproofing and Ongoing Maintenance
The ledger-to-wall junction is one of the most common points of moisture ingress on NZ decks. Ensure:
- Flashing is correctly lapped and sealed
- There is an air gap or drainage path behind the ledger where possible
- Timber is inspected annually and retreated or replaced if signs of rot appear
For guidance on moisture management, BRANZ publishes excellent technical resources for NZ conditions.
When to Call a Professional
Attaching a deck ledger to a concrete block wall involves structural and weatherproofing decisions that can have serious consequences if done incorrectly. You should engage a qualified professional if:
- Your deck requires a building consent (very likely if attached to the house)
- The wall shows any signs of damage or instability
- You are unsure of the block type (hollow vs solid) or wall construction
- The deck is more than 1 metre above ground
- You need an engineer's design or producer statement
A Licensed Building Practitioner (LBP) is required to carry out or supervise restricted building work on consented projects — see Licensed Building Practitioners. You can find experienced local deck builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
