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Raft floor slabs
We design raft slab floors for homes and buildings nationwide - from good ground through to TC1-TC3 liquefaction zones, CodeMarked slabs, expansive ground and complex sites that need specialist raft slab thinking.
We work across QPOD, EPS, XPOD and Maxraft slab systems, with more than 20 years' experience designing raft floors for New Zealand conditions.
Our focus is practical raft slab design that is clear, buildable and suited to the site - with straightforward advice, responsive communication and a team you can talk to.
Fast turnaround, consistent high quality details - one engineer from consent to code compliance.
Vert-X is QPOD’s nationwide design engineering partner for continuous and raft floors - the team QPOD relies on to make floors work wherever they’re built.
At QPOD we have been working alongside Vert-X to deliver our clients a cost effective and faster engineering offering. I’m genuinely loving the way it is going and always receive an outstanding service from Stephen’s team - my clients say the same. Our aim is to make this part of the project a painless one; Vert-X does that and more.
Peter Crawshaw — General Manager, QPOD Earth Friendly Foundations
Why builders and designers choose Vert-X
Fast raft slab design is only useful if it is also clear, coordinated and easy to build. At Vert-X, you deal directly with the engineering team doing the work - from design and consent documentation through to construction review and PS4 sign-off.
One engineer from consent to code compliance
The same design teams carries your slab from design through to construction review, including the PS1, Design, Memorandum and PS4 sign-off.
Fast turnaround, without the handoffs
Raft slab and simple residential designs are typically turned around in 2-5 business days, with clear communication from the engineer doing the work.
Nationwide inspection network
Backed by inspectors across New Zealand, we can review slabs on site and issue PS4's efficiently, wherever your project is located.
Full structural design when you need it
When a project needs more than a slab, we can also design beam, portals, steel and timber elements - keeping the slab and structure above it coordinated, documented and easier to build.
Geotech and slab design, coordinated
We can engage geotechnical specialists on your behalf, so the ground investigation and slab design are aligned from the start.
Raft slab systems we design
Examples of the raft slab systems we work across nationwide – showcasing different foundation options for residential, commercial and complex ground conditions.
QPOD Continuous
QPOD Slab Tank
QPOD Raft
EPS Raft
XPOD
Maxraft
Raft slabs FAQs
Answers to some of the questions we are most often asked about raft slabs.
A raft/pod slab needs SED whenever it falls outside its CodeMark certificate or if it is a proprietary floor system without a CodeMark certificate. Sites where the ground doesn’t meet the NZS 3604 definition of "good ground" Will normally fall outside CodeMark limits. Good ground means soil that can permanently carry an ultimate bearing capacity of 300 kPa (100 kPa allowable) and isn't soft clay, fill, peat or liquefiable. Common SED triggers: TC2 and TC3 Canterbury land, expansive/reactive clay soils, low bearing capacity, stepped or sloping floors, heavy point loads, and any build outside the CodeMark conditions. On good ground under CodeMark, no engineer or producer statement is needed and the council must accept the system. Where the Codemark limits aren’t met, a Chartered Professional Engineer must design the floor and issue a PS1. Vert-X provides QPOD, XPOD and EPS raft slab engineering nationwide. Vert-X can also draw standard pod layouts for QPOD and EPS floors that meet CodeMark requirements without issuing a PS1 statement.
On CodeMark "good ground" (TC1), the pod supplier (QPOD, Allied SuperSlab and similar) can often provide pod layout and materials estimates and CodeMark-compliant drawings – no Chartered Professional Engineer and no producer statement required, and the council must accept it. Off good ground – TC2/TC3, expansive soils, high loads, stepped floors – a CPEng Engineer must produce a site-specific specific engineering design (SED) with a PS1 producer statement, and a PS4 construction review after a pre-pour inspection. Vert-X provides QPOD, XPOD and EPS raft slab engineering nationwide. Vert-X can also draw standard pod layouts for QPOD and EPS floors that meet CodeMark requirements without issuing a PS1 statement.
A raft (or waffle) slab is a reinforced concrete floor that sits on the ground rather than in it, using a grid of pods to form voids so the concrete forms a stiff waffle of ribs and edge beams. Pods can be polystyrene (EPS) or recycled plastic (QPOD, XPOD). It's been increasingly adopted by New Zealand residential builders and the engineers at Vert-X have been doing pod floor designs since the early 2000’s.
We recommend you contact floor system suppliers such as QPOD to get the best idea of overall construction prices. Vert-X provides extremely competitive project-specific quotes for SED raft slabs and Codemark layouts.
Vert-X turns raft slab designs around in 2–5 business days once we have the full set of information. To design an SED raft slab or Codemark pod layout we need your floor plan, a site-specific geotechnical (soil) report and the truss layout showing line and point loads. Missing information – especially the geotech report or truss loads – is the most common cause of delay, so getting those lined up first keeps the turnaround fast.
These Canterbury technical categories describe liquefaction risk and drive how much engineering your slab needs. TC1: future liquefaction damage unlikely – standard NZS 3604 or CodeMark pod slabs are fine, no engineer needed. TC2: minor-to-moderate liquefaction possible – you need an enhanced, more robust slab (a waffle raft designed to the MBIE TC2 rules) by a CPEng. TC3: moderate-to-significant liquefaction possible – site-specific geotechnical investigation and fully engineered foundations are required, sometimes deep piles or a combination of a stiffened concrete floor on a geogrid reinforced raft.
Yes - the pre-pour inspection is the single most important checkpoint, done after all reinforcing, pods, formwork and in-slab services are placed but before any concrete is poured. The council building inspector checks the slab against the consent (reinforcing, cover, dimensions, damp-proof membrane). For an SED raft slab, the engineer also inspects before the pour, because that inspection feeds the PS4 construction review. Miss it, and the work can't be verified - which can hold up your Code Compliance Certificate. Vert-X provides pre-pour inspection and PS4 for the raft slabs we design.
A pod raft slab generally uses two forms of reinforcement: steel bars in the ribs and edge beams (which carry the structural loads), plus a mesh in the concrete topping over the pods (which controls cracking). For some slabs – particularly TC2 and TC3 sites, steel fibre reinforced concrete can be used in conjunction with this reinforcing for a stronger slab. Pods (EPS or recycled-plastic QPOD/XPOD) are just void formers and spacers – they carry no load; they space the mesh and shape the ribs. In an SED slab, the engineer specifies the exact bar sizes, spacing, laps and mesh for your ground and loads, rather than relying on the standard CodeMark layout.
Raft slabs can be insulated to meet the Building Code's H1 energy-efficiency requirements, which tightened in 2023 and vary by climate zone. Insulation is added under the slab (rigid XPS/EPS boards like QPOD SlabCore) and around the perimeter edge beam (systems like Firth HotEdge or Magroc Insulfound), which is where most floor heat is lost. The slab's R-value is a function of its geometry plus any added insulation, and the concrete's thermal mass helps stabilise indoor temperatures. Your designer calculates the R-value for your climate zone and most pod floor suppliers have R value calculators on their websites.
Yes - with the right engineering. On gentle slopes, minor steps can often be poured in a single slab; for bigger level changes, a retaining edge or stepped slab is integrated into the design. On poor or soft ground, options include a compacted gravel raft under the slab to spread load, ground improvement, or (for liquefiable TC3-type sites) deeper foundations designed after geotechnical investigation. All of this is specific engineering design by a CPEng, informed by a soil report. This is exactly the kind of work Vert-X has done nationwide on projects from Kerikeri to Queenstown.