THE FRIDAY DETAIL | NCC 2022 | PART 4
A concrete floor can hide quite a lot. Underneath the surface might be a network of beams, a line of strip footings or individual supports doing very different jobs. Before asking whether a detail looks right, it helps to know what system you are looking at.
This week we are bringing the related footing-system clauses together. We have already covered the ground preparation, membranes, concrete and reinforcement. Now we can connect those pieces to the structure beneath the home.
Check the support and what it stands on.
A stump and its footing do different jobs. The stump supports the floor structure; its footing spreads that support into the ground.
Clause 4.2.13 separates footing provisions from requirements for concrete, steel and timber stumps. The tables distinguish supported floor and roof areas, construction and stump material. A post that looks the same as another may carry a different share of the building.
Picture two supports beneath a floor: one also receives roof load, the other does not. Asking only for the post size misses that difference. Find the support identifier, what it carries and the referenced footing detail together.
For a homeowner, ask the builder to point out that relationship on the drawing. For a professional, reconcile the support schedule, load assumptions, footing reference and bracing details. Read the applicable table notes, not just an isolated cell.
The beams and slab work as a system.
Think of a stiffened raft as a slab with a structural beam arrangement, rather than a flat sheet of concrete with a thick perimeter.
Clause 4.2.14 groups its tables and figures by site class. Construction type also changes the table selection. Its notes require the internal and external beams to form an integral structural grid.
In practice, read the slab plan and beam sections together. The plan tells you where the beams run; the section explains their shape and reinforcement. A section copied without its beam layout leaves a major part of the instruction behind.
Fictional change: an opening is moved and a support shifts. The useful question is whether the revised load position is coordinated with the footing design. Moving a beam on site to suit a new layout is not an administrative drawing tidy-up.
Similar names do not make details interchangeable.
A strip footing supports a line of construction. The floor between supporting strips can be a separate infill element. Do not assume that an infill floor is the same structural arrangement as a raft.
Clause 4.2.15 provides strip-footing tables and figures for Class A, S and M sites. Read the chosen construction row with its notes and associated figure. Clause 4.2.16 is specifically the footing-slab route for Class A sites.
Fictional review: a drawing for a Class M project cites 4.2.16. That citation needs resolving. It does not mean the project cannot be built; it means the stated reference does not match the cited clause's site-class scope. Ask the designer to confirm the correct design basis rather than swapping in a table yourself.
Keep the full footing-system name on an enquiry or variation. “Change the slab” is much less useful than identifying the affected strip, beam, support or panel on the current drawing.
A wall change can reach below the floor.
Fictional exercise: a client replaces a framed external wall with masonry. The architectural drawing is revised, but the footing schedule still describes the former construction. A subcontractor proposes using the existing detail because the floor plan has not changed.
Write a short response that identifies the changed wall, the mismatch and the person who must resolve it. Do not choose a new footing size. The aim is to recognise that supported construction is an input to the footing details, even when the building footprint stays the same.
A useful request is: “Please confirm the footing design for the revised wall at grid B. The current structural schedule still lists the earlier construction. Which coordinated revision should we use?” Keep that question with the drawing revision and the response.
What to ask before the next step
Ask the builder to show you the footing system on the current drawing, explain what the relevant support carries and identify where the detail is documented. If something has changed, ask how that change has been carried through the structural information. Clear questions now are much easier to deal with than an assumption hidden in concrete.
For construction professionals, the same conversation becomes a coordination check: system, supported construction, drawing revision, table or design reference, then the unresolved change. A tidy drawing is useful. A coordinated drawing is better.
Keep learning
The companion interactive lesson has system comparisons, three practical scenarios, a drawing-change exercise, eight explained questions and a downloadable takeaway. Open the free interactive footing-systems lesson. Browse the existing Part 4 lessons for the earlier topics.
Next we will group the remaining clauses 4.2.17 to 4.2.22 into one topic on special construction and local slab details, before moving into masonry. This article does not replace those details or a project design.
Edition and application: this series teaches NCC 2022. Confirm the edition, amendment, state or territory variations and approved design that apply to the actual project. The scope and site-classification lessons cover the entry conditions; they are not repeated here.
