Day 19: We have a (Poly-)Waffle Slab

The first major building milestone has been reached today. We have a slab! Sure it’s not quite “move in” time, but it is very exciting to see actual build progress after many months of planning.

Our Slab
I had two initial impressions when I first saw the slab:

  1. It’s big!
  2. I hope it’s not too far back from the street.

It’s always different seeing something physical as oppose to looking at it on plan after plan after plan. It’s something that I constantly preach at work – make it tangible in order to get real, solid  feedback about an idea or concept.

Point 1 is a no brainer really – of course the slab is going to be big – its a big house. Der.


Regarding point 2, our original land siting had the house with a seven metre setback.  Once we got into the tender process we were informed that, unless we wanted to go through a planning dispensation process, we would need to adhere to a nine metre setback (which is the average for our streetscape).  Neither of us are fans of houses butt up against block fronts; I have always thought that a large house requires a decent setback for aesthetic purposes. Hence, we were not concerned to move it to a nine metre setback.

It’s only when I saw it that I had some minor doubts. However, these initial concerns were short lived when others (more wiser) pointed out that once the frame is erected, and the height of the house is fully realised, it would provide a completely different perception. The house is nearly eight metres tall and combined with the setback will be the right proportion and perspective.

Our slab is a Waffle Raft design.  “What is a Waffle slab?” I hear you ask. Well let us start with the most common and simplest slab design; the stiffened raft design. It is represented as per below


It consists of trenched edge/internal beams and slab panels with steel reinforcement.  The concrete for this slab is poured in one operation. This design can be used on a wide variety of sites except for Class-P and Class-E.  These sites include:

  • Reactive clay sites
  • Soft soils
  • Areas subject to erosion
  • Soils subject to abnormal moisture conditions
  • Uncontrolled fill sites

For these environments a waffle raft slab design can be employed.


The slab “ribs” are created on top of the ground using a series of polystyrene “voids” (think blocks) that are laid out in a grid.  This addresses the problem of the stiffened slab having its beams embedded in reactive clays that are subject to expansion and compression.  The initial reaction of some people is one of horror when they think that a large portion of their slab is actually foam.  But this is actually what gives it its strength – It creates the form; the structure of cross beams.  The slab is also has steel reinforcement.


This represents the end of the base stage and is a trigger for our first stage invoice (ouch!). Next up is framing – looking forward to it!



One thought on “Day 19: We have a (Poly-)Waffle Slab

  1. Pingback: Day 24: We have Wood – Our Carlisle Canterbury 44

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