A formwork step to the edge of a raft slab

A couple of ways make a formwork step in a concrete raft slab.

I have had a question from Dave W. who lives in Wakefield in the UK.

"Hi Bill
Great site!
My problem is how to build shuttering for a raft foundation for my single skin double garage. The slab has been drawn with an outside step for the brickwork to sit on. (300mm x 300mm) with another 300mm below. If that makes sense? How do I construct the shuttering to get the concrete in the right place?
a step in the edge of a raft slab
Formwork step- The desired layout.

Dave sent me a sketch of what he had to do, it looked something like the sketch to the right.
I guess that if you had to build a formwork step in a slab, then around the perimeter is the easiest way. It gets fairly tricky supporting something like this in the middle of a slab.

a step in the edge of a footing
Sketch of the desired slab edge

  • This first layout is ideal when you already have standard made up edge form that you don't want to modify, or if you are hiring modular formwork for the main outer form.
  • In this case to form a large rebate in the edge of the formwork, then I'd say put a bottom in the form.
  • The reason for this is that if you don't put a bottom on the form, then you will have the concrete welling up in-between the supports and it will be hard to get level and neat for the next trade.
  • Also if the concrete wells up around the form it will be very hard to
  • If there is a bottom then you have to vibrate it well, and you will still get air bubbles trapped there but it should be no big deal.
a step on the edge of a slab
A possible method of doing it if it is indeed a proper exposed step.

  • If the flat surface is to be exposed to view, then a different layout should be used to enable the struts etc to be stripped so that finishing tools are able to get in to the surface.
  • In this case the outer form would be cut to the reduced height and the step form would be braced as in the sketch below.
  • The vertical face of the timber that forms the step has a 45 deg. bevel on the bottom to enable the concreter's tools to right into the corner.
  • The surface of the step would be treated the same as if it was a stair.
  • Doing it this way, like a stair step, the excavation would probably have to be wider, and more formwork material would be used.

Please don't take this as the best or only way to do a step like this. Here I am just thinking, scratching my head, taking a pencil from behind my ear and sketching.
On a real job my first thoughts are what materials I have to hand. What is available for hire etc. Sometimes it saves money to be a bit creative.

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