Concrete shuttering design?

[Ian White]

I'm about to make a concrete base for a 4.5m x 3.0m workshop. The timber shed will be supplied in panels by the local sawmill, who know how to design them for the severe wind and rain we often have around here.

Their recommendation is to form the base with an upstand as shown below, so that the wooden frame sits on top but the boards can overhang to prevent water driving in underneath.

(view only in a fixed pitch font) _______ ______ ___| |___ | |

To complete the picture, the base will be in a corner between two dry-stone walls. One side and one end of the foundation will be poured right up to the bases of the walls, so vertical shuttering at the lower level will only be needed for the remaining side and end. Also there is about a 0.3m fall in the ground from end to end. The soil cover is very thin, so that will be removed to expose the very firm rocky subsoil. The fall will then be mostly levelled up with solidly rammed hardcore, but with an adequate cover of concrete all over.

The other obvious requirement is that what's drawn as a horizontal level of concrete should actually slope away slightly all round, to provide a runoff.

My question is: how to shutter and pour the concrete?

There seem to be altogether too many alternatives... shutter it to cast the entire shape in a single pour, or various options for two pours and/or adding the sloping skirt later. I'd very much welcome comments and suggestions to narrow down the range.

[Rick]

What I would do

1) Shutter it (but not the upstand), and put the slight slope into it in one pour, unless the mix if very runny this won't be a problem to get the slope into it. 2) Add the upstand with a row of 7N dence concrete blocks, or concrete bricks round the edge 3) put down a DPM, and insulation for a quality job, of an insuated floor.

Rick

[Rob Morley]

By far the easiest in terms of shuttering will be to pour a flat slab, then add the raised bit, and finally slap on a bit of mortar to provide the runoff.

[John Cartmell]

Why not pour the central raised bit (flat) then the outer rim at a lower, sloping, level?

[Rob Morley]

If you pour the central bit first you need to shutter the full depth on four sides, then two sides for the second pour. Doing it the way I suggested only needs shuttering on two sides for the main pour, then minimal shuttering for the top slab.