A reader's excavation question: what size footings?

Shawn G   from   Broadstairs UK   asked what size footings to use.

Hi Bill. Great site you have there. I have just started to build an extension on the back of my house. Plan A was it was going to be a glass conservatory between 2 existing brick built walls.. Plan B now is brick built side, glass front and a tiled roof. I have just started to dig the footings. My question is how deep and wide should they be. Also at the base of the footings do i lay a layer of hardcore.
Many thanks
Shawn

Bill's answer

Hi Shawn,
That's a hard one for me at this distance, as I have no information to work on.

• Your best bet is to get somebody local to advise you.
• Do you have to get it inspected etc. ? If so the guy or the department that does the inspecting is the one to ask.
• He may of course just pass the buck and tell you to see a structural engineer (we get a lot of that here).
• It sounds like it is only a small job, but even so don't run foul of the system.
• You can live in the place for years, but when you come to sell then it may be a problem if you didn't sort out the paper work at the right time.
• We have really good foundation ground where I am , but anywhere else could be totally different.
• Again we don't use hardcore.
• It really depends on the finish of the bottom of the trench.
• If it is rocky and bumpy or otherwise rough and uneven, then provided that there is still enough room, put in some fill to level it and compact it.
• For the most part we just use a compacted sand bed.
• Cheap unwashed fill sand that levels everything up, and saves on concrete. Fairly good sand can be flooded with a hose pipe and gets close to 100% compaction.
• With hardcore etc. you may have to use mechanical compaction like a wacker packer
• Then it would be a trade off between the time and cost of compaction and the cost of a bit of extra concrete.