I am about to embark on building a small extension, of size 1.5m x
3.5m, sited on an existing concrete driveway. I've never done
foundations before, and have heard that it is very hard work, so I
wondered about getting someone in to dig and fill the foundations, and
install a new rainwater gulley. I've had three quotes, £2100 (but
only as part of a build-the-entire-extension quote which I didn't
want), £2700, and £2900 (all inc VAT). They're roughly comparable so I
guess they are probably fair prices, but they are higher than I'd
anticipated when I first did the costings for the extension a year or
so ago. Have prices gone up a lot recently?
To keep costs down, I am thinking about doing it myself, but I'm
trying to work out whether it is worth the hassle. I think I've got
the rough idea of what needs to be done, but a few questions arise:
- If I cut out the perimeter with a concrete saw, are gas/water
services deep enough to avoid being cut, or must I locate them
precisely first? I know where they are already to about +/- 30cm.
- I will use a powered breaker to smash up the concrete in the centre,
taking care to avoid damaging the services. Is a mini/micro-digger
useful, or more hassle than its worth for a small site? Maybe a self-
propelled skip loader is a useful compromise?
- Will the BC surveyor want to see a test hole already dug right down
to 1m, or will they hang around and watch a test hole being dug until
they're happy it's deep enough?
- If I bought in ready-mixed concrete, will the operators level it out
for me, or would that be my job?
- Any other tips, or recommendations for making the job easier?
I'm sure other questions will arise as I think about this more :-)
, but are the savings really worth the effort?
Water SHOULD be. Electricity SHOULD be. Gas I dunno.
Both SHOULD have warning tape over the top.
Bothe will be deeper than the concrete for sure, so cut concrete and
then hand dig those areas.
Why? use it as a decnt sub base, put a DPM and insulatin down and
screed over surely?
To be honest, I think if you are fit, you are making a bit of a mountain
Yes, hand digging is hard work, but the risk of damage is a lot less.
Also falling into the pit you have just dug with the digger, with the
But a mini digger is fun and will do the job in an hour or so, BUT
beware those services. Dead easy to smash a service with one., No feel.
How deep are you having to go?
The BC will want to see the complete trenches, and will advise on the
depth to meet current requirements. If he is dubious because trees etc
are around, he may also want you to get soil samples analysed.
If he is already involved, direct most questions to him. They are
usually much friendlier when asked, than when being told...
Your job. Its not that hard. rakes and so on. And 'level' isn't that
necessary. Simply lay an underground course of concrete block up to a
decent level, using variable mortar beds of VERY strong cement (2:1)
However given the size, it may be just as easy to get a mixer and mix
your own, which allows a micro adjustment at the final stage by adding
little bits to the existing soggy puddle.
easiest way to get concrete more or less level is to hammer in pegs to
concrete top using strings and levels, then fill to the required depth.
Ask BCO whether you need compressible polystyrene sheet to line the
trench for heave..thi sis simply EPS pegged securely to the trench edges.
Ask BCO whether or not you can, if they are exposed, lay foundations
directly round services. can't remember rules on this one. They may
want them in a flexible sheath first.
Yes. Also you can adjust to taste, like using rebar to strengthen it
You may also decide to try and use bow ties or rebar to tie into holes
made in existing foundations.
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