There are some parts of your new house extension/build you may be able to cut corners on, but the foundations should definitely not be one of them. If your foundations are wrong the whole build will be wrong.
After having worked alongside dozens of builders for many years, I can say with a degree of confidence that the majority of the smaller sized ones (who most of you will no doubt be using) have no great interest in the groundworks/foundations part of your project.
They just want it over and done with as quickly as possible so they can get started on the real building part for them, i.e., the kit and this is where the problems can arise.
In my experience, builders can be broken down into three categories.
1) those who price the build to do the works properly - and do:
2) those who price the build to do the works properly, but still cut corners wherever possible to save extra money:
3) those who deliberately or through ignorance, price the build a good bit cheaper than anyone else to definitely get the job and are then forced into cutting every corner possible, or try to make as much as possible an extra:
If that sounds familiar... read on!
Below are some of the most common mistakes made when doing the foundations part of a new house/extension that I have personally seen frequently occur over the years.
The strip foundations not dug to correct width due to incorrect buckets or more likely to save extra money.
The strip foundations not dug to correct depth through lack of experience or again to save the cost of extra skips.
A less expensive size of mesh or believe it or not, quite often none at all to save time cutting it all up and extra money.
The incorrect depth of concrete poured into the foundations due to no laser or just wanting to save extra money.
The foundations are dug to correct depth but the formation is still very soft. Instead of digging it a bit deeper to get down to harder ground or asking the engineer to have a look, it is simply ignored and the concrete poured in — due to lack of experience, sheer laziness or perhaps to save time — which can cause structural problems down the line.
The area under where the concrete slab will be, is dug too shallow so that just a skim of hardcore is put on top instead of the required 150mm (in most cases) — again just to save time and money. Don’t forget — most companies just use shovels and wheelbarrows to bring the hardcore round which is hard backbreaking work, and therefore time consuming and usually more expensive.
Whatever depth of hardcore is used under the slab, it is not compacted properly or often not at all, which can cause a void and lead to structural problems later on.
Pouring the concrete slab in (which is actually a bit of an art) can easily end up in a total mess due to lack of experience and not using the proper equipment.
‘Keep It For Backfill’ — if I had £1 each time I’ve heard this... Again through lack of experience or more often wanting to save extra time and money, they want several tonnes of basically whatever kind of waste material is dug out the foundations left on site somewhere normally outside but close to where the blockwork is going. That would be fine if the material was suitable but in the vast majority of cases it’s just clay, soil, subsoil or sand which most certainly isn’t.
Once the blockwork's up to floor level, the substandard material is then shovelled into the foundation around both sides of the wall which over time can cause a void under the concrete slab and will cause the slabs for a pathway to sink on the outside. Type1 stone should be put back in and compacted in layers with the appropriate type of wacker.
The concrete is poured into the strip foundations when there is still a lot of water sitting there. Usually when there is a lot of rain, a good bit of waste material from around the edges of the foundation will fall in too. If that's all hidden under the water and the concrete is simply poured on top, as well as weakening the concrete, voids can also occur, causing structural problems down the line.
Another common mistake is when the concrete is poured into the foundations when the ground is frozen in winter time, which can again cause structural problems at a later date.
And last but not least - the foundations are initially marked out incorrectly, obviously making the whole build wrong if not spotted quickly. (don’t know how many times I’ve decided to check the measurements myself too, and had to change some of the lines)
Don’t forget, at the end of the day it’s you who'll have to live with any mistakes — not whoever makes them. So if you see any of the above taking place, don't be afraid to speak up... before it's too late.
Remember, if you're spending your hard earned money on such a costly investment, then you really need to make sure that you're working with professionals who will do it right first time.
Finally, it's worth bearing in mind that we do this type of work on almost a weekly basis, whereas your builder may only do two or three foundations a year.
Let Edinburgh Micro Diggers dig your foundations and your builder do the building.
Here’s an example of one of our typical foundation and concrete slab jobs that we recently finished at Craigleith Edinburgh.
This extension is 29m2 and will be a lounge after completion hence no drainage required.
We dug the strip foundations, poured concrete back in and put in the concrete slab leaving it around 210mm below finished floor level to allow for 150mm insulation, underfloor heating pipes and a 50mm finished floor screed.
Four full skips were filled from the foundations and underneath the concrete slab area, with another three and a half filled from around the extension to allow for a patio and pathway resulting in a total of about 60 tonnes of waste material taken off site.
Here’s pics from start to finish of this project! Use the navigation dots or arrows to see the whole project in sequence.
Edinburgh, Musselburgh, Penicuik, Dalkeith, Biggar, Peebles, Tranent, Haddington, Dunbar, North Berwick, Livingston, Bathgate, Broxburn, South Queensferry, Lanark, Carluke, Galashiels, Boness and Falkirk.
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