Correct size of concrete block foundation?

[BML]

Some years ago when building an extension I used 3 inch concrete blocks and was told by the Building Inspector to replace them with 4 inch. I've just visited my daughter who is having an extension built and the builders are using 3 inch concrete blocks. So, does anyone know the correct size of concrete blocks used in foundations?

 

[Spectric]

Depends on what you are building, where you live and what the ground is like, most will go OTT if trees nearby. Used to be a concrete pour with building inspector there or use 8 inch hollow blocks and fill. These days it could be anything they can away with.

 

[Lons]

It should make absolutely no difference whatsoever as long as the blocks are specified for below ground.

What is critical is the depth and overall width of the found depending on ground conditions, tree roots and what you are building on top of the foundation, e.g. if you are building a 300mm cavity wall then the found should be around 400 mm wide minimum and it doesn't matter if you're using 75mm or 100mm blocks as the centre of whatever you're building must be filled with concrete mix to ground level and therefore solid. I don't see the point as the extra concrete would offset the block price difference and I never used them.
By far the most common trench type found these days is a reinforced full fill concrete in any case or double size aerated trench blocks.

My experience with BOs has always been good and the vast majority are helpful but you can get the odd dork among them.

 

[Keith 66]

It is always best to be nice to your building inspector, when we built our large extension we had two, One was an ex builder himself & was good as gold, the other was rather more officious but okay.
A friend crossed swords with the same bloke a year later, while digging footings he argued the toss got beligerent & tried to tell the bloke how to do his job. So when they hit a lump of concrete that intruded into the next trench & crossed where the drain would go he made them dig it out rather than re route the pipe. It turned out to be a WW2 anti aircraft gun base & was about 12ft square & three ft thick, very hard concrete with lots of rebar, It took the builder nearly two weeks to break it up. An expensive lesson.

 

[HappyHacker]

I have come across a variety of Local Government building inspectors over the years a few were excellent but unfortunately many were idiots.

The village I live in is on a sandstone outcrop. The old houses are built on solid sandstone foundations that are above ground and below ground, some had bedrock floors until recently. Solid sandstone is visible above ground throughout the village. While building an extension and digging the foundations we hit solid sandstone at a foot down, the inspector insisted on a metre depth otherwise we would get frost heave. He was unable to explain how you would get frost heave in sandstone. The JCB managed to scrape it out with the bucket teeth about 1/2 inch at a time.

While building a basement the approved plans showed walls build up off a concrete slab. The last 300mm of diging was again in solid sandstone. Another building inspector insisted on the floor, laid on solid sandstone, being reinforced as the walls would impart a turning moment on the concrete and lift up the middle. He was unable to explain how this was going to happen unless the sandstone moved but kept on repeating there will be a turning moment and it must be reinforced.

A friend, who used to be a building inspector, was building foundations on solid granite. The building inspector insisted on one metre foundation depth. My friend told him NO, the granite has been there for millions of years and it is staying there and left undisturbed is not going to move in the next few million years, he would not have it dug out and if the BS refused a completion certificate he would sue the council as he was talking rubbish. He got his completion cert.

I have also been working on a customer site when the BS was so rude and unhelpful to the very nice cooperative customer that if he had been talking to me like that I would have helped him off the premises without opening the door.

I have managed to get the occasional very helpful and pragmatic inspectors but it does appear to be pot luck.

 

[Inspector]

Just curious if you have a building inspector "problem" like has been described above if you can go to an engineer and have them sign off on the change or variance to the plans and code? Lots here would just require rebar pins in the stone to lock the work to it and sign off on it. The building inspector has to accept the engineers change and stamp, making sure the work was done to the drawing. Engineer is responsible for the structural and related stuff. The inspector makes sure it complies to the drawings. Engineer will charege for it but it is a lot cheaper than beating your way through the rock.

Pete

 

[Spectric]

I think in the Uk the building inspector would inspect the digout to ensure it meets his requirements and then watch the pour so that the builder does not fill in some of the trench to reduce the cost of concrete. There has been cases where builders on large sites have used very little cement in the mortar so that you can just scrape it out with a pen, on others spacing out trusses to save cash. You need inspectors but good honest ones because so many houses are just thrown up and when the builder thinks it's time they disolve the business and start up againto get out of being chased for bodges.

 

[Inspector]

Inspectors here specify at what stages they want to inspect and can stop work if there are problems or the builder has proceeded beyond the inspection point. For my house there was an inspection of the concrete forms, then when the framing was done, plumbing and wiring rough in, insulation and vapour barrier. Then the final inspection when everything is done. He could "red card" the job if he found issues, stopping all work until remedied and re-inspected. Final electrical inspection and plumbing was by different inspectors and they could sign off without looking depending on which electrical or plumbing company pulled the permits. Good companies get spot checked new or bad ones get full inspections until trusted. If there is an issue that crops up with the foundation, like the bedrock or high water tables etc, then the inspector can suggest acceptable remedies but if the contractor doesn't like it then they can get the engineer to provide the remedy that the inspector has to make sure is complied with. The bigger and more complicated the job the more involved and frequent the inspections. It is everyones best interest to get along with one another without being onerous.

As for shady builders and trades. They are everywhere but laws allow them to "reinvent themselves" and start anew. I think that they shouldn't be allowed to have or be part of another business again if they pull that stuff anywhere in the country.

Pete

 

[HappyHacker]

Historically UK Building Control are part of the Local Authority and they pass the plans as meeting the appropriate Building Regulations and employ the Building Inspectors who then made sure the plans were being adhered to. Now there are private companies providing the same function but they often rely on photos as well as inspections.

The whole system is open to abuse both historically and now. On large sites the Building Inspector would be taken into the site office while on site drains were not connected or connected to the wrong sewer, cavity wall ties omitted and other examples as listed in other posts.

The older Inspectors are often the best as they know the ropes, the younger inspectors often know the theory, or some of it, and have yet to learn the ropes and some are on a power trip. In the example I quoted of the rude inspector he was doing a final inspection. He said "I cannot pass this it is not ready for occupation", it was all finished apart from the joiner who was hanging the kitchen cupboards and as it was a granny flat planning permission had specified a full kitchen was not allowed. But being ready for occupation is not a Building Regulations requirement.
He then said as he had not seen the insulation in the stud walls he would not pass it and the plasterboard (plastered and painted) would need to come off so that he could see it. The customer pointed out that they had a visit from one of his colleagues, with his name and date, who had seen the insulation and told them they could continue. He replied "There is nothing in the notes so I will need the plasterboard off to see the insulation". Eventually he agreed they could sent the photos taken with the insulation installed before the plasterboard was put on. He said a lot of other stuff and was a complete pillock but the customer got his completion cert after sending the photos in.

I have also been on site when a competent inspector was checking on progress, giving helpful advice to the customer about roof penetration for a chimney and asking me sensible questions about what certification I would provide for the electrics I was installing. I have also had the helpful inspectors who have provided good advice and made pragmatic decisions when a builder has got things wrong.