Extension Foundations

[ianm75]

Hi All,

I have recently been quoted in excess of £35k for a single storey 24sqm extension by 3 independant builders. This cost is just for foundations, block work, roof external render. I have already paid for the architect, party wall surveyor, council fees and utility checks so i suppose you can say thats another 3k

Because i see this cost as excessive and therfore not financially worth doing on my house i've decided to look into doing it myself with the help of sub contractors.

The Architect has stipulated 1000mm deep foundations because of the clay area. I have worked out i will need 10sqm of foundation concrete.

The question i have is if the architect has stipulated 1m x 600mm foundations is this 1m below the current 150mm DPC level?

Also will i have to calculate so that lay 1m of concrete then use a couple of layers of engineer brincks to bring upto DPC, lay the DPC membrane and then use blocks for the rest.

Thanks Ian

 

[GaryinAshby]

Ian, I found myself in the same situation last year! I am not a builder, but I have a couple of builder mates that taught me bricklaying in the past. I now have a few garages & various patios built by myself. I also have a "sound" knowledge of building regs from the same folks. But, I'm no expert!

That didn't put me off. I drew and submitted the plans myself for both planning and full building regs approval. If you've only gone for a building notice, talk to the building inspector, as a building notice really is geared towards the "experts" where a B.I checks your work after you've done it, whereby full plans submission ensures that a B.I is checking what you want to do BEFORE you do it!

Like you, I had 1000mm deep (x 450mm wide as its only single storey) foundations and the 1000mm refers to the depth of concrete below normal ground level. I then used about two courses of 7kN blocks and three courses of class B engineering bricks to bring it up to DPC height.

Ask lots of questions, do lots of research & then do it yourself. I had quotes of £32K plus for my work. I'm doing all of it myself, buying lots of materials off EBay & so far it looks like it will all come in for under £10K.

 

[GaryinAshby]

The pics below are of our extension taken a while ago. It is around 30m² and is a staggered "L" shape. The biggest saving was the bricks. 1500 Baggeridge solara multi-buffs for £100 from EBay!!!!!!

 

[^woody^]

Hi All,

The question i have is if the architect has stipulated 1m x 600mm foundations is this 1m below the current 150mm DPC level?

Also will i have to calculate so that lay 1m of concrete then use a couple of layers of engineer brincks to bring upto DPC, lay the DPC membrane and then use blocks for the rest.

If you've paid for drawings, then why don't the drawings show you, in detail, how the foundations and substructure are to be be built?

Foundation depth is below ground level, DPC 150mm above the highest ground level. Normally you would have three course of engineers below ground, then a minimum of two above upto the DPC level. The DPM is then lapped into the DPC and then the main structure can be built

 

[^woody^]

The pics below are of our extension taken a while ago.

Whats happening on the rear elevation with 1/4 bricks on the end of the middle pier

Its a bit of a no-no - should be header 3/4.

Otherwise looks a nice job

Edit: or is it the front elevation

 

[GaryinAshby]

Do you mean the pier thats tied into the house? The reason for the smaller (1/3) brick length is due to a monumental cockup with measuring for the french doors..... , necessitating a bit of cutting & chopping to fit the pier into the allocated space. Never realised that was a problem & the B.I never said anything (she probably didn't notice it, seemed more interested in making a fuss of the dog!).

 

[softlad1971]

I think he meant the 1/4 bricks on the window reveals. Should not have been done like that.

 

[GaryinAshby]

At the risk of sounding thick....... any chance you can explain a bit more? Can't quite see which bit you mean. Is it the front view (with the two smaller windows) or the rear view (where you can see the back of the house with the patio doors).

Thanks

 

[GaryinAshby]

The (crude!) sketch below shows the window reveal in plan view of the two smaller windows, is this the area in question? This pic obviously omitts any wall ties or cavity insulation.

Is this questionable structurally or just aesthetically?

 

[^woody^]

Ends of a stretcher bond wall should not have a 1/4 brick - 1/2 or full brick only.

Its not structural but it is 'wrong', and the BCO would not comment

 

[ianm75]

Thanks All, especially GaryinAshby your reply was really helpful and i'm glad i'm not the only one receiving extortionate quotes for relitively small extensions.

I have done the maths and the shell with roof and 2 velux windows, foundation concrete ready mixed and delivered, all blocks, timber and tiles and so far i'm looking at 4k. A big improvement over 35k. Even if i bring in a bricky and roofer i will make drastic savings hence why i'm looking at digging and getting the foundations poured and then get the trades in after.

So Andy, you used two courses of 7kN blocks and three courses of class B engineering bricks to bring it up to DPC height. So assuming the actual trench you dug was more than 1000mm (obviously depending on surrounding height)

Also, why use the 7kn blocks and not just engineering bricks to build from foundations to just over DPC? I have several copys of Architects diagrams and was looking at an older one! The ones with the building reg on do state pour foundations to 150mm below DPC so should i then use 2 layers of Class B engineering bricks and then DPC and then 2 further layers of engineering bricks then the the rest of the build on block?

I like the ebay idea, i have just seen 600 tiles, the type i need for £9, could get a bargain. Like your Extension Gary, looks like you've done a great job, maybe take it up as a full time job. Would'nt be hard to undercut the competitors!!!!

Thanks Ian

 

[noseall]

how far you need to dig down, as far as finished concrete is concerned is irrelevant.

building control will ok a 1m dig provided the ground conditions are suitable. if not, they will ask you to dig deeper.

we normally mass fill the trench with concrete. we never fill any higher than 21" below dpc. this translates to: two blocks and one course of bricks (internal masonry) or 1 course of blocks and 4 courses of bricks (external masonry).

this allows for the fitting of drainage gulleys, and allows for moderate ground level deviation. just showing two courses of bricks below damp is asking for trouble.

 

[GaryinAshby]

just showing two courses of bricks below damp is asking for trouble.

Yes, it makes it difficult to get any fall on surrounding patio/driveway etc without showing the concrete blocks beneath. Guess how I know that one................................

 

[ianm75]

how far you need to dig down, as far as finished concrete is concerned is irrelevant.

building control will ok a 1m dig provided the ground conditions are suitable. if not, they will ask you to dig deeper.

we normally mass fill the trench with concrete. we never fill any higher than 21" below dpc. this translates to: two blocks and one course of bricks (internal masonry) or 1 course of blocks and 4 courses of bricks (external masonry).

this allows for the fitting of drainage gulleys, and allows for moderate ground level deviation. just showing two courses of bricks below damp is asking for trouble.

Thanks for the info noseall, So what you saying is that if ive been advised by the arhcitect to dig 1000mm then i dig the 1m which is likely to strart about 150mm below DPC (i havent checked yet but looks that way. Then basically only fill the trench with concrete to about 21" below DPC giving me enough for 2 lines of block and 1 line of bricks. This option means that the foundation trench may not have the full 1m of concrete it will be approx 620mm of filled concrete then the block 2 lines of block and 1 line of brick making up the reminaing 380mm or do i have to take theses into concideration and dig deeper?

Thanks again for all your help!!

 

[noseall]

you are getting slightly confused about concrete thickness.

there is an accepted minimum thickness (where applicable) of only 225mm of concrete. anything thicker is a bonus.

our average thickness of concrete is 650mm.