Conservatory foundation dilemma

[garyo]

I'm building a conservatory which needs a 13.7 metre long foundation trench. It's not going to be under building control, but I want to do a proper job so I'm expecting foundations to be around 650mm deep based on my initial dig/examination. The cavity wall will be 255 wide, plus 150mm either side, giving a foundation width of 555.

If I was to build up from 15cm below ground level, I'd need 4 cubic metres of concrete. This seems way too much to mix in my belle 140, but ready/mini mix at ~£120 a cube seems like a lot too, so now I'm considering something like a 200mm foundation and then block work up to ground level. The obvious dilemma is that I spent a day or two laying the blocks rather than buying the readymix.

Any thoughts? Am I under/over estimating the work/money involved anywhere?

Gary

 

[Robert too]

If it is a machine dig , then a 400 wide bucket would be perfect for a trench fill foundation rather than a strip footing.

You mention you want to do it " right " The minimum depth for a foundation under building regs is 1 m

 

[garyo]

The top 8 inches of the site has already been excavated and filled with a concrete patio, on top of hard core, so I don't think a machine dig is worth it unless I'm going to go much deeper than 650mm.

1 metre. That's a lot of digging. And a lot of concrete!

 

[diyisfree]

You could lay a 200mm strip foundation and then come up from that just bellow the finished level with trench blocks not concrete blocks.

Simple to use, builds inner and outer skin so to speak in one go and doesnt need the lean mix infill.

Just a thought and what i have done in the past.

 

[garyo]

The current plan is a 50mm cavity, total wall thickness 255mm, so a 250mm wide trench block would be ideal. I have four 45 degree corners to deal with so they migiht be more awkward to cut/size I suppose.

Cheers

 

[diyisfree]

you can simply use the handle of a saw to mark the 45 degree cut. the great thing about trench blocks is that you can cut them with a saw

nothing difficult about it

 

[simhar]

Thats thing with conservatories most people dont realise - most of the effort goes into the foundations (the thing you dont see !)

 

[garyo]

absolutely. I've been planning the base, wall, and floor construction for a couple of months now, and haven't even thought about what happens when I get the 600mm wall finished (apart from having a beer)

I've got to the point now where I'm going to use thermalite trench blocks to 150mm below ground level. From that point the outer skin will be brick, and inner skin will be thermalite blocks. The problem I'm having is that the height of the thermalite blocks doesn't divide nicely into the space:

1) from 150mm to the DPC (total = 300mm)
2) from the DPC to 600mm wall height (total = 600mm)

so I'm having to use a course of bricks to make up gaps here and there. Is this usual?

Diagram below - appologies for the photobucket resizing:

Link to image is : http://www.photobox.co.uk/album/album_photo.html?c_photo=1312143026[/url]

 

[Robert too]

it is normal , if you havent planned it correctly ,

you can get thermalite " bricks " too but if you dont fancy ripping a course of blockes down , just go with brickwork


or how about just laying a row of blocks on their side across the whole wall width ?

 

[garyo]

I'm not sure where I'd lay them?

One solution would be to raise the initial trench block level up 7.5cm, and compensate with a thicker foundation strip. That would remove the single course of bricks below the DPC, but only leave me with a single course of brickwork below ground level outside, which I thought was bad practice for frost, and the risk of exposing the blockwork if the outer levels change slightly.

I take it there's no way I can lower the inner DPC 7.5cm , which would leave me with three courses of bricks above the DPC which could be replaced with a single thermalite?

Cheers

Gary

 

[garyo]

Another question, how should I tie the dwarf wall into the existing house, and prevent damp bridging into the conservatory by running along the bricks/mortar in the existing house?

I suppose the same question applies to the conservatory frame itself too?

Gary

 

[diyisfree]

wall starters are the easiest option.

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=101892&ts=96757&id=41317

 

[garyo]

Cheers. Would I put a vertical DPC behind those?