Base for a full height conservatory

[elsmore1]

Hello,
I am in the process of building a base with a builder friend for a 4m long by 3m out full height conservatory. The plan is to build strip footing with A142 mesh 300mm by 300mm, then about a 3 brick high wall, fill the centre with 150mm hardcore, DPC, insulation and 150mm of concrete.

My question is as there is a slight slope I have dug out the trench and at the wall of house the earth will cover all of the concrete, but as the ground over 3m falls away a bit when I reach the furthest point away from house the concrete will only be about 100mm under ground and about 200mm will stick out, this is at worst point.

WIll it be ok to use heavy boards to contain the concrete while setting (only over part of the footings) and try and back fill with some mud or will I need to dig the whole trench deeper by about 200mm this will of course mean a higher brick wall.

I hope you can understand but basically the 300mm by 300mm concrete footing will be exposed at bit at the ends. I am sure the concrete will be strong enough as if you just built a 300mm by 300mm block of concrete and left it on open ground and put a haevy load on surely it would still hold, Is the earth actually doing much?

 

[hotrod]

I understand what you're saying, you want to erect some formwork for your footings that are going to be exposed due to the gradient of the site. What are the ground conditions clay/sand and what's the thinking behind using A142 mesh?

 

[kevindgas]

dig your trench to allow for the gradient then use some formwork to step the concrete at the top

 

[elsmore1]

Thanks for replies I will respond to hotrod first. Yes I do want to ercet some formwork i.e heavy wooden planks, also the site is quite heavy so mixture of clay and topsoil, it has been ok to dig out so not too much clay that we cant get it off the shovel! I would say its pretty solid ground and does not dry out in the summer.

The guy who sold me the conservatory did not suggest the mesh but the polish builder helping me did, I have read its not necessary but some people do use it, I guess it will do no damage and I will only need about 2.8m square to use in the trench, and maybe soem for the base 12m square it does not cost too much either.

Now reply to kevindgas. I understand and I am doing this, just halfway through digging the trench.

Many Thanks

 

[RedHerring]

Perhaps your conservatory is not subject to Planning or Building Control but I would still follow the 'Regulations' anyway. And,as I understand it, according to the following (page 36 and table 10) http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf
the minimum width of foundation should be 400mm based on the fact that you can easily dig with a spade and minimum depth should be 450mm.

Have I read it correctly?

Also, do foundations have/need a vertical friction element to them?

 

[Chukka63]

id just dig it all the extra 200mm and be done with it.

 

[RedHerring]

It might also be worth checking Diagram 11 page 24 Differences in ground levels. ( http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf ). Assuming you are using 100mm thick masonry, you'd only be allowed 400mm difference in ground level. (Not that it applies if you're not subject to BC)
There is a note (note 3) that says it only applies to walls with full height of masonry. I don't know whether that means that other superstructures reduces or increases the allowable differences in ground levels.
I've recently completed a garage and I have greater differences than 400mm (probably about 600mm) with a 100mm thick wall and the BC never mentioned it if he was aware of it.

 

[elsmore1]

Again thank you for the replies, from the table in page 36 I think I fit into 300mm wide foundations but I have dug mine about 350mm to 400mm, also I will fit a A142 mesh.

The depth I will keep to at least 300mm, as the conservatory people advised this is ok and it is a full height one so I have no dwarf wall. Also on any exposed area I will back fill with some topsoil so any concrete is not exposed to frost or reduced.

Also the slope is only about 350mm at worst so about 80-90% of the strip footing will be totally under ground level.

Its not subject to planning permission so I think i may overcomplicate issues and I have seen foundations built in various ways!

 

[hotrod]

Dig your footings to a sufficient depth so that when they are poured the top face is still below ext.GL. I would query the wisdom of digging 300mm trenches - but you haven't stated the wall construction. How wide is the wall actually going to be? If you're building a 100mm cavity wall you want more than a 300mm strip.

 

[kevindgas]

also be careful of measurements as a lot of cons'y manufacturers use internal or external measurements not centre line.

i found that out the hard way once!

 

[elsmore1]

Thanks for that, I was always a bit waery of conservatory dealers, but they have supplied me with plans for the base with exact external dimensions so as long as I build to that any errors hopefully i can contribute to them!

 

[elsmore1]

Also reply to hotrod I am only building a flat concrete base and the wall of about 3 bricks high and 1 wide is only an edging to contain the concrete it will have no cavity and I estimate I will only need 130 standard house bricks to build the whole wall, So i will end up with what looks like a concrete square with a brick edging.

 

[Chukka63]

Thanks for that, I was always a bit waery of conservatory dealers, but they have supplied me with plans for the base with exact external dimensions so as long as I build to that any errors hopefully i can contribute to them!

make sure whether the measurements are the external frame or the cill

 

[elsmore1]

Ok I will double check the plans and can call the guy as hes quite helpful so will confirm that to me.

regards