Conservatory Base

We started work a few weekends ago on a dwarf wall type conservatory approximately 3m2. After reading several posts on here we opted to dig the foundations to the same depth as the house foundations. Concrete was poured level to base of 3rd brick below exisiting damp course.
Outer wall of Conservatory is constructed of 3 courses of engineering bricks then DPC followed by 8 courses of face bricks. Inner wall has a course of concrete blocks to start then celcon blocks to bring it level with the outer wall (50mm cavity filled with insulation).

Our question relates to the oversite/base etc....after googling conservatory bases several sites show a similar cross section through the base consisting of the following......
Minimum 150mm Hardcore
Sandbinding
DPM
100mm minimum concrete
50mm screed

All of the aforementioned diagrams show the hardcore starting at the level of the concrete foundations with 3 bricks on the outer wall to the DPC.
If the height of 3 bricks is approximately 220mm............how do you squeeze 150mm hardcore, 10mm sand binding?, 100mm concrete & 50mm screed in?????.
I had also intended to install 50mm insulation in the floor as well....which would make the above even more impossible........

Answers on a postcard please........or alternatively.....on here would be most appreciated (am starting to think that this manual labour stuff is affecting the brain).

You work down from the finished floor level (ffl) not up

And it would be a good idea to incorporate some insulation to keep the floor a bit warmer than it would otherwise be

And you don't need a screed - you can trowel the slab smooth

so its

100mm concrete
50mm insulation (celotex)
DPM
25mm sand blinding
100mm hardcore - well compacted

This assumes the ground is firm. If not, you increase the hardcore as necessary and compact it in layers of 100mm if its thicker than 150mm

But you've gone way too deep with the foundations by the sound of it if you've gone more than 600mm in good ground

Whether the hardcore is at the same level of the concrete foundation depends on the foundation design - deep strip or traditional, so don't rely on any internet images depicting a "typical" section

Thanks Woody, apart from a slight difference in measurements that was pretty much what I was hoping to do (or believe to be acceptable).
So if I lower the current ground level between the footings by 50mm to give me 275mm to DPC I should be Ok?
Would you also advise putting 25mm thick insulation (celotex) between the 100mm slab and the inner wall? (sorry something else I read while perusing the net).

Yes run some of insulation around the slab (and any screed) where is meets an external wall to remove a possible cold bridge. 25mm of polystyrene will do and will be cheap. There is also available some 10mm dense foam on a roll which will do for this.

If you plan for this to be purely for summer use, then you could do away with the insulation, which will reduce the digging. But otherwise go for 275mm of floor structure

Presumably you are having the floor level with the existing house floor - and not a naff step down?

The idea was to use it all year round, gone for the Pilk K energy glass, triple poly roof etc.
With regards to your last comment this poses another minor question, we had planned for the floors to be level (the 275mm previously mentioned would bring us up to existing floor level).....but we have existing (and are keeping) patio doors which have an external cill. The bottom of this cill is probably 10mm below existing floor level and we were planning to use ceramic tiles within the conservatory.

This will probably mean having to dig an additional 20mm (making 70mm) so that the concrete floor finishes 10mm below the cill to allow for tiles/adhesive etc (and will probably result in a step down into the conservatory of approx 10mm).

you may actually be able to lose most of the existing sill within the screed, provided your new conservatory external door threshold is a standard variety.

upvc sills have a fairly tall internal upstand and may allow you to do this and have room for floor tiles also.

Thanks noseall, do you mean to just bury it within the screed or to cut it away?

colport27 said:

Thanks noseall, do you mean to just bury it within the screed or to cut it away?

Click to expand...

either/or.

fill yer boots. it is exactly what i have done at home. i've swallowed mine within the screed.

just ensure that you have the tolerance for the tiles. if the new doors open out then i'm fairly sure you will be able to do it. if the existing doors open into the connie use these as the guide.

Thanks again for all the replies, sorry for posting this next question as im aware similar have been posted so many times before but I cant seem to find online calculators that give the same results.

Hardcore (crushed type from builders merchants), how much do we need?
Internal measurements of the walls are 2393 x 2616mm and we were proposing 100mm depth (0.63m3)....can it be ordered in m3 or does it have to be weight?
100mm concrete floor......how much balast, how much cement?

Sorry again for posting this but we only have tomorrow to order the materials to be able to commence the floor on Saturday.

you will need about 1.5 tonne of mot type 1 stone. you may not use it all but a lot of b.m's will only deal in as small an increment as 0.5 tonne.

this will need compacting and subsequently blinding with 25-50mm of building sand. it will need compacting again once blinded.

you will also need the same amount of ballast for the floor slab (1.5 tonne). same story regards quantity, so i guess you will have a bit left over.

you will need about 9 x 25kg bags of cement to mix with the ballast, for your floor area.