OT: Some concrete leads

[whiskychaser]

Can't you just place planks on all four sides, like a mold or form, and seal them at the bottom so there's no leaks, then fill the inside with fresh concrete until it's flat all the way across?

That would be the idea. The big problem is how you seal the bottom to stop the new concrete running out

[whiskychaser]

If the surface is that bad you have a couple of options. Scribe the sleepers to match the concrete under(Who poured this mess?) or form around the existing pad and pour a level layer on top.
If'n it were me I'd level the concrete.

Thanks Ross but there isnt an existing pad to form round. The mess was poured many years ago and foul water drains run shallow underneath. I've done them before but risking hitting one isnt something I need. So we come back to scribing sleepers as you suggest -or planing 32ft of timber into wedges. Which is where I came in. To be honest I'd normally just get on with it but the glass wont fit in the frames if I get this wrong. All this cos she wants to grow her own vegetables

[Ross]

Did someone just pour the concret onto the ground?
If you are going to build a glass structure you will need a stable surface.

[AngryPopTart]

Then just build a form around the concrete, even if it's larger than the existing slab, with the boards in the ground and pour a completely new slab to cover over the old one.

[Tiger]

From reading all the replies you wrote, seems like your slope is quite alot over long span... That's such a large greenhouse. A couple of options...

1. Hire a concrete leveler specialist... What they will do is come to your site... drill a couple of holes and pump in grout or new type is polyurethane to lift up your concrete to almost level. Most likely grout in your case as you have to raise quite a bit. This is costly for big concrete as you describe. After this is done, you can pressure wash the concrete and lay a thin layer of concrete resurfacer to level out the rest of concrete.

2. Create a level form all around your existing concrete base, support it well so it doesn't blow out. Pressure wash the concrete and then put a layer of concrete conditioner so it will bond to new concrete well... and hire a truck to deliver and pump concrete into the form.

3. Pressure wash your concrete... Lay the perimeter where your new greenhouse will sit on... create a 8" perimeter level form all around it. Use concrete conditioner... then pour concrete into the form. If the bumpy surface bother you for your greenhouse floor, surface concrete leveler or sand and lots of patio blocks.

4. The right way is as you know... jack hammer all concrete out and relevel the ground, and form... but this is practically #2.

If the issue is you don't know how to level that concrete form or concrete over such a large span, use water level or transit... water level is much easier to use.

[Tiger]

Another thought... I think the concrete was level for your original greenhouse... the land settled as rain wash away soil so that's why you got a sloping concrete. To prevent this from happening again, you have to do the retaining wall around the slope, or your greenhouse will do the same thing over long time.

[pingu]

No, the concrete - if it's nice and runny (but not so runny that it runs past the foam) - should find its own level.

If you want a really nice job then you could first make a platform of concrete by relying on the concrete to find its level. Once it has set, take off the planking and attach fresh 4" high planking (or whatever, depending on high thick you want the second concrete layer to be) to the side of the concrete using Rawplugs etc.

Then pour a second layer of concrete over the first layer and tamp the second layer down, using the new planking as a guide, to get a nice flat surface. Job done.

[Tiger]

Similar to what I said... my way is safer and more true level. Remember... glass will crack under stress of unlevel foundation.

[leicesterboy15]

Have you got a picture of the location? Might be a bit easier to visualise then.

From what you describe I think we had something similar with my workshop shed, we built a brick border and concreted them in, then filled the hole with hardcore (well, stuff that was left from removing the fireplaces) and sand. We then leveled it off and flagged over it leaving the old concrete hidden and a nice level square base. If your greenhouse is normal shed size, you don't even need to flag it, you could run beams off the brick base under the greenhouse supports, that may be quicker.

But I would have thought the easiest way would be what has been suggested and make a wood border and fill it with concrete. As long as the consistency is correct and the wood is attached so it can take the weight it doesn't need to be completely tight, it won't run out of all the gaps unless they're large! If they do just run around it scooping up the excess until it starts to dry or hammer off the excess once its dry.

[Ross]

"No, the concrete - if it's nice and runny (but not so runny that it runs past the foam) - should find its own level."

It's this sort of method that likely got him the surface he has.
Either form it correctly, pour on top and finish, assuming it's clean and rough enough to stay put. Or sleepers, either scribed or shimmed as Paul says.
A glass(any) structure on a Mickey Mouse foundation is just asking for it.
If it is attatched to the existing structure it should also have some foundation below the frost line to prvent the addition from moving independantly of the house.
If you put it on a pad it shouldn't be attatched to the house as it will move around more than the house.