Extending a concrete base

[Plextor]

Outside our back door we have a concrete slab (around 7 inches thick) that used to be the base for a lean to greenhouse I want to extend this by a one to two feet so that it can then be used to support a PVC conservatory

I have been told that no matter how well I prepare the extra base there is bound to be some minor settlement along the join between old and new bases which could for instance cause any tiles laid across the gap to crack in time to come Should I be concerned about this and if so could I just not wait a while before tiling

Many thanks

 

[Deluks]

Better if you can wait a few months for the concrete to fully dry out if tiling into it.

As for movement, If you drill into the edge of the existing with a 12mm drill bit, you can hammer in 300mm lengths of rebar so the it stitches the 2 slabs together.

If you have a grinder you can cut a 'ledge' in the existing, so that the new concrete will partially cover it. If you've 7" to play with then allocate about 3 of that for the depth of the cut.

Be sure to dig down and put a well tamped hardcore layer+mot underneath.



You also have damp proofing to consider, as the existing probably won't have any underneath. Sure it's 7" all the way through? That might be the edges only?

 

[Blagard]

You should pin the two slabs together.

Scabble (Roughen) the edge of the existing slab where the joint will be. Drill about 200mm into the edge of the slab at say 500mm centres and fit some 400mm long reinforcement bars that then stick out 200mm and finally cast your slab extension. Make sure you fully consolidate the hardcore under the new bit of slab and there should be no problem.

Your other concern should be a DPM. You could apply one over the top of the slabs before laying a screed. I doubt that you would be able to extend/lap to any DPM under the existing slab if there even is one.

Edit" Oops! Deluks beat me to it!

 

[Plextor]

Thanks for the quick replies So if I follow this advice how many months should I wait before tiling And suppose I did not tie the the two bases together but waited the same length of time Could I not then make good and then tile Surely any movement would be minor

Thanks again

 

[Blagard]

So if I follow this advice how many months should I wait before tiling And suppose I did not tie the the two bases together but waited the same length of time Could I not then make good and then tile Surely any movement would be minor

You are waiting for the moisture in the concrete to dry out. Ballpark figures for a concrete slab would be one month per inch thickness. But if you have no DPM it may never dry out fully. There are tile adhesives you can use in damp conditions to fast track the works and perhaps this may be your best option anyway. Obviously the slab will not be drying out very quickly while exposed to the elements and once roofed over, heat and ventilation will have some effect. Having a DPM over the concrete with a screed on top would mean drying time for the screed only.

I would not recommend you avoiding to tie the slabs together. It would be very bad practice in my opinion. You would have a very high risk of cracking even if you waited a year! Additionally, ceramic tiles have no tolerance for movement