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I have a 18x20 second floor deck with three 6x6 support posts. Under the deck is a 40-year-old, well-settled, 12x20x6" concrete patio. The deck posts are set outside of the patio, set on footings underground - so the posts are buried. I'd like to extend the concrete patio so it matches the dimensions of the deck. That makes for a 6x20 extension.

One of my concerns with the concrete extension is that the new pour would surround the posts and I want to take the necessary precautions so I don't cause any future problems.

As for the extension, my plan is to implant short lengths of rebar into the side of the patio to help keep it and the new pour even. I will make sure the new pour is deep enough keeping the frost line in mind and using compacted aggregate underneath. Any thoughts or concerns with this approach?

I've read where I should use a 1/2" rubber wrap around the posts to accommodate post and/or concrete expansion. Is this advisable? Can I (or should I) seal the area with silicone to prevent water from seeping down?

Just to make it more interesting, and another of my concerns, is that the old patio drops about 5-6" over 12'. I'd like to reduce that slope. I was thinking of pouring the extension higher than the old patio and having it feather up the slope. Probably using an aggregate-less mix for the area on top of the old patio.

To finish it off, I'll either paint it or cover it with pavers - with the preference being the pavers.

I'd rather not demo the old patio, as some may suggest, but I'm open to all advice. The desired end result is that I want a more level, larger patio, under my deck, that will sit several inches above ground at the far end.

I have considered decking over the concrete but I have concerns with airflow, critters, and maintenance that I believe to outweigh the concerns of a concrete extension.

Thanks in advance for taking the time to read this and for responding.

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Sounds to me that you have covered all the bases. Maybe you should pay attention to few details:

Connecting old and new concrete; you have mentioned that you plan to do it, make sure that they are well stuck inside of old concrete (you drill slightly smaller hole than bar and hit the bar it with hammer when placed inside a hole. Also, you can chip old concrete (inch or two) so it will connect better with new one. When it comes to posts I haven't seen the picture so it's hard to tell, but if you want to separate it from slab, you can do it by placing thin Styrofoam around posts before pouring concrete. Later on you can leave it or take it out and put some commercial products (make sure that those are for outdoor usage). When it comes to changing the slope I really don’t see why would you do it? Accept if you don't like it. Anyway, if it's not causing any problems my advice is not play with it....I’m not sure it can be done properly.

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  • I'm not a concrete expert, but drilling a small hole and driving rebar into it sounds like a recipe for cracking the slab and having concrete shrapnel flying about. I would think that drilling a larger hole then installing the rebar either with the new concrete flowing in to hold it in place or, more likely, an appropriate epoxy mixture to hold it securely in the old slab would be a better approach.
    – FreeMan
    Mar 10, 2015 at 18:35
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    Good advice. I'll probably drill larger holes and use epoxy to hold them in place.
    – Chris
    Mar 10, 2015 at 20:06
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    No, no and no....concrete will not crack, and shrapnel will not fly about. I'm telling you to do this so you would connect old and new concrete. And if hole is bigger than bar it will not work.Method I have described is cheaper and works 100%; I have done it many times before. If you want you can drill holes same size as bars (or 1mm bigger) pour in some of the special products (sika-anchorfix),but it is not necessary really since this is just a ground slab.Don’t fool yourself new concrete will not fill the hole you have drilled,unless it is inch or so wide, but making holes this big is wrong.
    – python starter
    Mar 11, 2015 at 13:22

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