Extending Concrete Base for Garden Office

7 Oct 2008
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United Kingdom
Following an earlier post on the same subject I am hoping someone will be able to advise on the following: -

The existing concrete base is topped with patio slabs some of which are loose. When getting the concrete pumped in for the extended base the aim is to cover the sabs with approx. 50mm of concrete. Should all the slabs be removed first?

For the new part of the base I have dug out approx. 8m x 1.5m to a depth of 300mm. I am now putting in some hardcore from an old path. This will be compacted down with a whacker plate. I then intend getting a groundwork company to oversee the levelling etc.

The garden building will then have a timber base structure fixed to the base after DPM has been layed.

Is there anything I missed? Grateful for members views
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Just a few queries.

Firstly joining 2 slabs of concrete often doesn't work well as they will move and settle at different rates. Also consider that the old base is perhaps not still level.

If you are not having the base as your finished floor and will be building timber over the top then it doesn't matter much.

50mm of concrete is too thin and will crack up. you need to remove the slabs and clean the top of the concrete base thoroughly. This will give you at least 100mm which should be enough.

You may be better not trying to bond the new slab to the old one and use a bit of dpm to create a slip plane between them.

are you planning on putting steel mesh into the new slab?

A 125mm or 150mm slab with steel mesh would be best if you are doing a long term valuable structure on top.
Thank you for the response r896neo.

The base will not be forming part of the finished floor as timber sole plates will be used for a timber deck for the building wall panels to be fixed to will be laid. The idea behind 'extending' the current base was to have a solid structure to reduce the chance of vermin attack if I was just to extend the 1.5m using a plinth system.

Given that the majority of the building will be supported on the existing base I was not planning on using steel mesh but could perhaps use that between the 2?

I will look to get estimates for pouring the new base and then a further 100mm for the entire area. I am hoping that the firm supplying the concrete can give me some useful advice too!! Many Thanks

Firstly I would advise removing all the paving before concreting. Unless you can qualify what paving they are and what they have been laid on. Often you will find that the paving are non-pressed and my have been laid on 5 spot thus leaving voids.

Secondly 2" (50mm) of concrete is not thick enough to hold its own when there may be sway in the ground as you are building above it.

I would remove the paving and whatever they have been laid on and increase the thickness of the concrete to at least 3" (75mm) if not even 4" (100mm)
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Many thanks for your reply. I have now removed all the slabs and in most places the slab is very solid. I am just in the process of calculating the amount of concrete I will need but was looking at 150mm to go on top!!

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