Building on existing shed base that is half concrete slab and half paving stones

[guir78]

Im looking to build a 15ftx9ft workshop on an existing concrete base and paving slab base. Ideally i'd like to not have to remove the current base and paving and install new larger concrete base

Current concrete base is 8ftx6ft and then another behind it 8x6 (for a greenhouse). These 2 bases seem to be somewhat level with one another.

Next to the concrete base are paving slabs, I would estimate 10-20mm height difference but hard to tell.

Could I build the base over these 2 surfaces and pack it out under the paving slabs?

I was thinking of just buying these jacks to to raise up the part of the bearer that will be on the paving slabs -

 

[freddiemercurystwin]

I would, although probably find a cheaper alternative to jacks you mooted.

 

[JohnD]

Will the new building have a floor?

 

[guir78]

Will the new building have a floor?

yes, I'll be buying a summer house from Dunster House. They come with pressure-treated bearers and a 19mm T&G floor. Ill be laying Celotex 25mm sheets in between the bearers for the floor insulation

My idea is to use some joist cradles like this on the paving slab section as some parts are 20-30mm below the concrete base. On the concrete base section I will use packing shims to bring everything up just 3-5mm from the concrete base so it's not sitting in water

Do you think that will work or should you leave the bearers in contact with the concrete base?

 

[JohnD]

As you will have a floor, I would prefer a new perimeter strip foundation, or pads for posts. Then you will know it is deep and stable and will not shift with ground erosion, and you can cast it to the ideal height and level, raised above watersplash, and with DPC to protect the timber.

For mine I used concrete pads, with steel post feet. They need thorough protective painting before you fit them. You can put simple square formwork round the upper part so it looks neat, benched to throw off water, and position them inside the footprint of the shell

I don't think your 5mm gap is enough. 100m is better and will allow airflow.

 

[guir78]

As you will have a floor, I would prefer a new perimeter strip foundation, or pads for posts. Then you will know it is deep and stable and will not shift with ground erosion, and you can cast it to the ideal height and level, raised above watersplash, and with DPC to protect the timber.

For mine I used concrete pads, with steel post feet. They need thorough protective painting before you fit them. You can put simple square formwork round the upper part so it looks neat, benched to throw off water, and position them inside the footprint of the shell

I don't think your 5mm gap is enough. 100m is better and will allow airflow.

i cant add a perimeter because the current concrete base is already butting up against neighbours fence, only space I have is on the paving slab side. Digging foundations is out of my budget (and skill) so I'm looking for the most simple, robust method.

Im also very restricted by height. The summer house is 2.45m high. Building regs allow 2.5m so I cant raise up much fro the current concrete base at all.

 

[JohnD]

The paving slabs are more likely to shift. Maybe lay an edge strip to match the level of the concrete. For a shed, a strip a foot deep, and wide, will do. If you have a spade, you can dig a trench.

You will need gutters and a downpipe to carry rain away and prevent it falling or pooling on the base.

 

[guir78]

The paving slabs are more likely to shift. Maybe lay an edge strip to match the level of the concrete. For a shed, a strip a foot deep, and wide, will do. If you have a spade, you can dig a trench.

You will need gutters and a downpipe to carry rain away and prevent it falling or pooling on the base.

I see, have I understood what you mean with this sketch below? It does add a lot of work to the process, I've never laid concrete and would rather get the shed built over a weekend or 2.

Feel like its going to be a real PITA to get this strip poured and level with the two current concrete bases and would probably mean pulling up all the paving stones and relaying. From what I've read, paving stones do seem to be a very popular alternative to a concrete base, would it not be enough?

Update: Pulled up a paving slab, this is what I'm dealing with, approx 25mm thick slabs

 

[cdbe]

Presumably you will have access to the Packers? Any movement just repack - a five minute job, I'd try and get the timber as high as possible (within your constraints - measured from the highest adjacent ground) as it may be treated but it'll still be cheap timber.

 

[JohnD]

I see, have I understood what you mean with this sketch below? It does add a lot of work to the process, I've never laid concrete and would rather get the shed built over a weekend or 2Update: Pulled up a paving slab, this is what I'm dealing with, approx 25mm thick slabs

Oh, I pictured the strip as going round the edge of the slabs.

When you build a shed on a slab, if any part of the slab is exposed, rain will fall on it and tend to pool under the shed.