Cost of concrete base in NW London

Heyyouguys

Hi All

Hoping you can help with this.

I have a large garden, and at the back I want to create a concrete area 9m x 5m (so 45sqm in total) with a depth of 150mm - right now for a shed and planter area but in future can be used as a base for an outbuilding. I have had quotes for this ranging from £2000 to £8000, and am based in North West London.

The job would involve excavation of the area with a small digger (due to access), supply and pouring of the concrete and waste disposal.

Could someone please advise how much I should be paying for this as I am very confused?

Thanks in advance!

FreeBear

You won't need a 150mm thick base unless you are planning on installing industrial machines.

100-150mm of MOT1 or crushed rubble topped with 75-100mm of concrete - Some would still regard that as over the top.

£2000 sounds a bit on the cheap side for such a large base - You'll have something in the region of 18-20 tonnes of spoil to get rid of - A mini-tipper to get the waste to a suitable area close to a road, and a grab lorry to uplift (possibly multiple trips). Then you have to get 10-14t of hardcore in, and top it off with 3.4-4.5m³ of concrete. That is a lot of concrete that you don't want to be mixing on site either by hand or with a cement mixer. Ready-mix is the answer, and you still have to get it to the work area - Pumped will be costly, a mini-tipper will take time (and need a darned good clean afterwards).

On balance, £8K might not be far off.

stuart45

Who's doing the sub base and the formwork?

Heyyouguys

Thank you both for your help, especially the comment about 100mm vs 150mm.

They will be doing the sub base and formwork as well, they are doing everything as part of a larger project involving a porcelain patio creation as well.

I have someone who has come in about £4500 who seems very reputable, so from your feedback, definitely seems like this is a good price. 

Section62

Heyyouguys said:

Thank you both for your help, especially the comment about 100mm vs 150mm.

 

What kind of outbuilding were you planning for the future? Depending on the final dimensions you may need building regs consent (as well as possibly planning consent).  If BR applies then make sure the type of slab (and thickness) is appropriate for the type of building.

Also, 9m is a fair length for a single slab - you might want to take advice on incorporating a movement joint, or alternative approaches for crack control.

FreeBear

Heyyouguys said: I have someone who has come in about £4500 who seems very reputable, so from your feedback, definitely seems like this is a good price. 

You need to ask about specifications - i.e. thickness of sub-base and concrete on top.

At 9m by 5m, you will need expansion joints at (typically) 3m spacing on a 100mm slab (~2.4m @ 75mm). This is to prevent the slab cracking. If the slab uses reinforcements, expansion joint spacing can be increased,

Heyyouguys

Thank you again for your fantastic advice.

It will be reinforced with steel but I will definitely bring up the expansion joints and thicknesses with the landscaper I choose. 

stuart45

What's the reason for using steel? Tensile strength because of poor ground conditions meaning possible failure of the sub base, crack control at the surface or both?
If it's for tensile strength the rebar needs to be in the bottom half. If it's for crack control it goes on the top half. Or it goes in both.
Using expansion joints and control joints in concrete means you normally need a break in the rebar. If you have continuous rebar through the slab, it can crack in different places. 
Most landscapers aren't concrete specialists, so they normally don't really have the knowledge to understand the properties of concrete, when rebar is required, where to place it and whether to use expansion or control joints.

Heyyouguys

Thank you all for your input. I know how annoying it can be for people to ask a question, get answers and then disappear, so thought I would give an update. 

We went ahead with the whole project in the end and I picked one of the landscapers @ circa £4500 for the concrete. The whole garden is now done and they have done a great job, but as @stuart45 says, it seems they are great at porcelain but less so with concrete.

I wanted your thoughts on this concrete finish, especially the stones showing - I expected it to be smoother and "cleaner" in terms of the finish. I understand it has been tamped but even then it still seems super rough. Any thoughts would be much appreciated.

Thanks in advance!

  

FreeBear

For a shed base, it looks to be perfectly acceptable. Yes, it could have been given a smoother finish by running a float across it a few times, but that would have probably pushed the cost up. If you hadn't specified a smooth floated finish, then you wouldn't get one.

If you want a smooth finish, you could but down a ~25mm thick screed over the top. But I don't see much point in that if you are putting a shed on top.

Section62

Heyyouguys said:

I wanted your thoughts on this concrete finish, especially the stones showing - I expected it to be smoother and "cleaner" in terms of the finish. I understand it has been tamped but even then it still seems super rough. Any thoughts would be much appreciated.

As FreeBear said, it looks fine to me.

If the whole area is being covered by a shed/outbuilding then nobody will see the concrete when the building is there.  If an area isn't going to be covered by a shed/outbuilding then you need the finish to be reasonably rough to provide slip resistance - a perfectly smooth concrete surface can be very dangerous in wet/icy conditions.