Raft Slab - DIY or not?

[Tony K]

Hi All,

Leaving aside the footings themselves (which might be piles, steel screws or concrete trenches), I need to construct two 5m x13m raft slabs as shown in this sketch:

 

 

From the top down it is:

250mm poured concrete slab with A393 mesh top and bottom, and rebar to SEs design.

85mm clayboard, which will collapse leaving 75mm void.

20mm blinding sand used just to ensure a level base to lay the clayboard on.

 

I will be need shuttering / formwork to create the edges as shown.

 

I have fallen out of love with local groundworks firms who, based on their inability to even quote for work when they say they will, I am not thrilled about employing for anything that matters. Like this.

 

Question is, can an amateur like me create the above? I'm already excavating the area. How difficult is....

 

Creating level formwork to the edges. What would I use? Timber? Hired road planes?

Getting the sand base level?

Installing the clayboard? 

Placing the mesh and rebar in?

Accepting the concrete when it arrives?

 

Cheers

[JFDIY]

It's all do-able, depends on how much you want to take on.

 

Main thing you'll need is a decent laser level and receiver to save time getting everything level. Straight timbers and stakes work fine for forming the edges, as will hired steel forms. Benefit of timber is you can do it at your own pace and not be on the hook for weekly costs. 

 

If you use a mix on site concrete firm you can dictate the pace and you only pay for what you actually use.

[Dreadnaught]

I am on a similar journey, @Tony K. I too will be taking the advice of some of those on this site who have done it before us.

[Russell griffiths]

This is hard to answer really depending on you. 

How will you move the mesh?

what type of piles? If you have continuous augered piles how will you break the tops down?

are you laying the concrete?

how will you get the concrete to all areas of the slab?

what is the finish on the concrete?

if You are supposed to have a 20mm layer of sand is you excavated area that good or will you need something else to get the levels right. 

It is all very doable on your own except laying the concrete, you will need lots of help for this, moving the sheets will be very awkward on your own they are very heavy. 

 

I work on my own every day and just plod on but I have an excavator for moving heavy stuff and would call on 4-5 other blokes when I need to pour the slabs. 

You will need a good bunch of tools

and don’t underestimate how hard work it will be. 

[Miek]

2 hours ago, Tony K said:

 

Creating level formwork to the edges. What would I use? Timber? Hired road planes?

Getting the sand base level?

Installing the clayboard? 

Placing the mesh and rebar in?

Accepting the concrete when it arrives?

 

All of the above apart from the last are doable and fairly straightforward because you can do them at your leisure, BUT, 16m of concrete is a good size pour so I would get an experienced hand in when you do that unless you have prior experience yourself. Concrete is very unforgiving if things go wrong. Access for the concrete needs to be considered. You can hire a pump for about £450 if it's tricky and they take a lot of the hassle out of the equation. I would strike roller it if it's a simple rectangle.

 

 

[Triassic]

I did my own insulated raft foundation. I did  the whole job myself, the only bit I had help with was the placement of the concrete. I got a local man with a concrete pump to help.

 

Why the piles?

Where is the insulation?

Why not go the whole hog and install an insulated raft foundation?. 
 

Hit the search button above and look at insulated concrete rafts.

[Tony K]

Thanks for the advice. 

Any idea what I should be looking at to pay a groundwork firm to do it for me? I've priced materials already, but not sure what labour costs might be. 

 

Not that any of the firms round here even respond. 

[Steve247]

All good and valid statements made by others. 
I would however say that the bearing integrity from (as you say) concrete trenches, screw piles or concrete piles will add complexity each in their own right and how you integrate in to your raft slab. So it’s critical you review and understand each option and who’s going to do what. 
piles will in my experience need specific rebar tied into your raft and not just placing standard mesh alone. 
if you go for concrete trenches, they are likely to be 1m deep for BCO and if the conc strip is say 350 thick you’ll need trench blocks or other method to reach up to and integrate your raft. 
a raft as you refer is rather basic and doable if your hands on and have done some research on principals of how to do it.  However, ordinarily a basic thing like mesh panels might be bigger and heavier than you might expect.
Btw, I would doubt that 20mm sand would level out a reduced dig of 13m or so and you ought to be considering a compacted hardcore base and DPM over your levelling sand.  Hopefully your SE is doing a detailed design for you.  Get that right and then analyse how you are going to undertake each element. 

[Tony K]

29 minutes ago, Steve247 said:

Btw, I would doubt that 20mm sand would level out a reduced dig of 13m or so and you ought to be considering a compacted hardcore base and DPM over your levelling sand.  Hopefully your SE is doing a detailed design for you.  Get that right and then analyse how you are going to undertake each element. 

 

Cheers. What sort of depth would you advise?

 

2 hours ago, Triassic said:

I did my own insulated raft foundation. I did  the whole job myself, the only bit I had help with was the placement of the concrete. I got a local man with a concrete pump to help.

 

Why the piles?

Where is the insulation?

Why not go the whole hog and install an insulated raft foundation?. 
 

Hit the search button above and look at insulated concrete rafts.

 

I can't get a positive result from doing that. Is there a link or suchlike? Ta

Edited May 2, 2020 by Tony K

[Tony K]

2 hours ago, Russell griffiths said:

 

I work on my own every day and just plod on but I have an excavator for moving heavy stuff and would call on 4-5 other blokes when I need to pour the slabs. 

You will need a good bunch of tools

and don’t underestimate how hard work it will be. 

 

Did you find that what you saved in costs doing it yourself you lost in time and therefore mortgage payments?

[SuperJohnG]

@LA3222 recently completed his insulated raft  foundation. Took him 6 weeks to complete, though he reckons it could have been done in 3 had he done some things differently with the benefit of hindsight and if the weather was better. 

 

Do a search of his posts to learn more. 

 

I'd also recommend, as others have, you go the extra step and make it an insulated foundation.

[Tony K]

2 minutes ago, SuperJohnG said:

@LA3222 recently completed his insulated raft  foundation. Took him 6 weeks to complete, though he reckons it could have been done in 3 had he done some things differently with the benefit of hindsight and if the weather was better. 

 

Do a search of his posts to learn more. 

 

I'd also recommend, as others have, you go the extra step and make it an insulated foundation.

 

Thanks. 

What benefits are there to an insulated foundation as opposed to what I've currently got, where insulation will sit on top of the slab once the outside walls go up?

[SuperJohnG]

Just now, Tony K said:

 

Thanks. 

What benefits are there to an insulated foundation as opposed to what I've currently got, where insulation will sit on top of the slab once the outside walls go up?

 With an insulated foundation you are completely isolated from the ground as the insulation sits on the sand blinding then is filled with concrete to completely eliminate any cold bridges. Then will then form your floor which you can then fit your flooring to. The further added benefit is that you can install the UFH in the insualted raft and avoid the multiple steps required with a standard type raft foundation. 

 

With the standard raft you have mentioned you will fit insulation on top but the walls will still be cold bridging to the ground.

[Tony K]

1 minute ago, SuperJohnG said:

 With an insulated foundation you are completely isolated from the ground as the insulation sits on the sand blinding then is filled with concrete to completely eliminate any cold bridges. Then will then form your floor which you can then fit your flooring to. The further added benefit is that you can install the UFH in the insualted raft and avoid the multiple steps required with a standard type raft foundation. 

 

With the standard raft you have mentioned you will fit insulation on top but the walls will still be cold bridging to the ground.

 

OK thanks 

[Tony K]

3 hours ago, Dreadnaught said:

I am on a similar journey, @Tony K. I too will be taking the advice of some of those on this site who have done it before us.

 

@Dreadnaught I don't know about you, but I'm already getting the feeling I need to persevere with getting groundwork firms to do it. 

 

Have you got any quotes for your job so that you at least know what savings you'd be making by doing this yourself?

[Dreadnaught]

18 minutes ago, Tony K said:

@Dreadnaught I don't know about you, but I'm already getting the feeling I need to persevere with getting groundwork firms to do it. 

 

Why do you think that? Does the example of people like @LA3222 and @Triassic, among others, not inspire you?

 

18 minutes ago, Tony K said:

Have you got any quotes for your job so that you at least know what savings you'd be making by doing this yourself?

 

I have had quotes for the screw piles. And I have a quote for the raft from a timer-frame company that does a combined solution of raft and frame. My engineer is currently finalising my raft design (or at least will be when the frame design is finished) and I will then explore the best approach to building it.

 

If I can find someone, I would much prefer to partner with someone who is more experienced than me with such things and build the raft with them as a team. And then hire a specialist team only for the concrete pour and power float.

Edited May 2, 2020 by Dreadnaught

[Dreadnaught]

By the way, @Tony K this is what @LA3222 managed to achieve … Very impressive!

 

 

Edited May 2, 2020 by Dreadnaught

[Tony K]

33 minutes ago, Dreadnaught said:

 

If I can find someone, I would much prefer to partner with someone who is more experienced than me with such things and build the raft with them as a team. And then hire a specialist team only for the concrete pour and power float.

I might have an option like that. The guy in question is a piling contractor recommended by someone on here, and though I probably won't now use him for driven piling (because I probably won't be using driven piling) he also offered his services on a day rate to help with the slab. I can't vouch for him myself other than to say he seems nice and helpful, but @Patrick speaks very highly of him. You're welcome to his contact details if you like. 

 

In terms of doing it all myself, the concerns for me are around the time it would take me to get it right and how unforgiving the concrete is if I make a mistake, though I will look at @LA3222 very impressive work.

[Dreadnaught]

6 hours ago, Tony K said:

I might have an option like that. […] You're welcome to his contact details if you like. 

 

Many thanks @Tony K, I've sent you a PM.

[LA3222]

@Tony Kan insulated raft can easily be done pretty much solo. Areas where a bit of help is good are setting your profiles out - using a tape measure solo is a long and tricky process, much easier with two. The concrete - I had 3 blokes turn up to do mine, after watching them I am glad I didn't try it myself. The finish wasnt laser flat but flat enough that laying floor finishes should be fine.

 

I had a mix of A393 and A292 mesh in mine. Thankfully the widest A393 strip was 600mm so I could move it myself - a full sheet is 70kg, not too heavy but the shear size makes it extremely difficult to move solo. A292 is about 40kg so even full sheets can be moved solo (it does get snagged etc but doable).

 

If you can use an insulated raft rather than piles it should be doable. Piles I have no experience of, but if you have a good set of drawings showing how all the steel ties together I'm not sure why you couldn't tackle it. Cutting steel and placing it is easy - just keep checking the drawing. Another mistake i made between layers was to plough on to the top layer assuming A393 was top and bottom in the thickened strips - it was actually A292, by the time i paused to check I'd cut all my A292!! A lot of unnecessary work created having to lap and tie them all back together.

 

I just used a 9" grinder to cut the steel and a potato stack tie twist to do the ties. Dont follow this approach, there are thousands of ties and it takes ages, hire one if those hand held tie twist machines, I wish I had - it can be used to tie the UFH pipe too.

 

I should add some pics of finished slab to the other thread, my house is going up at the minute though so very busy!

 

Good luck with whatever you decide, contractors are a PITA and I've had the same difficulties you are facing, you'll get there in the end.

[Tony K]

4 hours ago, LA3222 said:

@Tony Kan insulated raft can easily be done pretty much solo......

Good luck with whatever you decide, contractors are a PITA and I've had the same difficulties you are facing, you'll get there in the end.

 

Very useful advice, thanks @LA3222

 

I should also add that I will be using a concrete pump simply because there is no vehicular access to the site. Don't know if that makes the job any easier or harder. 

[Big Jimbo]

Could be worth talking to a couple of local smallish concrete firms. I have One near me, that will bring a few bods, pour and level.

[Tony K]

56 minutes ago, Big Jimbo said:

Could be worth talking to a couple of local smallish concrete firms. I have One near me, that will bring a few bods, pour and level.

 

I was wondering if that was an option. That way I can diy the formwork if needed and then hand over to the experts for the pour. 

 

That said, I presume I'll be getting the formwork level all around the top edge, and that when the concrete is pumped in I would just have to slide a long straight timber across the top of the formwork to get a fairly level top to the slab?

 

Or is it not that simple?

[Russell griffiths]

Have you considered your drainage needs to go under the slab as well. 

[Tony K]

3 minutes ago, Russell griffiths said:

Have you considered your drainage needs to go under the slab as well. 

 

I have thanks, yes. 

It isn't too complicated, waste pipes and a soak away connection. 

 

I plan to put the pipes in after the piles are in, then cast the slab.