Best stage to lay a concrete garage floor slab.

[epsilonGreedy]

I have been looking at instructional videos about how to lay a ground bearing concrete slab and noticed how much effort is put into tamping down to a level finish (prior to smoothing).

 

Would I be correct in thinking the task is simpler if the footing walls are no more than 1ft higher than the FFL of the concrete as this allows direct external access round the perimeter of the slab?

 

The other option would be to crack on with the walls, put the roof on and then lay the floor slab when protected from the weather.

 

The garage floor is 4m x 5.2m and will have a 2.5m or 2.8m opening.

Edited February 20, 2019 by epsilonGreedy

[scottishjohn]

9 minutes ago, epsilonGreedy said:

I have been looking at instructional videos about how to lay a ground bearing concrete slab and noticed how much effort is put into tamping down to a level finish (prior to smoothing).

 

Would I be correct in thinking the task is simpler if the footing walls are no more than 1ft higher than the FFL of the concrete as this allows direct external access round the perimeter of the slab?

 

The other option would be to crack on with the walls, put the roof on and then lay the floor slab when protected from the weather.

 

The garage floor is 4m x 5.2m and will have a 2.5m or 2.8m opening.

heres a thought for you 

a layer of blocks all round --be particular on them being level to the level you want the FFL

then fill it in with the concrete --don,t forget your dpc--you could fit it below the blocks

 you should be able to use a tamping bar /length of wood ,what ever ,across complete width and 2 people  can do it no problem 

Edited February 20, 2019 by scottishjohn

[PeterW]

Build your first row of blocks to the level required for the floor. Bridge the opening with a piece of 6x2 braced to 2” square pegs. 

 

Lay the DPM over the blinded hardcore, add the rebar if necessary and then pour the floor and tamp it flat.

 

Give it an hour then bull float it flat, leave it to go off. 

 

To remove the need for rebar or mesh I tend to use fibres and set the floor at 150mm depth. On your floor that would make the difference between 2.2-3 CuM or so of a C25 mix which will be £75-80. That more than offsets the cost of using RC25 and getting A142 mesh. 

[Jeremy Harris]

12 minutes ago, PeterW said:

To remove the need for rebar or mesh I tend to use fibres and set the floor at 150mm depth. On your floor that would make the difference between 2.2-3 CuM or so of a C25 mix which will be £75-80. That more than offsets the cost of using RC25 and getting A142 mesh. 

 

That's what I did.  Just as strong, perhaps slightly more resistant to cracking, but most of all quicker to lay, and no faffing around with steel fabric.

 

Our guys used a smooth mould for the front lip, where the door goes, which was a part of the steel shuttering system they used, so there's a ~100mm section with a very smooth finish and a rounded edge at the doorway.  The rest of the slab was tamped, levelled and power floated, like the house slab, as I wanted a smooth floor that would be relatively easy to paint later (still haven't got around to it, though).

[epsilonGreedy]

35 minutes ago, PeterW said:

To remove the need for rebar or mesh I tend to use fibres and set the floor at 150mm depth. On your floor that would make the difference between 2.2-3 CuM or so of a C25 mix which will be £75-80. That more than offsets the cost of using RC25 and getting A142 mesh.

 

 

That will also save me from starting a thread about galvanized rebar as recommended on YouTube videos produced in the United States.

[epsilonGreedy]

18 minutes ago, JSHarris said:

That's what I did.  Just as strong, perhaps slightly more resistant to cracking, but most of all quicker to lay, and no faffing around with steel fabric.

 

 

Re. resistant to cracking would this include coping with a few mm of ground heave?

 

My subsoil is mainly silt with an element of clay and I have a willow tree 10 meters away.

 

[epsilonGreedy]

49 minutes ago, PeterW said:

Lay the DPM over the blinded hardcore

 

 

Do you use sand-only blinding or a weak mix concrete?

 

https://www.designingbuildings.co.uk/wiki/Blinding

[Jeremy Harris]

17 minutes ago, epsilonGreedy said:

 

Re. resistant to cracking would this include coping with a few mm of ground heave?

 

My subsoil is mainly silt with an element of clay and I have a willow tree 10 meters away.

 

 

 

Probably not.  A few years ago I had a workshop slab that was a few metres from a large willow tree, also on clay.  That used to move up and down by around 5 to 10mm in one corner, taking the whole workshop with it.  There weren't any cracks, but the movement was a real nuisance and something I only discovered when I found that a wing jig I'd built was going out of true.  Heave will happen unless there is some provision to allow the soil to expand and contract without moving any structure that's laying on it. 

 

[PeterW]

Sand is fine as it’s only to stop stones coming through the DPM. 

 

I’ve used 25mm EPS before too and it is just as good, if not better, as it takes out the undulations and is quick and cheap. 

 

10m from a willow is close, how big is it though ...??

[Mr Punter]

52 minutes ago, PeterW said:

10m from a willow is close, how big is it though ...??

 

As far as foundations and NHBC / LABC go, for retained or proposed trees it makes no difference on the size of the tree, just species, proximity and soil type.

[epsilonGreedy]

3 hours ago, PeterW said:

10m from a willow is close, how big is it though ...??

 

 

Just been out with my laser to double check, said willow is actually 11.5 meters away from the nearest corner of the garage.

 

If I packed the outer perimeter of the slab with the same stuff used to insulate the edging of an internal screed floor would that help the slab slip against the footing blocks should the oversite soil heave?

 

Size wise it is difficult to say because it has been pollard'ed in its life.

[PeterW]

Ok so it can’t come out then ..? Just wondered as if it’s less than 300mm circumference I would have it out. 

 

You could use clayboard but it depends on the soil - is it really a clay soil ..? 

 

What founds has the house got or is it further away from the tree than the garage..?? 

 

If its single storey simple garage then I would be tempted by a reinforced ring beam slab for the garage unless there is little sign of movement 

[epsilonGreedy]

15 hours ago, PeterW said:

Ok so it can’t come out then ..?

 

 

I wish, it is 2m beyond the plot boundary in my neighbour's garden.

 

15 hours ago, PeterW said:

You could use clayboard but it depends on the soil - is it really a clay soil ..? 

 

 

According to the building inspector who spent a long time examining subsoil samples on foundation dig day it is principally silt with a few veins for silt. He then said 450mm non rebar concrete would suffice, actually I think he said 225mm would pass. Anyhow enough concrete for a 600mm x 600mm semi trench fill had been ordered so that is what I have in the garage and house foundations.

 

15 hours ago, PeterW said:

What founds has the house got or is it further away from the tree than the garage..?? 

 

 

Yes further away from the willow.

 

15 hours ago, PeterW said:

If its single story simple garage then I would be tempted by a reinforced ring beam slab for the garage unless there is little sign of movement

 

 

The 600mm x 600mm garge foundations were poured 7 months ago during that dry spell and while cleaning out the footing trenches over the winter I have been looking out for any fissure on the concrete. So the good news is the foundations have coped with a full, very dry to winter wet subsoil swell, without trouble arising.

 

Edited February 21, 2019 by epsilonGreedy

[Mr Punter]

Have a look at https://www.labcwarranty.co.uk/resources/foundation-depth-calculator/

 

These are, I think, quite risk averse, but that is no surprise.

[epsilonGreedy]

15 hours ago, Mr Punter said:

Have a look at https://www.labcwarranty.co.uk/resources/foundation-depth-calculator/

 

 

I had a look but have to question the validity of a calculator that estimates soil characteristics across a whole 2-character post code. I am at the edge of PE (Peterborough) which is one of the largest postcodes in England and covers a wide range of soil types. Lincoln is closer and in that case the calculator recommenced more shallow foundations at 0.3m deeper than my foundations.

 

You cannot beat the intuition of my private, 70 something building inspector, who for 10 years was the UK's representative at the annual European congress of building inspectors.

[Mr Punter]

Did the tree make much difference?