Looking for advice on laying 900x600x50 slabs

[Piper22]

I am lookin for some help regarding laying 900x600 slabs. I am using marshalls slabs and every time I get the same issue

I lay a row of 6 for example, all level both ways (actually have a slight run one way but for making the question simpler say level both ways)
Then when i come to lay the second, third fourth row the level will start to read further and further off even though each slab is flush to the next.

I dont understand what I am doing wrong, I would assume if I lay a slab level and then lay one next to it with the edges flush (no lips) then the level should read the same on both slabs.

I recently did a patio of 8x10 slabs, first row level and by the time i got to row 10 matching each slab to the next I was over 50mm off level over 3 slabs width

 

[noseall]

Use string lines and long straight edges like I did on this BIG patio..

Slabs2

• noseall
• 15 Oct 2015

Slabs

• noseall
• 15 Oct 2015

Porchpatio2

• noseall
• 15 Sep 2015

Patio14

• noseall
• 17 Oct 2015

Patio13

• noseall
• 17 Oct 2015

Patio4

• noseall
• 17 Oct 2015
• 2

Patio5

• noseall
• 17 Oct 2015

Patio6

• noseall
• 17 Oct 2015

 

[^woody^]

Lay a straight edge across the surface.

Prepare the base so that the surface is even with the fall you need, and the slabs can just drop down with little tapping.

 

[Ian H]

Are you leaving a gap to point up after?

 

[Blagard]

I dont understand what I am doing wrong, I would assume if I lay a slab level and then lay one next to it with the edges flush (no lips) then the level should read the same on both slabs.

Maybe you are using a short level on its own. If so either buy a longer level or use a long straight piece of timber with the level on top of that. The longer the timber the better

 

[noseall]

Your slabs should be descending to a fixed point. This point can be a mocked up bedded slab. You use your long straight edge to set the fall of your first slab, to aim for this point. You can then use a level to come off this slab (sideways to the fall) and set the first row with same amount of tilt or use a line instead.

A level is more useful when laying a random pattern. A line is better when laying same size slabs.

This one was mostly done with a level and straight edges.

Pat3

• noseall
• 26 May 2017

Pat2

• noseall
• 26 May 2017

Pat1

• noseall
• 26 May 2017

Wave3

• noseall
• 22 May 2017

Wave2

• noseall
• 22 May 2017

Wave1

• noseall
• 22 May 2017

 

[stuart45]

I like the idea of finishing the slabs inside the brickwork instead of on top. Used it myself on the last patio and the HO was really pleased with it.

 

[noseall]

I like the idea of finishing the slabs inside the brickwork instead of on top. Used it myself on the last patio and the HO was really pleased with it.

 

[Piper22]

Are you leaving a gap to point up after?

no not leaving a gap. The only thing i can think of is the slabs are not all the same and not perfectly flat on top. Last patio i did my first row had a slight run one way and by the time i got 8 rows down they were running the other way even though all the edges were flush with the next slab. Surely if a slab is level and you lay one next to it with edges flush it should be at the same level but it doesnt seem to work that way

 

[Blagard]

Surely if a slab is level and you lay one next to it with edges flush it should be at the same level but it doesnt seem to work that way

That is like saying if you lay one brick on wall level and the next one to it the wall will be level - It won't and is why bricklayers work to a string line. Just a small deviation between units be it bricks or slabs means that you can accumulate a large error by the time you get to the other end. By far the simplest solution is to do what noseall said. Set temporary a slab (or more) a good way off at the finished level and use a long straight edge to line your slabs into it. That way you should not get any errors at all. That temporary slab is simply lifted as you get to it and relaid next the the others but still lining through the level across multiple slabs with a straight edge.

The job is made really easy by having the sub-base correctlly levelled though to start with as that means what ever you use to set the slabs in place will be the same thickness across the whole area.

 

[cjard]

Surely if a slab is level and you lay one next to it with edges flush it should be at the same level but it doesnt seem to work that way

Indeed, masonry units are seldom perfectly regular and you can't guarantee, in a row of ten, if you level #2 to #1 and #3 to #2 and so on, that by the time you get to #10 that #10 will be level with #1

To see it in action, take two things of equal height; wood offcuts, bricks, pool balls, whatever. Place them in the centre of adjacent slabs and put the level between them, now do two slabs apart, now 3..

Consider also testing your level: place it on top of another level(that is upside down) and check the bubbles read the same. Rotate one level 180 degrees horizontally (so that was was the left end is now the right but the top is still the top) and check it against the other level again. If the bubbles ever read differently, one or both levels may be out

 

[^woody^]

Pressed concrete slabs are all the same size with no appreciable size difference. If you level one and line one line through then all the others should be level and in line too.

The errors are with the slab layer.

 

[noseall]

The errors are with the slab layer.

I would never say that.....

 

[cjard]

Pressed concrete slabs are all the same size with no appreciable size difference. If you level one and line one line through then all the others should be level and in line too.

Note of course that woody is saying here that laying the start and end slabs and putting a line between them and laying to the line will produce a level result - it's true. What seldom produces a whole job level result in laying any kind of masonry unit, pressed, moulded, baked, cut, cast or otherwise, is the practice of levelling the one you're laying now, against the last one you laid and expecting the tenth/20th etc to line up with the first..