Do patio slabs really need to be placed on mortar?

[BlakesHeaven]

I've read lots of posts and articles and understand the normal approach is to wack and compact and then put the slabs on mortar, however I wonder how this can make them stay in place any more than laying them straight on the compacted MOT1 & ballast. I was thinking that the main benefit of using mortar would be if the foundation is a bit rough and not so flat and you then use the mortar to ensure a uniformly flat slab surface. I don't see that mortar will stick to ballast and hold the slab in place any more than the weight of the slab will hold itself in place. Read on and I explain it will be jointed and retained.

Below is my prepared area which runs at a 1:80 slope, left to right and on the right 1:80 away from the house towards the drainage corner.

I went to a lot of trouble to ensure it was perfectly flat, compact and with the correct slope. Also, because of the slope, I figure that a spirit level isn't much use as none of the slabs will be horizontal. If I start putting mortar down, I'd have to fiddle around ensuring each slab continues to be 1:80 down the slope.

The front and side retaining walls will be smartened up with a single-skin brick fascia that will be taken up above the patio height with the part above the patio being doubled up. This means the slabs will be encased by walls and once all the joining compound has gone off the slabs on the slabs will be locked in place.

So right now I'm thinking that the use of mortar will be an added complication and is why I'm thinking of just placing the slabs straight on the ballast.

Hence my question; do patio slabs really need to be placed on mortar if the foundation is perfectly flat, well compacted, jointed and retainded by a wall?

All opinions are welcome, especially those that also say why?

Many thanks in advance.

 

[Munroast]

NO

dry mix a bit of cement into the sharp sand that you tap your slabs into to stop it being washed away by rain or dug away by ants. But the main thing is to have a good constancy equal density base for them to sit on. Don't have any organic material like top-soil that will change over time and allow the slabs to sink

your biggest worry there is, the outside will never move by a mm, but the inside could


edit - if that answer is illogical, its cause i'm completely pished

 

[jj4091]

I would have thought it is as much to do with assisting levelling as fixing.

 

[robinbanks]

I figure that a spirit level isn't much use

If you tape a packer to the level at the right distance (e.g a 1 mm packer, 80mm way from the end of the level) you create a level that measures a 1:80 slope, if you place it so the packer end is on the downhill part

Not entirely sure I'd bother as rainwater will typically escape down the grout gaps and if you're using riven paving it'll sit on the slabs anyway.. but if you're looking for most of a flash downpour to quickly escape down the drain I get it..

The only fault I'd raise with your logic is the development of hard and soft spots from rain and walking on the patio over time. Worms also seem to love sitting under things placed on whackered MOT. The mortar isn't aiming to stick flags down to whacked MOT, it's aiming to fill in surface undulations and voids in the MOT to help reduce formation of points that could make the flag rock/wobble

If you think you've got the world's best, level substrate you have little to lose other than time by trying your theory out; they will be quick to lay and quick to take up, clean and bed on mortar at any point in the future if you're unhappy with the result. You might even get half way through the job of laying and end up with one that just will not stop rocking, and change plans..

Everyone does it one way, but doing it another may work for you, and maybe even everyone if you can get the idea out there. That's how innovation occurs!

 

[freddiemercurystwin]

You never see the council doing these vast repaving jobs putting concrete slabs down on mortar.

 

[StephenStephen]

Slightly off topic but hopefully helpful:

Measuring a slope with a spirit level....
TIP: An old builders trick to getting the right level is to place the base of the spirit level in with the right level, then you can draw on to the top of the vial with a felt pen where the bubble comes to rest. Now as you work across the surface you can ensure that the level comes to the same point, so the bubble comes to rest at your felt pen mark. Remember to wipe the marks off when you are finished.

How to Use a Level Including the Types of Levels

The spirit level is a useful tool and is used everywhere so learn how to use one and also about the types of level that are available

www.diydoctor.org.uk

 

[Cheslea2010]

Mortar creates suction between the mortar and the slab ensuring slabs stay in place it also ensures the finished surface adheres to sub base. A sand base will shift over time due to ground water and ants which will result in an uneven patio down the line. You will also have trouble with weeds taking root in the sand which will exacerbate the issue further.

 

[robinbanks]

and the slab

Argument here is that it's not a slab, it's whackered MOT