How level/flat should my sub base be?

[Matt706]

Hi, I have layed my sub base (Mot type 1) to around 100-120mm thickness for my patio. I have wackered it down a fair few times but now I'm wondering how flat it has to be before laying slabs.

I have gone over and filled in divets and areas where it seemed low however it's not perfectly flat.

Having not layed paving before how accurate do I have to be at this stage? Also and tips to help level at this stage?

If I could upload images it would be better for you to see.

Some additional information:
-Paving slabs are a 3 size mix at 40mm thickness and are a compressed concrete/aggregate type.
-they will be layed on a full mortar bed.
-there is a slight fall towards the house however I have installed a linear drain.
-area to cover it around 40sqm in a L shape

[rab1]

I'm not a pro but you should be fine. Just run string lines from the lowest to highest point of your falls and follow these for heights etc.


A Pro will be along soon.

[Matt706]

Rab1 this was my thinking. String line at either end and then get the sub following this line within 10-15mm.

Then do the same again when laying the slabs but folwing the line exactly with the mortar taking up the inconsistences in height.

Also I'm going to be leading with a spirit level across the width/back of house and measuring with line the fall away from the house. Is this correct.

[jwill]

Have you incorporated the fall with your sub base? I normally aim for around 10mm tolerance but can probably be worse on tricky jobs. Might be easier to use a straight length of timber to see how flat it is

[seanandruby]

Try not to overthink it. As long as the difference is not to excessive you will be fine.

[Tony McC]

The usual standard for sub-base level accuracy is ±10mm

What you are aiming for is as uniform a bed thickness as possible. It's when we have significant variation in the bed depth that problems develop in years to come.