Patio quotes and sub-bases

[peteski2017]

Hi all

Over the course of the last few weeks I've had a couple of quotes for a patio (very small area). Interestingly both came back with quotes that when I asked them did not include laying any MOT1 and just bedding them on compacted sand with a wet mix above. In the forum posts I've read through on here and elsewhere it seems that there is no common consensus on what to do. In I think most cases MOT is recommended but it seems there is no definitive yes or no/use or don't bother. Both said given it's only for light foot traffic laying MOT is not really necessary. For everyone out there who does this work and has experience of having had a patio laid what are people's thoughts on this? My feeling is MOT should be used for greater stability and for longevity but is it overkill for a 3m by 3m area? Seriously confused on this one! Am I being judgmental in thinking its them using short cuts to get the job done quicker?

Cheers!

 

[Notch7]

Whats the patio going on?

A general rule of thumb is that top soil is unsuitable to lay a patio on, because its a a soft material with organic matter.

So best practice is to remove the top soil, usually about 125mm to 250mm thick.

Then on top of the sub soil, a sub base is added. MOT type is good.

A patio with a sub base of compacted sand on top soil will move a bit, how much depends on your soil type.

 

[lonner]

As above

If its a slab with 40mm thick slabs id expect at least a 150mm dig.
75mm base mot is perfect and a sand cement bed, or mortar.

If its block paving you nedd 200mm dig for 50mm paving, 100-120mm hardcore if you want it to last.

 

[Ian H]

They might as well put the wet mix on compacted MOT rather than compacted sand. It’s less likely to get washed out.

 

[doogyscoot]

The paving expert section on sub bases suggests that for light for traffic a sub base is not always required, depending on the depth of top soil of and type of sub soil. Where we are the sub soil is near solid packed red sandstone/clay soil. It compacts as firm as type 1 so if your sub soil is like that you might be alright. Mind you'll maybe have to take a fair bit of top soil off and then use type 1 to make up the level anyway...

 

[TeeDub]

I've had a patio poorly laid by some contractors which is just indian sandstone on sharp sand. The sub-grade is clay-based. It has ended up with the sand filling with water and it either washing away or causing some form of liquifaction effect where the slabs sink in strange areas. See https://www.diynot.com/diy/threads/help-needed-dodgy-gardeners.524875/ for potential effects.

If it's an area that will have some foot traffic, or storing something like heavy bins, water butts etc then a compacted MOT-1 subbase is recommended. If it's already compacted subbase and you're pretty sure there's no organic matter that may decompose or wash away then you may get away with it, especially if the slabs are concreted and joined in well and all the water goes off the surface and flows away, rather than penetrating into the subbase.

How long do you need it to be in place and will it be used regularly? Is it already well drained and what's the existing subgrade made of? It's fairly easy to ask them to quote with it. They'll probably do a sharp intake of breath, say it's not necessary and massively increase the cost because they don't really want to do it, because that's more effort. Get a quote from someone who will include it from the beginning to compare reasonably.
They'll have to dig down 6 inches (possibly with a machine?), lay down 4 inches of MOT-1 hardcore and compact with a tool (which they may need to rent), lay the concrete bedding and then slabs. That's way more effort than sprinkle some sand, stand on it and then put some concrete and slabs down!

I have, on my front garden, cut down a couple inches into the soil/lawn and put gravel, compressed with the top of a mallet, then sand (and compressed) and then a heavy duty slab on top. This has lasted several years so far, but the area isn't trafficked and isn't liable to getting wet or frosty either.

Have they also considered other things like drainage (if they need to) and gradients of the patio?