‘It’s all about the base’

 

The good old sinking patio, I’ve fixed loads of them. There are several ways to overcome this problem, but first, let’s talk about why it happens. The main cause of a sinking patio is a lack of sub-base, so it’s best to get it right before it’s laid. When laying a patio, you must first dig the area out. This depth is usually around 250mm or 10 inches below your finished level. This allows for 7 inches of base and 3 inches for your mortar and paving. Any less than this and you may get sinking (subsidence). There are also cases when you put different depths to accommodate different situations. For example, if you find that you are laying a patio on a solid slab of rock or concrete, you need very little or no sub-base. On the other hand, if the ground is very soft or boggy, you will need to put much more depth of sub-base. Getting the correct thickness is a crucial component of a strong and non-sinking patio. There are a few names and types of sub-base that you can familiarise yourself with, which you can check for if you’re getting a quote. 

Sub-base Types (All prices are for loose quantities; bulk bags will be more expensive)

Average UK prices

 

Scalpings – £30 – £40 per tonne 

This is a byproduct of other quarrying and varies in quality. It has a maximum stone size of 20-40mm, ranging down to dust particles. It can sometimes have mud mixed in, which is not ideal. It can be used for domestic purposes like a patio, but it’s not good for things like roads. 

Recycled stone – £5 – £20 per tonne

This is a mixture of concrete, bricks, natural stone and slabs that have been put through a crushing machine. The quality of the end product depends on the type of crusher and the type of material. All sorts of rubbish could be mixed into it depending on the cleanliness of the original material. It is often used by farmers to make roads through fields, but occasionally, you can get your hands on it if you know the right place. I would only recommend using it as a base layer and then going over the top with something better. As you might find that it doesn’t compact very well.

Type 1 –  £30 – £45 per tonne 

This is a quality sub-base that is widely used by landscapers and builders across the country. It’s clean, consistent and compacts really well.

MOT – £30 – £45 per tonne 

This is just Type 1 that has been certified by the Ministry of Transport. This is used for public highways and is guaranteed to be ‘the good stuff’ 

All prices depend on how much you’re buying and how good a deal you’re getting. Prices always vary depending on location in the UK. Buying in bulk bags is always more expensive.

If you’re planning on laying a patio, make sure that you follow these simple procedures.

1. Dig the area out down to 250mm below your proposed finish level
2. Lay a membrane (this will stop the mud from mixing with the base)
3. Spread the sub-base evenly over the area, making sure you allow enough height for your slabs and mortar.
4. Compact the area using a vibrating plate. This will lower the level of the sub-base by an inch or two, so make allowances for that when getting your levels right.

If you follow these instructions, you will have a solid base to lay a patio on, and you will get no sinking at all.

What if the patio is already laid and it’s sinking in one area?

If the budget is tight, I would suggest taking the following steps to repair the sunken area, rather than replacing the whole thing.

1. Lift the sunken slabs
2. Remove the cement from the slab (if there is any attached)
3. Remove the existing base material from underneath the slab (150mm/6 inches deep)
4. Add 100mm/4 inches of concrete or post mix and water (ready mixed concrete)
5. Mix up some cement powder and water so that the consistency is like thick paint
6. Using a large paintbrush, cover the back of the slab with the mixture 
7. Make some mortar and spread it into the area 
8. Lay the slab onto the cement and tap it down until it is the same level as the existing slabs surrounding it.
9. If there is any pointing missing, you can use the remaining mortar to fill these joints
10. Once the mortar has started to set (the setting time depends on the weather; Hot weather dries it quickly, and cold weather slows it down). Scrape off any excess mortar and brush the joint with a soft brush until it looks smooth.

If the budget does allow it, replace the whole patio. Usually, if there’s a sinking area in one spot, then the rest of the base is probably terrible as well. 

My advice is that a sinking patio can be fixed, but if you’re not handy with tools. Get a professional in! 

 

By Rick Labbẽ – GardenWise founder

Design  –  Insight  –  Guidance