How to Lay Paving Slabs on Concrete (Guide)

When it comes to laying paving slabs, one of the key decisions you'll need to make is whether to lay them on concrete or an MOT Type 1 sub base. Sometimes the decision is already made for you if you already have a concrete base in place, but if your're working with a blank canvas, what should you use - MOT Type 1 sub-base or concrete?

Both options have their pros and cons, and the choice you make will depend on a number of factors, including the size and weight of the slabs, the intended use of the area, and your budget.

Pros and Cons of Laying Paving Slabs on a Concrete Base

One of the biggest advantages of laying paving slabs on concrete is that it provides a stable, level surface for the slabs to sit on. This can be particularly important if you're laying large, heavy slabs or if the area will be used for heavy foot traffic or vehicles.

Another advantage of laying paving slabs on concrete is that it can be a relatively quick and easy process. Once the concrete has been poured and allowed to set, you can lay the slabs directly on top, without the need for any additional preparation or materials.

However, there are also some disadvantages to laying paving slabs on concrete. One of the main drawbacks is that it can be more expensive than laying on an MOT Type 1 sub base, as you'll need to purchase and pour concrete.

Another potential issue is that concrete can be prone to cracking over time, particularly if it's exposed to extreme temperature changes or heavy loads. This can lead to uneven slabs and a less attractive overall finish. It's also not permeable which means it can be prone to puddling in wet weather and so it's often essential to install additional drainage channels to lead any surface water away.

Pros and Cons of Laying Paving Slabs on an MOT Type 1 Sub Base

MOT Type 1 sub base is a type of crushed stone aggregate that is commonly used as a base layer for paving projects. One of the main advantages of using an MOT Type 1 sub base is that it provides a stable, compacted surface for the slabs to sit on, without the need for concrete.

Another advantage of using an MOT Type 1 sub base is that it can be more cost-effective than laying on concrete, particularly for larger areas. This is because the materials required are generally less expensive than concrete, and the preparation process is less labour-intensive. Another benefit of MOT Type 1 is that it is permeable which when combined with a 6 to 1 mortar bed for your paving, allows a certain amount of water to permeate through. Do not confuse this however, with a fully SUDS compliant permeable paving solution.

However, there are also some potential drawbacks to laying paving slabs on an MOT Type 1 sub base. One of the main issues is that it can be a more time-consuming process, as you'll need to excavate the area and compact the sub base properly before you can lay the slabs.

Another potential issue is that if the sub base isn't compacted properly, it can lead to uneven slabs and a less stable overall surface. Additionally, if the area is exposed to heavy loads or extreme temperature changes, the sub base can shift over time, which can also lead to uneven slabs.

Creating a Concrete Base for Paving Slabs for Patio Use

If you've decided to lay your paving slabs on a concrete base, you'll need the following tools:

For creating the concrete base:

Shovel Wheelbarrow (optional, if you're mixing concrete by hand) Concrete mixer (optional, for larger projects) Cement Ballast Aggregate (e.g. crushed stone or rubble) Water Formwork (e.g. timber planks, nails or screws, saw) Spirit level Square Float

Project Overview

The aim of this project is to create a concrete base suitable for patio use at a thickness of 100mm. The final height of the base plus the paving slabs needs to be two brick heights lower than the damp proof course.

• When laying porcelain paving, this can be laid straight onto the 100mm concrete using outdoor tile adhesive and therefore the final depth will be 100mm + porcelain thickness (usually 20mm), so 120mm. 
• With natural stone or concrete paving, this should be laid on a 30-50mm mortar bed on top of the concrete bed, in which case the final depth will be 100mm + mortar thickness + slab thickness.

Steps Required

The first step is to prepare the area. Follow the steps below for creating a concrete base:

1. Start by removing any existing surface material, such as grass or gravel, and excavating the area to a depth of around 200mm, but calculate your total depths based upon the thickness of your paving stones as above. Remember, the finished height of your patio should be two housebricks (approx. 150mm) below your damp proof course. 

2. Once you've excavated the area, you'll need to lay a layer of hardcore (such as crushed stone or rubble) to a compacted depth of around 100mm. This will provide a stable base for the concrete to sit on.

3. Next, you'll need to create a wooden formwork around the edges of the area. This will contain the concrete and help to ensure that it dries evenly.

4. To create the formwork, cut pieces of timber to the desired length and hammer them into the ground around the perimeter of the area. Make sure that the top of the formwork is level and that the corners are square. You can use a spirit level and a square to check this.

5. Once the formwork is in place, you're ready to mix and pour the concrete. You can either mix the concrete by hand using a shovel and mixing tray, or you can use a concrete mixer for larger projects.

6. To mix the concrete, you'll need to combine cement, ballast and water in the correct proportions. The exact ratios can be dependent on the type of cement you're using, but as a guide, it's usually one part cement to six parts ballast and then gradually add the water to the mix. The correct consistency means it is a thick pourable mixture - not too dry and crumbly nor too wet and runny.

7. As soon as the concrete is mixed, pour it into the formwork, making sure that it's evenly distributed. Use a float to level the surface and remove any air pockets. The concrete thickness should be around 100mm. 

8. Once the concrete has been poured, you'll need to wait for it to set. This can take anywhere from 24 hours to several days, depending on the temperature and humidity.

9. As long as the concrete is set, you can remove the formwork and start laying your paving slabs.

Laying the Paving Slabs

In this section, we'll cover two processes for laying paving slabs on a previously formed concrete base - one for outdoor tile adhesive and porcelain paving, and the other for natural stone and concrete paving on a full mortar bed.

Process 1: Laying Porcelain Paving with Outdoor Tile Adhesive

Porcelain paving is a popular choice for outdoor spaces as it is durable, easy to clean, and requires minimal maintenance. To lay porcelain paving on a preformed concrete base, you'll need to use outdoor tile adhesive.

Step 1: Prepare the Concrete Base Ensure that the preformed concrete base is clean, dry, and free from any debris. Check that the base is level using a spirit level and adjust if necessary.

Step 2: Apply the Tile Adhesive Using a notched trowel, apply a layer of outdoor tile adhesive to the preformed concrete base. Spread the adhesive evenly, making sure to leave enough space for the joints between each paving slab.

Step 3: Lay the Porcelain Paving Slabs Carefully lay the porcelain paving slabs onto the adhesive, pressing down firmly to ensure a good bond. Check that each slab is level using a spirit level and adjust if necessary. Use spacers to create an even gap between each slab.

Step 4: Allow to Set Leave the paving slabs to set for at least 24 hours before walking on them. Avoid any heavy loads or traffic for at least 48 hours to allow the adhesive to fully cure.

Step 5: Fill the Joints Once the adhesive has fully cured, remove the spacers and fill the joints between the paving slabs with a suitable outdoor grout or jointing compound. Follow the manufacturer's instructions for mixing and applying the grout or jointing compound.

Process 2: Laying Natural Stone and Concrete Paving on a Full Mortar Bed

Natural stone and concrete paving can be laid on a preformed concrete base using a full mortar bed. This method involves using a layer of mortar to create a stable base for the paving slabs.

Step 1: Prepare the Concrete Base Ensure that the preformed concrete base is clean, dry, and free from any debris. Check that the base is level using a spirit level and adjust if necessary.

Step 2: Mix the Mortar Mix the mortar in a wheelbarrow or mixer, following the manufacturer's instructions. The mortar should have a consistency similar to that of toothpaste.

Step 3: Apply the Mortar Using a trowel, spread a layer of mortar onto the preformed concrete base. The layer should be at least 25mm thick.

Step 4: Lay the Paving Slabs Carefully lay the paving slabs onto the mortar bed, pressing down firmly to ensure a good bond. Check that each slab is level using a spirit level and adjust if necessary. Use spacers to create an even gap between each slab. Mix all packs of natural stone to be sure of an even mix of shades.

Step 5: Allow to Set Leave the paving slabs to set for at least 24 hours before walking on them. Avoid any heavy loads or traffic for at least 48 hours to allow the mortar to fully cure.

Step 6: Fill the Joints Once the mortar has fully cured, remove the spacers and fill the joints between the paving slabs with a suitable outdoor grout or jointing compound. Follow the manufacturer's instructions for mixing and applying the grout or jointing compound.

Conclusion

In conclusion, laying paving slabs on a concrete base is a popular choice for outdoor spaces such as pathways and patios. It provides a stable and durable foundation for the paving slabs, ensuring that they remain level and secure for years to come. The process involves creating a concrete base by mixing cement, ballast and water, and then laying the paving slabs onto the base using either outdoor tile adhesive or a full mortar bed.

While there are some disadvantages to laying paving slabs on a concrete base, such as the potential for cracking and the need for additional drainage, the benefits often outweigh the drawbacks. Concrete is a cost-effective and long-lasting material, making it a practical choice for many outdoor projects. Plus, with proper installation and maintenance, you can ensure that your paving slabs remain beautiful and functional for years to come. By following the steps outlined in this article and using the right tools and materials, you can create a stunning outdoor space that you can enjoy for many seasons.