Enhance your property by laying a new patio – part #4

Slate Tiles

With most of the below ground works finished it’s now time to start building the ground back up.
A green led laser level (more easily seen in daylight) was used to mark the highest point of the patio Then allowing for other factors like depth of service pipes below ground, slope of the lawn and distance of the slabs below the damp proof course (DPC) an amount of overall fall was calculated and then marked at the opposite end of the patio. In this case a fall of 40mm over 6 metres was achieved which is a little under the desirable gradient of 1 in 80.

Stakes were banged into the ground and string run between them to mark the top of the slabs from front to back. Allowing for the thickness of the slab, 40mm for the cement base and 80mm for the MoT type 1 subbase another string was run at the subbase level to show where the type 1 needed to be filled up to or ground shaved off to allow for the 80mm of type 1.

The ground was prepared with pickaxe and shovel to make it the right level for the Mot type 1. Excess spoil removed in the process was sieved through an automatic rotary soil sieve to create a pile of topsoil and another pile of stones which would be used as the first layer below the type 1 subbase. Any difficult areas of the old concrete patio base were broken up with a hex shank electric breaker using a wide flat chisel.

Once the ground was prepared the stones extracted from sieving the spoil were spread over the proposed patio area as far as they would go and then compacted using a petrol compactor plate (wacker plate). Then the rest of the required thickness was made up by the MoT type 1 and compacted using the plate.
The next step was to set the channel drain. This was installed 3mm below where the top of the patio slab would finish and laid in a bed of 4 to 1 sand and cement. End caps were put on at the highest ends using PVC pipe weld cement and the lengths were cut to clip into the 4 way corner unit which joins into the top of the soakaway pipe via the P trap through a double socket coupler joined to a piece of pipe. A gap of 10mm was left between the channel drain and the wall to allow gun injected patio mortar to be applied once the patio had been laid.

Now it was time to lay the first patio slab. The first slab required a cut to make it fit round the base of the concrete fence post. The cut was made with the combination of an angle grinder and an electric tile cutter using wet cut diamond blades. The electric tile cutter has a tray which you fill with water that the blade runs through to aid dust suppression. A facemask, face guard, protective goggles and ear defenders were used to protect the operators. Water was dribbled on to the slab while the slab was being cut to help the dust suppression process even more.

Type 1
Type 1 laid and pounded into flat surface.

Enhance your property by laying a new patio – part #2

subbase collapse under patio

This is part 2 of the patio replacement project which we are starting for the summer. We have pulled up the old patio slabs revealing the thin sand base layer that the original house builders put down.

A pickaxe is useful for levering up the old slabs and breaking up the ground while an electric breaker is useful for any concreted areas.

Be careful when pulling up the slabs and using hand tools like a pickaxe or shovel or power tools such as a breaker to remove earth that you do not damage any underground services like water supply or waste pipes. It’s always a good idea to know where the services run and how close to the surface they are when digging out spoil.

Here you can see a drainpipe which is quite close to the surface where it exits the down pipe.

Be aware of existing service pipes
Be aware of existing service pipes

Round by the rear of the garage the slabs had sunken quite badly where they butted up to the garage wall. This was most prominent near the rain gutter down pipe which could highlight that there is a leak in the pipe below ground level which is washing out the substrate causing the slabs to sink.

To investigate this a large trench was dug in order to find out the state of the pipework.  A visual inspection revealed the pipework looked undamaged and pouring some water into the gutter showed that there were no leaks.

What digging down did reveal was that the garage had been built using a block and beam construction and the void under the floor had been left open underneath the outer most precast beam. Because the garage is single skin the wall had been built on top of the beam which meant there was no outer facing brickwork laid down to the concrete foundation footing level and therefore the void was open allowing the collapse of the patio substrate into the void.

Patio sub-base collapsed into block and beam void
Patio sub-base collapsed into block and beam void

In order to remedy this some pressure battens were glued to the brick and block piers and pressure treated gravel boards were screwed onto the battens underneath the outer most beam. This will stop the hardcore and type 1 that we put back into the trench from collapsing into the void underneath the block and beam garage floor.

Shuttering to block and beam void
Shuttering to block and beam void

Then the trench was back filled with pea shingle, hardcore and sharp sand.

Back fill of pea shingle, hardcore and sand put back in service trench
Back fill of pea shingle, hardcore and sand put back in service trench

Once the slabs had been pulled up and removed the next step was to workout how much spoil to remove in order to get the correct fall on the patio. This was done using a long spirit level and some stakes banged into the ground to mark the level of the top of the slab that would allow a fall of between 25mm per metre and 25mm per 2 metres. Then dig out enough of the ground to allow the required 100mm of type 1, 50mm of mortar and then the thickness of the slab.