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There are two common ways DIYers lay flags.
The first is the spot method. The way to do this is to place five dollops of mortar on the ground (one in each corner and one in the middle) and position the paving slab on top of the mortar. Once the flag’s in place, use a spirit level and rubber mallet to get it level.
This popular way to lay flags can lead to problems over time because the mortar often doesn’t provide complete support. When you rest the flag on the mortar, the mortar spreads across the base of the flag but you can’t guarantee complete coverage. So small voids underneath the slab become weak spots when placing weight on top of them or during winter.
During wet and cold spells in winter, rainwater can gather in these small holes and freeze. When water freezes, it expands by 9% – 10%. As it expands, it’ll force the paving slab to move. Which, over time, causes paving slabs to become uneven and start to rock.
For these reasons, you should avoid the spot method for laying paving slabs.
The second way to lay flags is known as the screed method.
This way gives your flagstones 100% coverage ensuring your paved area is stronger and will last longer.
Instead of placing five dollops of mortar on the ground, you create an even bed on which to lay the paving slab. You can do this one at a time if you like, but it’s easier and more efficient to screed a larger area at a time.
The first thing you need to do is prepare the ground ready for the screed. You will need to ascertain if the flags or paving require a ‘fall’ to allow rainwater to drain away without leaving puddles. Once you have the ground levelled off correctly run a wacker plate over it several times.
How to calculate the fall on a typical patio or driveway
A typical patio or driveway should have at least a 1:60 fall. This means, for every 60 units, the area needs a 1 unit fall. So an area of 60 inches needs a fall of 1 inch, while an area of 60 metres needs a fall of 1 metre.
Now you need to set up two screed level boards.
I would normally put down a screed of approx 50mm (2 inch) so use timbers of 100mm x 50mm, (4 x 2 inch) set these level with each other using a piece of timber to span between them and a spirit level (1mtr level is ideal).
Cut another piece of 100mm x 50mm (4 x 2 inch) timber long enough to span the screed boards, notch this out so its edge is around 45mm (1 3/4 inch) below the top edge of the screed boards as shown in the drawing below.
This timber will be used to level off the screed mix.
The screed is made up of sand and cement. DO NOT USE BUILDING SAND as it will wash away and weaken the screed mix. Sharp sand (aka grit sand) is ideal to use and can be purchased for delivery in one-ton bags from larger DIY stores such as B & Q. This may sound like a large amount but with a dry mix ratio of 1 part cement to 6 parts sharp/grit sand at a 50mm (2 inches) depth it will cover an area of approximately 10 to 12 sq metres.
If you think the flags/paving may have to stand heavier use, such as a vehicle, then do a dry mix ratio of 1 part cement to 3 parts grit sand.
Sharp/grit sand does tend to be moist to the touch. This combined with moisture in the ground will set the cement which in turn binds the sand together and stops any movement.
So now you’re ready to screed and lay your flags/paving.
Fill between the screed boards with approximately 75mm (3inches) of screed mix, then slide your levelling piece of timber along the screed boards so you have an even layer.
Run over this layer with your plate vibrator once only.
Now top up the compacted screed with loose screed mix and slide the levelling timber over it again.
If your flags/paving stones are heavy, walk them (along the ground corner to corner) to the work area.
Make sure you have the flag/paving stone the right way round then place it down on the screed. Do this from the crouched position rather than the standing position to reduce any risk of back injury.
Once it’s in position knock it down level with a 24oz rubber mallet. The loose screed should allow the flag/paving stone to sink around 5 to 8mm (1/4 to 3/16 inch). Continue with each paving stone leaving space between each one of approximately 6 to 8mm, (1/4 to 3/16 inch). Use pieces of hardboard or anything that has a similar thickness to create uniform space around each paving stone.
Check your flags are level as you go. Adding or removing a little screed mix with a trowel as required.
When all your paving stones are in position, spaces in between can be filled carefully using mortar mixed with a waterproofer. It is a time-consuming task but a little patience and cleaning up as you go along can give a very professional finish.
Flags/paving stones can be finished off with edging stones to stop any screed from escaping and they can also compliment your flags or paving.
- Spot board
- Spirit level (600mm/1000mm)
- Tape measure
- Wood saw
- Rubber mallet
- Plate vibrator/whacker plate
- Sharp sand and cement for the mortar (how to mix mortar)