Tools required :- Spade / shovel, spot board, trowel, 1000 / 600mm spirit level, tape measure, wood saw, rubber mallet and plate vibrator (whacker plate).
There are two common ways DIYers lay flags, the first is the spot method where five dollops of mortar are put on the ground and the flag is positioned level on top. This method, which is used a great deal usually leads to problems with the paving / patios after a few months. The flags are not fully supported and may be prone to breaks if any load is placed on them, also water underneath the flags can cause them to start to rock and become uneven.
Safety tip:- Laying flags / paving and screeding is very heavy work, you should take great care when lifting any heavy weight and if possible have assistance when doing so. Wear appropriate footwear and gloves to suit the task.
The screed method gives the flags a sturdy bed on which to lay them and I think it produces a much better finished job. 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 will require a ‘fall’ to allow rainwater to drain away without leaving puddles, once you have the ground levelled off correctly run a plate vibrator over it several times.
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, grit sand is ideal to use and can be purchased for delivery in one ton bags from the 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 grit sand at a 50mm (2 inch) depth it will cover an area of approximately 10 to 12 sq mtrs. 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. 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 grit sand together and stops any movement.
So now your 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 is in position knock it down level with a 24oz rubber mallet, (screwfix sell them for around £5) the loose screed should allow the flag / paving stone to sink around 5 to 8mm, (1/4 to 3/16 inch) continue with each flag / 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 flag / paving stone. Check your flag / paving stone level as you go, adding or removing a little screed mix with a trowel as required. When all your flags / 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.