Laying paving slabs to slope down about 3" over about 20'

[DIY]

The edge of the patio is about 3 inches higher than the garden. Instead of having a step down from the patio to the paved path I want to lay a path of

2' by 3' paving slabs so that the slabs slope down slightly from the patio for about 20' with a fall of about 3 inches. How can I make sure that the fall is correct as I lay the slabs on wet mortar mix, assuming I start at the patio end and work down the garden?

I don't think that metal or wooden pegs in the ground with ropes as guides on either side of the proposed path will work because the ropes will sag over that length of run. How about wooden pegs in the ground with lengths of wood attached as guides on either side of the path? Or would a laser be a better proposition?

[The Natural Philosopher]

Not that noticeably.

Ive run strings for 60 feet or more. If you tension up hard they work OK., also what you do, is get a mate and sight down the string while he knocks in pegs under your instruction till they line up with the end points.

Lasers are pretty crap outside - you need a very bright one.

How about wooden pegs in the ground with lengths of

[Matty F]

If you have pieces of reflective tape as a target, you can see the laser spot even in sunlight.

[Harry Bloomfield]

The Natural Philosopher used his keyboard to write :

Or you could do what I did, do the setting out with a laser as the sun goes down.