Understanding Crossfalls and Their Role in Paving Design
Falls, or crossfalls, is the gradient that should be designed into a paving scheme, to allow rainwater to clear the area without leaving puddles likely to cause slip hazards during frosty weather. Some materials require greater crossfalls than others. Porcelain, for instance, should be laid with a minimum fall of 1:60, which means one centimetre per 50 centimetres of paving with the fall towards a drain, gulley or other area designated to receive water (shingle area/ rain garden or shrub/flower bed).
Due to the very smooth surface of porcelain, anti-slip texture and low water absorption, water is able to build up a film of surface tension, before it will start to run off. This is known as the longitudinal or endfall. A crossfall may also be known as a traverse fall in some regions.
Porous stones such as sawn granite, sandstone, limestone etc require a crossfall of 1:60 (which equates to 1 unit of fall per 60 units), even though it may appear that the surface will hold water to a greater extent than smooth porcelain, they are not liable to surface tension in the same way.
Sometimes, it is difficult to achieve these falls due to the site conditions and limited changes in levels, with fixed damp proof courses all at the same level within a scheme, and these may require a different logic involving gullies and inbuilt/underground rainwater channels (see Gullies and rainwater channels) to dispose of water.
Treads should have minimum 1 – 80 longitudinal fall from the rise to the edge of the tread, with the fall towards the front. (See Drip Grooves in Paved Treads.)
On footpaths, the minimum crossfall is 1:40, and is usually achieved with a crown or camber in the centre of the pathway i.e. slightly higher in the middle of the path, with lower edges either side to ensure water cannot lay within the path.
Certain projects that are constructed using fully permeable materials, sub-bases and bases, crossfalls may not be required, as rainwater will percolate through the paving, as part of a SuDS complaint scheme. (See SuDS Regulations.)
Recommended Falls Rates
Porcelain slabs 1:60
Sawn natural stone and precast concrete flags 1:60
Riven natural stone slabs and heavily textured slabs 1:50
Sawn and textured natural stone setts, precast concrete blocks and clay pavers 1:60
Natural stone setts having a cropped face 1:33
Steps and treads 1:40
Drainage channels 1:80
