Granite Step Drainage Slope: How Much Fall per Metre, and How to Write It on the Order
A granite step drainage slope is measured in millimetres of fall per metre, not in degrees: 1% is 10mm per metre, and sites usually work between 1% and 2%. That band is common practice, not a mandatory standard. Direction matters more than the figure, so the low point sits at the front edge and never toward the wall or the back of the step. Four lines go on the order, and a straightedge no longer than the tread checks them.
What drainage slope should an outdoor granite step have?
Set the granite step drainage slope as millimetres of fall per metre, not as an angle. A 1% slope drops 10mm over one metre. On site, outdoor steps usually sit between 1% and 2%, which is 10mm to 20mm per metre. That band is common practice, not a mandatory standard. The choice inside the band follows where the water leaves. If paving or a channel sits right below the bottom step, the 10mm end is enough. If water has to cross the tread to reach one end of the flight, take the 20mm end. Fall works together with direction, and the next section covers direction. The figure only binds anyone once it is on the shop drawing. If you only say “give it some fall”, the factory cuts square edges and the site pays for the rework.
| Fall | Drop per metre | Gap under a 300mm straightedge | Where it fits |
|---|---|---|---|
| 1% | 10mm | 3mm | Paving or a channel sits right below the bottom step |
| 1.5% | 15mm | 4.5mm | Water runs a short way along the tread to the outlet |
| 2% | 20mm | 6mm | Long runs, or a flight of several linked steps |
Which way should the fall run, and can it run toward the wall?
Put the low point at the front edge, the side people step down from. The fall must not run toward the building wall. It must not run toward the back of the step either. Neither place has an outlet. Water that arrives there sits under the slab or inside the joint, and then seeps into the bedding. A flight should form one continuous path. Every tread tilts outward, so water leaves the front edge and lands on the tread below. The same thing repeats step by step until the water reaches the paving or the channel at the foot of the flight. One tread with a reverse fall traps water on that step. That tread and the riser below it then stay damp. A landing needs its own fall direction, so water leaves at one side rather than at the joint between two steps.
Once the fall is built in, where do two details still fail?
At the front edge and at the joints. Without a drip groove the water does not break at the edge. It clings to the front face and runs down the riser, then reaches the back edge of the tread below. That spot is the joint, so the water goes down through it. The fix is a groove cut into the underside of the front edge, or a rebated front edge that makes the water break and drop clear. Joints are the second detail. A joint that is too tight holds water through surface tension and capillary action, and the water sits at the joint mouth. A joint that is too wide becomes a trough that collects water, sand and dirt. Work the width out the same way as paving joints. Fill the joint with a material that lets water through, and do not pack it solid with a rigid mortar.
Polished or flamed: does the finish change the drainage?
It changes whether you can see the water, not whether the water leaves. A polished face spreads water into a thin film. The reflection shifts, so the wet patch shows, and the surface is slippery underfoot. A flamed face is rough. Water spreads into a dull patch that looks dry, while the low spots still hold water. Roughness cannot replace fall. It only changes how the water spreads. A flamed step therefore needs the fall measured, not eyeballed. We keep Sesame Grey G602 in 30mm slabs, polished and flamed. Outdoor steps usually take the flamed face, with the fall built to the millimetre figure on the order.
How do the four lines on the purchase order read?
Four lines, one item per line, all written as numbers the shop can cut to. (1) Fall direction: write “falls away from the wall, low point at the front edge”. Do not write “mind the drainage”. (2) Fall per metre, and who builds it: write “15mm fall per metre; on a 300mm tread the front edge sits 4.5mm lower than the back, and the fall is taken up in the bedding so the slab stays 30mm”. The other option reads “wedge cut at the works, 25.5mm left at the front edge on a 300mm tread, drip groove to the node drawing”. If you choose the works-cut wedge, take the drip groove depth out of that 25.5mm first. The nose left behind is thin and chips in handling, so either go one thickness up or move the fall into the bedding. Pick one and mark it on the drawing. With bedding, mark where the fall starts and the minimum mortar thickness. With a works-cut wedge, mark the finished front-edge thickness and specify one block and one veining, because a wedge mixed across blocks leaves a visible line along the front edge. (3) Front edge detail: write “drip groove on the underside of the front edge”, and take the width and depth from the node drawing. For a rebated edge, write “front edge laps over the step below”. (4) Joint width and filler: give the width as a number and the filler by name, on the same line. Keep the four lines on one drawing, and check it once before it goes to the shop: an order written at 15mm per metre should read 4.5mm under a 300mm straightedge on site. When the two figures agree, the drawing is fixed. Sesame Grey G602 is stocked in 30mm slabs, starts at 100 m², and ships in 5 days once the drawing is fixed.
How do you check the fall on site?
Three tools: a short straightedge, a spirit level, and a tape or a feeler gauge. Pick a straightedge no longer than the tread: measure the tread depth first, then choose one that fits inside it. A 300mm straightedge suits a 300mm tread, and it also suits a 350mm tread, because it lies wholly on the stone. Lay the straightedge along the fall with one end at the front edge and the other toward the wall, set the level on top, and measure the gap at the low end. That gap is not the fall per metre, so convert it: gap ÷ straightedge length × 1000 = fall per metre. A 4.5mm gap under a 300mm straightedge reads 4.5 ÷ 300 × 1000 = 15mm per metre. Take three readings: the top step, a step in the middle, and the step nearest the ground. If all three converted values match the order, the fall passes. Check the drip groove at the same time, because a groove worn flat lets water climb the riser even when the fall is right.