I am about to build a small (3-riser) entrance stairway for my house guided by this how-to:

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This question is about the treads which appear to be consisting of three separate boards each. The text even specifies a gap between them:

Treads spanning 900–1000 mm between strings should be nominal 50 mm thick minimum and grooved or have a slip-resistant finish. Boards should have 5 mm gaps between them.

  1. Am I understanding it right that the treads are indeed to be of three separate boards each? Or are they just grooves in a single-piece tread?
  2. Why so? Is it so that water could drain in the 5-mm gaps? Or is it for better slip resistance? Or both?

Spacing between the tread pieces allows for water drainage and for the wood in that area to dry out. And yes the spacings between the tread pieces also help with the slip resistance.

It is common to use multiple narrower pieces for outdoor decking stair treads because they will tend to stay flatter overall when the wood has been exposed to the elements over time. If you use tread pieces that are full width lumber there is much greater chance that it will twist and warp over time. A wider flat piece can also get quite slippery when it is wet.

Another thing to consider, if you are using treated lumber, is that the chemicals in the wood will penetrate to a greater percentage of the cross section of narrower pieces. This will help to preserve the wood over a longer period of time.

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