Alternating tread stair
Approved document K, Protection from falling, collision and impact, defines an alternating tread stair as:
A stair with paddle-shaped treads where the wide portion is on alternate sides on consecutive treads.
In dwellings, alternating tread stairs may only be used in loft conversions where there is not enough space for conventional stairs and where the stair is for access to only one habitable room and, if desired, a bathroom and/or a WC (although this must not be the only WC in the dwelling).
Alternating tread stairs should:
- Make alternating steps uniform with parallel nosings.
- Have slip-resistant surfaces on treads.
- Should be constructed so that a 100mm diameter sphere cannot pass through the open risers.
- Provide a minimum clear headroom of 2m.
- Have tread sizes over the wider part of the step in line with the table below, and should comply with the diagram below.
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| Victorian timber alternating tread stair. This is sometimes referred to as a 'witches stair' because of the belief that witches are unable to climb them. |
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