Loft stair headroom: how much you need, drawn out

Headroom is the single thing loft stairs most often fail on, because it is a vertical measurement a flat plan cannot show. The rule is short; meeting it in a real roof is where it gets tight. Here it is drawn out.

The rule, and the loft allowance

Over the whole of a stair you normally need 2 metres of clear headroom, measured vertically from the pitch line, the line that runs across the tread nosings. Lofts rarely have that height, so a loft conversion is allowed a reduction: 1.9 metres at the centre of the stair, tapering to 1.8 metres at the side. It is a real concession, not a free pass. The reduced figure still has to be met at the tightest point, and plenty of loft stairs miss even the relaxed number. For the general rule across all stairs see staircase headroom requirements.

The loft headroom sandwich

A loft stair has to satisfy headroom in two directions, and only one of them gets checked. Everyone measures the height above the stair as you climb up into the roof. Fewer people check beneath it, because the loft stair lands on the floor below and needs clearance there too, over the flight or landing it lands onto. Squeeze the loft stair in without allowing for the space it eats on the floor below, and you solve one headroom problem by creating another. It is a sandwich: clearance above the new flight, and clearance under it over the old one.

Why a roof window does not always fix it

The common move on a drawing is to add a roof window over the tight spot, on the logic that a window means more height. It often does not, because headroom is measured straight up, and a roof window sits in the sloping plane of the roof with a sloping reveal, so the height it adds is at an angle, not vertically over the pitch line. It only helps if it is big enough, lands right over the pinch point, and works around the rafters. More on how a plan hides all of this in why a staircase that works on the plan can fail in reality.