What is the striking point of an arch?

The striking point is the centre from which the curve of a masonry arch is set out. Put the point of a compass — or the fixed end of a string — at the striking point and swing an arc, and you describe exactly the curvature the arch must take. In Dutch the term is porringpunt, from porring: the curvature of an arch or vault, meaning the height of the crown above the line joining the two springings.

Why the position of that point decides everything

The form of an arch follows directly from where you place the striking point relative to the springing line (the line joining the two springings):

• On the middle of the springing line → a semicircular arch. The radius equals half the span.
• Below the springing line → a segmental arch. The lower the point, the flatter the arch.
• Above the springing line → a stilted or raised arch.
• Two points, left and right of centre → a pointed arch. If they sit exactly on the springing points you get the equilateral pointed arch; closer together gives a drop arch, further apart a lancet.
• Several points → basket-handle and elliptical arches are assembled from three, five or more circular segments, each with its own centre.

Flat arch forms work the same way: the radiating lines from a striking point determine how the joints fan out and therefore how the load is passed along.

Setting it out in practice

1. Draw the springing line and the centreline of the opening on the substrate or on a sheet of board.
2. Decide the rise you want and mark the crown.
3. Construct the striking point: it lies on the vertical centreline, where the distances to the crown and to both springings are equal.
4. Drive a nail or screw at the striking point, attach a string or batten and swing the arc.
5. Build the centring (the temporary former) from that drawing, and lay the arch on it.

Points to watch

• Always work full size. Setting out an arch to scale and then converting almost always goes wrong; lay the drawing out 1:1 on a sheet of plywood.
• The striking point is often outside the work. For a shallow segmental arch it can sit metres below floor level. Use a batten pivoting on a fixed point, or construct the curve point by point.
• Joints point at the centre. In a correctly laid arch every course is radial: extended, the joints meet at the striking point. That is your check.
• Leave the centring in place until the mortar has gained sufficient strength. An arch only carries once the mortar has hardened.

Related terms

• Arch
• Springing
• Segmental arch
• Pointed arch
• Basket-handle arch
• Centring

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