What is a slotted hole?
A slotted hole is an elongated hole, rather than a round one, that allows the bolt or screw passing through it to shift position. It is also called an elongated hole; in Dutch it is a slobgat and in German a Langloch. You find slotted holes in brackets, channels, hinges, strike plates, cladding panels and plastic or aluminium trims.
What a slotted hole is for
- Absorbing tolerances. Small dimensional differences on site can be taken up, so a bracket or panel can be aligned before it is tightened.
- Allowing movement. Timber swells and shrinks with moisture, while plastics and aluminium expand noticeably with heat. Cladding boards, uPVC fascias and aluminium cappings are fixed through slots so they can slide instead of buckling or cracking.
- Later adjustment. Door hinges, strike plates and adjustable brackets often have slots so their position can be corrected afterwards.
- Larger structures. Precast elements are sometimes connected through slotted holes to allow limited movement while staying connected. A slip membrane or sliding bearing reduces friction.
Application
- Orient the slot in the direction of movement. A board that expands along its length gets slots running lengthwise.
- Fix in the centre of the slot when it is there for expansion and contraction, leaving room to move both ways.
- Use a wide washer (penny or repair washer) under the head so it fully bridges the slot.
- Do not overtighten where the component must slide. A screw that clamps the panel tight defeats the purpose. Follow the manufacturer’s installation guide for fixed and sliding points.
- Provide one fixed point (usually a round hole) so the component as a whole cannot creep.
Common mistake: screwing cladding panels or plastic trims rigidly through round holes. After a few summers and winters they crack, bow or shear their fixings.
Related terms
- Expansion joint
- Slip film and sliding bearings
- Cladding
- Shrinkage
- Facade anchor screw
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