Interiors & Surface Materials is one of Stone Federation Great Britain’s sector focus groups. Here the Group answers some questions that should be asked about including movement joints in stone floors.

Like all building materials, natural stone will be affected by changes in the conditions it is exposed to (temperature and traffic, for example) which is why it is often necessary to include stress-relieving movement joints in stone floors.

When designing a natural stone floor, the following questions will help you deliver a durable scheme and avoid failures in the floor.

Q: Do I need movement joints?

Where the distance between restraining surfaces, including perimeter walls, exceeds 2m a perimeter movement joint should be installed. Intermediate movement joints are required where the distance exceeds 10m.

If there is underfloor heating, the natural stone flooring should be divided into bays of up to 40m², but with no bay length exceeding 8m.

Q: Will the floor be subject to light or heavy loading?

The type of movement joint required is determined by the nature of the traffic load the floor will be exposed to.

Light loading is defined as normal, low-density pedestrian traffic with lightweight, soft-wheeled trolleys (eg domestic and office locations).

Heavy loading is defined as high density pedestrian traffic and/or heavy static, moving, dropped or dragged loads.

If the floor will be exposed to low loading, or in a low traffic and impact area, a non-leaching mastic would provide a suitable movement joint.

If the floor is in a higher traffic environment, a pre-formed movement joint, typically comprising metal side plates with a flexible synthetic rubber core, would be suitable.

Q: Is there underfloor heating?

If there is an underfloor heating system installed, the pipes or cables should be located to ensure that the system is contained within the pattern of the movement joints.

There should also be an assessment undertaken of the likely temperature range that the floor will be exposed to.

Q: Have you taken drying shrinkage into account?

Stone, like all finishing materials, reflects movements arising from supporting substrates. In the early period of a floor’s life cycle, movement occurs primarily from the drying shrinkage of the slab and screed. The Concrete Society says that, as a rule of thumb, a typical 10m span slab will experience linear drying shrinkage of 3.0mm, irrespective of design, depth, or amount of reinforcement used.

Q: What colour sealant should I use?

Because natural stones are absorbent to a greater or lesser extent it is advisable to seal stone floors as it will make it easier to clean them, especially in areas where food or drinks might be spilled. There are various sealants available, some of which are designed to enhance the appearance of the stone in some way. The same applies to the grout used.

• For more information on movement joints and natural stone flooring in general, architects, interior designers and clients can discover how they can purchase a copy of The Code for the Design and Installation of Natural Stone Flooring by sending an enquiry to [email protected].