Put cladding to the test

As members of the Centre for Window and Cladding Technology (CWCT), Aneira Beament of Ash & Lacy discusses standard test methods for rainscreen cladding systems.

In recent years, the construction industry has stepped up its scrutiny of testing, quality, certification and accreditation of products and systems; particularly within the external facades market.

Safety is paramount within our built environment and is firmly planted at the front of industry professionals’ minds when they are investigating and specifying materials. However, when presented with an extensive list of applicable British and European standards for compliance, it can be difficult to know exactly what to look for and how to compare the performance of similar products.

In this article, we focus on CWCT testing, a well-established test standard widely used in the rainscreen cladding industry.

What does CWCT testing assess?

CWCT standard test methods for building envelopes determine the level of serviceability of a facade or rainscreen cladding system when subjected to typical climactic levels of water, air and solid object impact.

The final classifications derived from the results depend on factors such as the maximum air pressure reached or the severity of damage after an impact. CWCT tests must be undertaken at specialist UKAS accredited laboratories or centres, which are experienced in the detailed requirements of the test setup and processes and are regularly audited by representatives from CWCT to ensure compliance to the prescribed standards.

Key performance criteria

A full CWCT test for rainscreen systems is made up of 4 key test methodologies. Wind load testing looks at resistance to wind load and is divided into two basic elements – serviceability testing and safety testing. Separate positive and negative wind load test pressures can be applied if the design wind load has different positive and negative magnitudes, this is particularly appropriate when testing bespoke cladding systems.

Serviceability wind load testing, where a component or sample of the cladding is subjected to both positive and negative pressure differentials equal to the design wind load (nominally 2400 pascals), to ensure that when the 1-in-50-year wind load occurs, the cladding system or component neither fails (by moving too much – this assessment is usually deflection limited) nor ceases to be weathertight.

The deflection of parts of the component or system are monitored and compared to pre-defined limits. An excessive deflection may lead to damage to fixtures and fittings, failure of joints, or may simply be unnerving for the occupants of the building.

Safety wind load testing, where the objective is to determine whether the cladding has a factor of safety beyond the design wind load (nominally 3600 pascals). This test is usually only performed for flexible cladding systems, where stress limits may be exceeded and permanent deformation occurs. The test sample is subjected to positive and negative application of 50% above the design wind load. A limit is placed on the residual deformation of the cladding system. It is important that fixings are capable of passing this test, as the cladding should not fail structurally during this test. The wind load tests are structural, and are related to issues of health and safety, and so must be performed to the full wind load (and beyond for safety).

Typically, a lower deflection or deformation figure indicates superior performance.

Dynamic water tightness testing; determining a suitable test pressure for watertightness testing is complex as the watertightness test is not undertaken at the design wind pressure – the positive wind pressure. A particular issue is that a short-term gust has little relevance to water penetration – the quantity of water that can be blown into an opening in a few seconds is very small, and unlikely to cause problems. It is sensible to use standard values, so for this reason watertightness is typically assessed at 300 or 600Pa. Remember, ventilated rainscreens in isolation are not intended to be fully watertight, but it is important that any water penetration is able to drain out at the base of the cavity and that no water permeates to the backing wall.

This test requires an impactor to be suspended in front of the test rig. The impactor is then raised and allowed to swing in a pendulum manner so that it strikes the test sample. The severity of the test is given in terms of the impact energy which is given by the relationship: E= mgh, where: E is the Impact energy in J (or Nm) m is the mass of the impactor in kg g is the acceleration due to gravity in ms-2 h is the height through which the impactor falls. The impact performance of a cladding panel may be influenced by many factors including the flexibility of the support structure.

What to look out for

  • Read the report in detail, don’t assume that because it’s tested it covers all eventualities
  • Look for the tested spans or rail centres – this will be the maximum permissible span capability for the system. You can reduce the rail centres, but cannot increase them.
  • If the CWCT test is relevant to a panel system, check with the manufacturer to confirm the largest panel size that can be supplied. It is not necessarily the largest panel size tested.
  • Check to see if larger panel sizes are reliant on additional stiffening or bracing to achieve wind load/deflection performance criteria.
  • Engineering calculations can be used to extrapolate the test data to alternative panel sizes and layouts. Ensure that any engineering calculations are undertaken by qualified individuals carrying PI insurance.
  • Certain performance criteria (such as impact resistance) is graded rather than awarded a pass or fail. Make sure that the grade achieved during the test is suitable for the intended building use, location and height.

Aneira Beament is group head of marketing at Ash & Lacy