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What housebuilders need to know about NHBC deflection limits

26th January 2024

Deflection limits measure the degree to which a part of a structural element is displaced under a load without causing damage.

Timber structures must satisfy deflection limits to ensure their serviceability, which means they can perform well and provide comfort and safety to the occupants and users. Deflection limits depend on the span, the load and the stiffness of the timber joists, and they are set to avoid excessive deflection that can cause problems such as cracking, malfunctioning, sagging, vibration or structural instability.

The deflection limit

As a reminder, the NHBC Standards 2023, brought in the requirement that the deflection limit should not exceed 0.003 x the span for the combined bending and shear based on the total dead and imposed loads, with a maximum deflection of 14mm where strutting is provided, or 12mm where strutting is not provided. This applies to the instantaneous deflection, not the final deflection.

This requirement did not apply to plots where the foundations had been concreted after 1 January 2023, but the floors have been supplied, stored, or ordered before this date, acceptance of floors designed to previous NHBC Standards was accepted until 30 June 2023.

This meant that from 1 July 2023, all new floors were to meet the new requirements.

Roof designers can work with housebuilders on deflection limits

Housebuilders should work closely with roof designers to ensure that the roof structure satisfies the deflection limits and provides adequate serviceability. They must follow the NHBC Standards 2023 to design pitched roofs, which set out the technical requirements and guidance for roof structures.

They also need to consult the technical manuals of the timber joist manufacturers, which offer helpful information on deflection limits, span tables, load calculations and design details.

Moreover, they need to ensure that the roof structure meets the serviceability states and deflection criteria for metal roof sheeting, sheeting rails and profiled metal wall cladding, as recommended by the Metal Cladding and Roofing Manufacturers Association (MCRMA).

Going above and beyond

While the NHBC standards set a minimum benchmark for structural integrity, by working within the limits, you can offer your customers numerous advantages, including:

  1. Enhanced Occupant Comfort: Exceeding the NHBC deflection limits leads to stiffer, more stable roof structures, minimising perceptible bounce or movement during regular use. This creates a more comfortable living environment for occupants, reducing concerns about potential structural instability.
  2. Reduced Risk of Damage: Excessive roof deflection can strain and damage non-structural elements such as ceilings, drywall, and flooring. Roof truss designers and manufacturers can help prevent these costly and inconvenient issues by adhering to stricter deflection limits.
  3. Long-Term Structural Integrity: Further reducing the NHBC deflection limits contributes to the overall longevity and resilience of the roof structure. By minimising stress and strain, stiffer roofs are better equipped to withstand the cumulative effects of long-term loadings, such as snow, wind, and other environmental factors.
  4. Futureproofing: By surpassing the NHBC deflection limits, roof truss designers and manufacturers are effectively future-proofing the structure against potential changes in building codes or loading requirements. This proactive approach ensures that the roof remains compliant and well-suited to accommodate future needs.
  5. Enhanced Reputation: Adhering to stricter deflection limits demonstrates a commitment to quality and craftsmanship, distinguishing roof truss designers, manufacturers, and housebuilders from competitors. This can lead to a stronger reputation and a competitive edge in the industry.

For more support or advice on the NHBC deflection limits, contact the TRA Technical Committee, or for more information on the NHBC’s deflection limits, visit the NHBC website.

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Trussed Rafter Association