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The NHBC burns a hole in fire-rated down light certification.

Updated guidance to housebuilders using engineered timber joists and recessed downlights is causing problems for installers.
By Gordon Routledge
​Thursday  27th February 2020
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Around seventy per cent of new-build homes use engineered timber joists as part of floor construction. Commonly known as i-joists, these engineered solutions offer significant advantages to house-builders compared to traditional timber joists. Manufactured offsite to exact lengths, they don't twist or bow like conventional timber.  They are also easier to handle and drill through for electrical and plumbing services.

Video report: Gary Hayers and Joe Robinson drill in to the issue

The burning issue?

I-joists don't perform the same as traditional timber construction in a house fire. There is less material to burn once the protective layer of plasterboard has failed, causing a floor to collapse sooner. I-joist and plasterboard combinations have been extensively tested and certified to achieve 30, 60 and 90-minute fire ratings. However, adding recessed down lights into the I-joist ceilings has not been adequately assessed.

Plasterboard does the heavy lifting of fire performance. By drilling large holes for down lights, the resistance to fire is severely impacted, unless the down light can preserve the integrity of the ceiling.  Hence the requirement for down lights to be fire-rated. 

This issue caused the NHBC to re-issue their guidance to house builders.  As a result, the NHBC now requires test evidence showing that recessed downlights are suitable for a matching I-joist and plasterboard combination.   In other words,  if you're installing a downlight which was tested within a traditional timber construction, the NHBC won't sign the off the completion certificate. 

I-Joists used in the majority of new build residential construction projects.
Punishing process.

Engineered joists come in lots of variants: different sizes and build material combinations.  At the time of writing, we understand that the NHBC wants evidence of a downlight tested with every different manufacturer's engineered joist.  They will not accept a generic "worst-case" scenario. 


The tests to validate the performance of a recessed downlight is a punishing process.  For the manufacturers, it's especially punishing in terms of cost; a test cycle can cost around £15,000. Multiply this for each downlight, joist and manufacturer combination, and it amounts to a substantial investment.


The NHBC updated their guidance notes in August 2019. Several key lighting and joist manufacturers started to invest in re-testing straight away.  Others, we believe, have taken a wait and see approach hoping for a less draconian approach from the NHBC. This hasn't happened, and the guidance is now being rigorously enforced.


In the post-Grenfell world of risk-averse fire compliance, the test laboratories which carry out the tests are full. Lighting products join a queue of other building materials under investigation.  As a result, only a few lighting manufacturers have the required I-joist test data requested by the NHBC.

6.4/10 i-joist guidance issued by the NHBC - August 2019
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The task force arrives.

We have learned that The Lighting Industry Association (LIA) has set up a dedicated task force to tackle the issue. The Fire Rated Downlight Task Force (FRD-TF)  comprising LIA members who manufacture downlights and technical experts from the LIA.

The task force aims to coordinate a response to the NHBC and attempt to resolve discrepancies between test methods and the various European and British standards.

It appears that the testing of downlights is a smoke-filled grey area. Lighting manufacturers would rather see a well-defined test at a product level rather than as part of a complete ceiling assembly. 

The FRD-TF has also noted that audio and ventilation products, which mount within fire-rated ceilings,  are not being subjected to the same level of scrutiny. 

The FRD-TF is looking to engage with other building product associations, such as the Timber Association to further asses the issue.

So expect a flotilla of associations arriving at the NHBC's door for a meeting on the 4th March. 

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We have asked the NHBC to comment.

KNOW HOW: How are fire rated down lights tested ?

The 30 minute fire test for a recessed downlight is a challenging process. A real-life ceiling is constructed. Then it's subjected to a severe fire with temperatures quickly exceeding 600 degrees Celcius.

Ceiling build

4 x 3 meter ceiling structure built to joist manufacturers instructions.

Ceiling finished with specified plasterboard. gaps and screws sealed. Down lights installed.

Ceiling structure loaded to mimic real-life floor conditions - e.g furniture / people.

Fire test starts

1 Minute


Paper on plaster board starts to blacken.

6 Minutes


Filler peeling away.

17 Minutes


Plasterboard starting to deform.

33 Minutes

Flames in the test chamber - ceiling timbers starting to burn.

42 Minutes

Smoke escaping from upper floor level

45 Minutes


Ceiling collapses 

A test failure occurs if the ceiling structure collapses before the prescribed test duration. In the example above the ceiling collapsed at 45 minutes, a considerable margin over the 30-minute test requirement.

Standards to look out for:


BS EN 1365-2:2014

Fire resistance tests for load bearing elements. Floors and roofs


BS 476-21:1987
Fire tests on building materials and structures. Methods for determination of the fire resistance of load bearing elements of construction


BS EN 15725:2010

Extended application reports on the fire performance of construction products and building elements.


NHBC Guidance notes  - Here >>

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