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News - 11/03/2017

Web Joist Fire Rating

It has been brought to our attention that there is an issue surrounding the use of engineered joists and particularly with open web joists in relation to compliance with TGD Part B 2017.

This is an issue that is being worked through live on many sites now.

Building Standards will release further guidance on this matter before Christmas and we understand that some of you are already aware of the situation.

However, it’s clear there is still some confusion and misinformation as to where this has come from, how it applies to ventilation and how some of these solutions work.

 

This aims to cover the main points.

There have been no changes in TGD Part B in relation to the load bearing capacity and integrity of the floors. It was always 30mins.

TGD Part B 2017

TGD Part B 2006

What has changed is the standards by which they are assessed.

All structural elements designed to the Eurocodes and requiring a fire performance must achieve that performance when tested in accordance with the EN fire test methods.

Web joists require a 30min fire barrier be provided to the underside of the joists. This is required to maintain the load bearing capacity of the joists.

In the event of a fire, the fire could spread through any opening in the ceiling through the open joists and reduce its loadbearing capacity.

When traditional timber joists were tested, and fire spread was limited to the exposed joists only, which gave the 30min fire resistance without the need to provide 30min barrier on the underside.

It is worth noting that the original IAB approvals for Web Joists made reference to penetrations: Eg.

“b) 30 minutes fire resistance: No apertures (downlighters, service penetrations etc.) should be present except where fitted with an approved light fitting or an appropriate proven penetration sealing system.”

See attached IAB cert for example, which dates back to 2004.

Penetrations in the plasterboard.

Partial penetrations of fire resisting construction such as down lighters, ventilation ope’s, etc. are required to provide the same fire resistance performance as that of the main construction, in certain circumstances.

This is particularly so in the case of “Metal Web Joist” or indeed in any floor scenario in which the construction allows a total open floor void.

The fire performance may be achieved by the product having an approved fire resistance or by protection of the ope’ by approved products.

It should be noted that there are no specific standards, European harmonised, BS/BS EN or otherwise for the performance of these products. They are tested by approved testing centres under the conditions of the BS/ BS EN standards only.

The fire performance of such products may be proven by tests to the relevant parts of either BS 476 or the BS EN 1365 conditions/test regime.

Either small scale tests under the conditions of the BS/BS EN standards that test the specific performance of the product or full scale structural tests under the conditions of BS/BS EN tests are acceptable.

We have confirmation of this from the Building Standards Section of the Department of Housing, Planning, Community and Local Government.

Aereco have investigated a number of products on the market that will give the 30min. fire resistance for duct penetrations through plasterboard ceilings.

 

Below is our recommended solution

 

Firefly 109 Vent Sleeve from Tenmat.

This product is tested to the relevant standard, BS 476.

It can be fitted from the underside of the ceiling and is tested up to 60 minutes.

The use of a 100∅mm duct connection to the Aereco BXC is required, as this product is not suitable for ducts greater than 100∅mm.

The sleeve is only 35mm deep and requires an ope’ of 127∅mm.

We also recommend that the surface mounted, bracket version of the BXC is used.

 

Expansion of the product

These products contain a reinforced outer layer that allows for inward expansion only.
Therefore, no additional sleeve is required.
The product was tested with a specific ceiling construction using a single 15mm layer of plasterboard.

 

 

Installation of the Firefly 109 with the Aereco BXC extract unit bracket version.

This solution requires very little additional labour and are relatively inexpensive, costing less than €10 per ceiling penetration. We can supply these as standard with the BXC extract units.

This is an issue not just for ventilation installations, but any other services greater than 25Ømm that penetrate the ceiling.

Please note also that there are no implications with regards to DCV and Part F compliance. This is solely related to TGD Part B.

 

We will endeavour to keep you informed of any further developments in relation to this.

Should you require any additional information, please do not hesitate to contact us.