SFPE Discussions

Intumescent Expansion of protected steel

Do we have any acceptance criteria re the gap required around the intumescent painted steel and if we can have timber framing or plasterboard hard against the steel.

I have a response from Nullifire UK as below
In general, a minimum gap of 30mm for up to 60 min and 50mm for up to 120min or 50 x DFT of the intumescent is recommended to allow adequate expansion in a fire.

This obviously poses a challenge as Architects & builders do not consider this requirement which increases the overall thickness of walls where the steel column may be imbedded in. The arrangement of intumescent steel within a wall is generally used to avoid strapping and lining steel to achieve fire protection without increasing the depth of the wall.

One way of protecting steel columns embedded in a wall cavity is to fire rate the wall with one way fire ratings on each side of the wall. This should alleviate the requirement to paint the steel with an intumescent coating. Your post indicates that if you do not have the specified clearances then the adequacy of the intumescent coatings may be questionable. Someone who has done a lot of testing of intumescent paints should comment.

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I have had this issue in the last few months as well. It is usually overlooked by structural engineers who are responsible for it under NZBC B1 -1.3.3i but aren’t aware of it. Some architects have become aware and have raised it recently.
It is a challenge to detail the construction, especially on outside walls with cavities, which are usually lined with a fibre cement RAB board.
I am not aware of any fibre cement boards that have been tested for structural protection as they are poor insulators. You can use calcium silicate boards (Promatech etc) as one means to protect steel where you can’t use plasterboard. Promat (and others) have details for steel protection with their boards. They are thicker than RAB, and more expensive.
There are some issues to watch with using a universal wall system. The wall is obviously thicker, due to the thicker plasterboard. This then causes problems with window reveals and door frames, especially with rebated jambs which are not made for the increased thickness of the plasterboard or the wall. it also means that you have an entire wall lined with the thicker plasterboard (at increased cost) as you would have a step otherwise.
I have usually called for the gib to be fixed to the stud either side of the post so it is encased 4 sides and it then doesn’t matter about penetrations into the wall elsewhere, or if they use a standard plasterboard in the rest of the wall.

Note that a steel post in the middle of a wall dividing a firecell won’t necessarily be protected. Looking at this some time ago when a contractor made a mistake on site, Hans gave me a lead on some information on the temperature profiles inside the wall during the fire test of a 60min wall system. From memory the cavity temperature got to around 6-700 degrees, which depending on what assumptions you make around the heat transfer to the steel gives a steel failure time in my project of around 30-45 mins. The wallboard thickness of the 60min system equated to a universal wall system of 30min, which goes to show Winstone Wallboards know something about plasterboard when they recommended a universal wall system.


I agree that burying a steel post in the middle of a fire wall does not work. GIB have clearly indicated that if we use this approach then the steel shall be strapped and lined as per the GIB guide for protection of steel. You have correctly noted that fire rated linings do not necessarily provide the insulation requirements. One needs to be aware of what the contractor does on site.

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further to this conversation, the ASFP in the UK have some good info on passive fire protection and particular issues which are worth a read:
ASFP tech guides
Their FAQ states ASFP FAQ

> G2. Can intumescent paints be used to protect beams and columns where external cladding systems are intended to be abutted to the steel surfaces?

In general, a minimum gap of 50 x dry film thickness of the reactive coating is recommended to allow adequate expansion in a fire, This guidance applies to the gap between the cladding and the flat surfaces of the protected steel section, as well as the flange tips unless specific fire tests prove otherwise.
Gap sizes may be reduced where manufacturers have specific fire test evidence.
Continuous linear fixings [timber or metal] should be considered as part of the main beam, and duly protected from fire., unless other supporting fire test evidence can be provided.

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I have had a similar issue on a current project, and after some inquires, Azko Nobel (International Paints) sent me their "technical note TN/F/IC/011 - Over-Cladding Passive Fire Protection " and they have allowed me to post it here as below.
They did a test of an I beam with 12.5 standard plasterboard around it, and the result was that the clad beam had a lower temperature in the early stages and once the plasterboard had burnt off, the intumescent expanded and the final time to critical temperature was the same so the non fire rated plasterboard didn’t stop the combined system working.
Of course different construction details, maybe type of beam and different paint systems may have different results and the always encourage designers to allow for a nominal expansion zone when considering cladding detailing. .

At an NZIA seminar it was stated that some intumescent paint systems allow for timber to be fixed to the steel without compromising it’s performance. Does anyone know which paint systems allow for direct timber fixings?

Altex have done some testing with timber battens fixed along a column and unrated plasterboard boxing. Resene’s can provide manufacturer statements which present the test method and results, however this is a manufacturer opinion rather than a test report from an approved authority, so could be challenged. My recollection is that they used a 90min or 120min paint system and achieved ~80min in the test, but that the unrated plasterboard boxing played a big role and the performance depends on how quickly this boxing falls off.

They have the same issue that they have done a test on a particular column size with blocking in a particular layout and a particular thickness of their product. They have suggested this could be extrapolated up to 250x250mm boxing, but trying to make a judgement about whether the tested arrangement is similar enough to your situation is shaky ground.

There seems to be a large portion of the industry who are happy direct-fixing timber over the intumescent steel for applications up to 30mins, would be interested to hear others thoughts on this. Certainly would save a lot of headaches in terraced housing developments.