A fire door is not a smoke-tight door, and a smoke-tight door […]
A fire door is not a smoke-tight door, and a smoke-tight door is not a fire door. A lack of knowledge about the difference remains a challenge for fire safety in construction, but divided classification shows how smoke-tight doors are tested.
When working with fire safety in buildings, fire doors are often perceived as a combined solution that protects against both fire and smoke. But the two do not automatically go hand in hand. A fire door can be correctly fire-rated without simultaneously providing effective protection against the spread of smoke. This can create uncertainty in both design and door selection.
“Many people take it for granted that a fire door is also tight against smoke. That’s a natural assumption, but it doesn’t necessarily hold true. Fire and smoke are two different issues, and they are tested in different ways,” says Trine Dalsgaard Jensen, Resistance to Fire Engineer in DBI’s fire testing department.
The difference between fire and smoke is partly related to the door’s construction. Fire doors are designed to withstand fire exposure, and many are equipped with materials in joints and seams that are activated by heat and expand during a fire. This is effective against flames, but it also means that smoke from a fire elsewhere in the building can pass through the door long before the temperature rises.
“A fire door typically works as intended once it gets hot. But smoke can spread much earlier, and therefore a fire classification in itself is not a guarantee that the door will also hold back smoke in an early phase,” explains Trine Dalsgaard Jensen.
Differentiated classification creates clarity
A significant part of the discussion about smoke-tight doors has concerned testing for smoke in cold conditions – that is, at the same temperature as the surroundings. In this type of test, it has been, and still is, possible to tape the air gap between the door leaf and the floor during testing. In such cases, the gap is not included in the measurement, even though smoke can in practice spread precisely there. The crucial new development is that it now appears directly from the classification whether the bottom edge was included in the test or not.
“If you tape the bottom, you’re not really measuring whether smoke can pass under the door. That means there’s a risk that the test doesn’t give a true picture of the door’s real-world performance,” she says.
The consequence has been that doors with different properties could end up with the same smoke classification, depending on how the test was carried out. This made it difficult to use the classification as a practical decision-making tool.
With the updated designations, greater transparency has been created. The smoke classification in cold conditions is now divided so that it is clearly stated whether the door was tested along three sides or along all four, including the bottom.
“The key point is that you can now see the difference. When a smoke-tight door is classified as Sa3, it means it has been tested along three sides. Sa4 means it has been tested as tight all the way around,” explains Trine Dalsgaard Jensen, adding that these will be the new classifications stated on the CE marking.
Industrial doors get their own classification
At DBI, a restrictive approach has been used for many years when it comes to testing with the bottom taped.
“In our assessment, using tape at the bottom does not provide a representative test result. That’s why we have always recommended testing solutions that ensure tightness along all four sides. This means that many already have documentation that can be used for the new classification,” says Trine Dalsgaard Jensen.
With the new classifications, S200L has also been introduced for smoke testing in hot conditions. This class is aimed at large industrial gates and doors, which have different prerequisites than ordinary hinged doors and where national special requirements have previously existed.
“Large industrial doors simply cannot be compared directly with small, hinged doors. By gathering the special requirements in a common supplementary class, you get a more realistic and comparable system,” says Trine Dalsgaard Jensen.
The existing S200 classification remains unchanged for ordinary hinged doors.
Confusion in the Danish Building Regulations (BR18): Cold smoke or hot smoke?
Lack of knowledge about smoke-tight doors is one challenge, but the area is also unclearly described in the building regulations. In the guidance texts to the so-called pre-accepted solutions in the Danish Building Regulations, requirements are mentioned for tightness against cold smoke, while the technical specifications simultaneously refer to the smoke class S200, which covers smoke testing in hot conditions.
This creates uncertainty about what is actually meant by a smoke-tight door in terms of the building regulations, and whether the door must be documented for cold smoke, hot smoke, or both.
“It’s unclearly described, and that creates confusion in practice. In our assessment, it is the S200 requirement, hot smoke, that is the intention, while the reference to cold smoke is more an expression of an ambiguity that has not been corrected,” says Trine Dalsgaard Jensen.
Although the introduction of Sa3 and Sa4 makes it clearer how doors are tested for cold smoke, it does not change the fact that the Danish Building Regulations can still be interpreted in several ways when it comes to requirements for smoke-tight doors.
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