Protected Stairwells and Thermoplastic AOV Roof Lights

We’re talking about BS 9991:2015 and Approved Document B (ADB) Volume 1 specifically here.  Both these regulations have the concept of a protected stairwell that is used as a means of escape which should be shielded from fire as much as possible.

Protected stairways often need to have smoke ventilation, and a common, widely practiced option is to have a polycarbonate AOV Roof Hatch installed at the head of the stairwell. So how do these two standards compare regarding the use of thermoplastics, namely uPVC and Polycarbonate in this protected zone?

The two standards differ substantially in this area, with BS 9991 stating that “Plastic roof lights should not be used in protected stairways”, and ADB permitting their use if they meet the C-s3, d2 fire rating classification.

There’s one obvious ambiguity that both regulations don’t address though…

Does the term “Roof Light” apply to an AOV smoke vent?

There are some clear distinctions between typical roof lights and AOV vents, the obvious one being that AOV’s are designed to be in the open position during a fire event.  In this position the opening leaf is typically automatically opened to at least 140°, this removes the polycarbonate thermoplastic away from the heat and greatly reduces the risk of burning droplets entering the stairwell.

This assumes that the system is in working order which highlights the importance of regular servicing and testing of any AOV system.

There’s no official answer here that we’ve been able to find.  Although, a polycarbonate AOV roof hatch is a very common solution that has been widely installed across the country and each time it would have been given an “okay” by the building control officer.

What other AOV options are available?

If you are required to construct your protected stairwell to BS 9991:2015 standards and/or your building control officer considers the AOV to be a roof light, then there are still some options for a roof mounted AOV.

• Glazed AOV Roof Hatch or Roof Louvre with no thermoplastic components. This option maintains light entry into the stairwell and will often look better than a polycarbonate dome as well.
• Aluminium AOV Roof Hatch or Roof Louvre, again no thermoplastic components but without the light entry you may have difficulty getting an architect to agree with the change.

It’s important to highlight some challenges that come with both these options.

• Extra weight, aluminium will be heavier that the polycarbonate and a glazed option will be even heavier than the aluminium one. This could present an issue for actually getting the vent on the roof to begin with.
• Motor electrical current draw, with heavier vents you’ll need bigger and more powerful motors installed to achieve the required 60s opening time. You’ll want to ensure that your smoke vent control panel can handle this before connecting the AOV up to it.

How does this affect the products Rocburn Limited supplies?

We’ve been supplying AOVs manufactured with either uPVC or aluminium for many years, both glazed and not.  We’ve vetted our manufacturers to ensure the uPVC/polycarbonate options they produce meet or exceed the minimum C-s3, d2 fire rating requirements of Approved Document B.  We’re confident that the products we’ve sold to date all meet the relevant requirements and aim to continue their supply for suitable projects as usual.

In Summary

The use of AOV roof hatches manufactured from thermoplastics like uPVC and polycarbonate is a murky area within the regulations.  Whether you can use them will depend on your project and building control requirements.  If your building is higher risk (over 18m tall or 7 storeys high) then the Golden Thread applies, and you’ll want to achieve the safest solution possible.  If building control will sign off a solution that meets Approved Document B regulations, then Rocburn can supply AOV roof vents that meet or exceed the minimum fire rating it states.

As always, if you have any questions then reach out to us and we’ll do our best to answer them.