Guidelines for Steel Roofing

 

Over the last decade building design in New Zealand has been evolving and this requires a review of building methods that we have come to think of as normal. One of the parts of the building needing fresh thought is how we construct steel roofs.  Following are guide lines on matters that need to be considered when designing and building steel roofs.

Although the building industry works in different trades, from a building performance point of view, the performance of the roof is linked to the performance of the building below.  The design must take a holistic view of the building and roofing trades have to consider the work of others.  It is not realistic to draw a line across the ceiling and expect the roof performance to be independent of the building below.

Firstly, decide whether the roof will be a warm roof or a cold roof.  This has little to do with the materials used but is about where the insulation is located.

Warm or cold

A cold roof has the insulation located close to the ceiling lining and the roof structure is maintained at a similar temperature to the exterior environment.


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A warm roof has the insulation located outside of the structure so that the structure is maintained at a similar temperature to the interior.


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Most of what has been built in New Zealand historically has been cold roofs but increasing problems with condensation, mould and air quality has caused some review of historic practice.

A warm roof is a less familiar construction method but provides better building performance and minimises the risk of future problems.

What needs to be ventilated?

There is a popular view that all roof problems can be solved with ventilation. However, incorrect ventilation practice can make building performance worse.

 

There are 3 spaces to be considered when thinking about roof ventilation.  Each space effects every other space.

 

Space 1 is the interior

Space 2 is the roof space below the roof underlay

Space 3 is abovethe roof underlay but belowthe roof steel

 

 
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Space 3

With steel roofing the most important space to ventilate is space 3; abovethe roof underlay but belowthe roof steel.

Steel will always generate condensation on the underside of the steel.  This is because the steel is frequently colder than the surrounding ambient air so humidity (water vapour in the air) will condense on the underside of the steel.

The roof underlay has 2 functions.

  • Firstly to absorb the dripping condensation and store it until the air warms up and the water can evaporate.

  • Secondly to collect any excessive water present and allow it to drain down the underlay to the gutter.

However, if the underlay is pressed tightly to the underside of the steel there is less opportunity for the underlay to freely dry out and the underlay may stay saturated for long periods.  If that happens the underlay is prevented from performing the intended function.

Kraft paper based underlays suffer particularly from this and will tear apart if remaining saturated for too long but still subjected to normal movement of all of the materials in the roof structure.

Excess saturation of underlays is generally less problematic with corrugated profile steels because the surface area contact between steel and underlay is low, maybe only 20% of the entire steel surface area.  However trough profile steels have a high surface contact with roof underlays, likely greater than 80% of the steel surface area.  Trapezoidal profiles are somewhere in between.

To mitigate the surface area contact issue and avoid lengthy saturation of the roof underlay then providing a space between the steel and the underlay is a good practice.  There are various ways to do this including a number of proprietary products that allow a free air flow between the underside of the steel and the top of the underlay. 

Note however that the space should be formed using a non absorbent material and in a manner that does not prevent flow of liquid water down the underlay. 

This ventilation can be added in both warm or cold roof construction.


 
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This type of ventilation does not necessarily demand a special ridge vent. Most steel roofing is sufficiently air leaky to allow air to flow out under standard ridge vents but some enhanced ventilation may be helpful.

 

So that deals with space number 3 where ventilation is desirable.

Space 2

Moving to space 2 matters get more complex.  

Most roof spaces (below steel roofs) will be naturally air leaky because of normal small construction gaps.  Therefore it is necessary to evaluate whether specific ventilation is needed in space 2 at all and this depends largely on space 1 below.

Consider the following issues about space 1 (the interior)

  •  Will it be air conditioned?

  •  Will it be temperature controlled?

  • What is the expected average interior temperature?

  •  What is the likely difference between interior and exterior temperatures?

  • Will there be sources of high moisture generation present?

  • Will the interior have adequate natural ventilation?

  • Will the interior have passive or mechanical ventilation?

 

All of those questions will fade into less significance if the simple step is taken to prevent the passage of air from the interior (space 1) to the roof space (space 2).  This is called an Air Control Layer.

An air control layer may be painted plasterboard, plywood with taped joints or a wrap with taped joints.  Anything with NO holes can be regarded as an air control layer but it cannot be an air control later if holes are made for downlights, sprinklers, services or negative details.

 

 
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The purpose of the air control layer is to prevent the movement of air from space 1 to space 2.  This will significantly reduce the passage of both heat and moisture (humidity) from space 1 to space 2.  If that is achieved then the need to ventilate excess heat or moisture from space 2 (roof space) is greatly reduced or eliminated entirely.

Most heat and moisture are transported vertically by convection (the movement of air).  If movement of air is restricted then heat and moisture can still be transferred, heat by conduction and radiation or moisture by diffusion but these mechanisms are less significant.

 

Matters that influence the need for specific ventilation provision of space 2 include;

 

  • A particularly air tight roof construction.

  • Very high humidity in space 1 that may diffuse into space 2.  However this can be controlled by use of a vapour control layer that is like an air control layer but with vapour management properties.  A vapour control layer is not necessary in most of New Zealand most of the time and is beyond the scope of these guidelines.

  • Large difference in temperature between space 1 and space 2, although this should be properly addressed with the correct amount of insulation.

Space 1

The living space should be different to the outside environment.  It is not healthy, in New Zealand, to live all year at the same temperature and humidity as outside so heating and ventilation are needed.  To save energy and enjoy a comfortable healthy environment it is better to not allow all of the heat to escape into the roof space where it will cause a problem. Also it is better to ventilate through windows, doors or ducted systems and not allow the roof to be the place where stale air is dumped. 

The issues, listed above and repeated here, all have an influence on how space 1 is designed;

  •  Will it be air conditioned?

  •  Will it be temperature controlled?

  •  What is the expected average interior temperature?

  • What is the likely difference between interior and exterior temperatures?

  •  Will there be sources of high moisture generation present?

  •  Will the interior have adequate natural ventilation?

  •  Will the interior have passive or mechanical ventilation?

While the answers to those questions will influence design decisions there is one method that is fundamental to solving most potential problems.  Use of an air control layer to clearly separate the roof space from the interior space is useful.  Having mechanical ventilation or heat recovery systems located in the roof space may be a useful design feature but for these systems to be effective there still needs to be an air control layer present.  

Insulation becomes largely worthless if air is permitted to travel through or around the insulation carrying most of the interior heat with it.  For insulation to be effective there needs to be some control of air movement.

Do not allow under any circumstances allow extract fans from showers, clothes dryers or kitchen cooking to discharge into the roof space.  If that is happening then an enormous amount of roof space (space 2) ventilation will be needed.  

Acoustics

Sometimes mass in the roof space is required to dampen rain noise on steel roofs. This is often sought by the use of plywood located under the steel.  More value can be obtained from the presence of plywood if it is located in a place where it can also function as an air control layer.

Things to remember

Loose fill or batt type insulation do not prevent the passage of air.  

Suspended ceilings are not air barriers and allow air to pass thorough.

Most heat and moisture travels with air particularly in a vertical direction.

Placing a vent at the ridge of a roof is okay but if there is no air control layer from space 1 to space 2 all that happens is the warm interior air is vented to the exterior. 

Allowing air to wash through spaces containing insulation diminishes the value of the insulation.

An air control layer can be made from a variety of materials but it must be continuous. This means wraps must be taped at joints and penetrations.

Conclusion

Design evolution is demanding a review of how roofs are designed and built.

Identify whether a roof is intended to be a “cold roof” or a “warm roof”.

Roof ventilation is not the answer to all issues.  First question what space is to be ventilated.

The need to ventilate space 3 above the roof underlay but below the roof steel is influenced by the steel profile and the type of roof underlay being used.

Separate the interior (space 1) from the roof space (space 2) with an air control layer.

An air control layer may be painted plasterboard, plywood with taped joints or a wrap with taped joints.

Ventilation of the roof space (space 2) is unlikely to be needed if an air control layer is used to prevent the passage of interior air into the roof space.  Normal construction air leakage to the roof space will most likely be adequate.