Keeping skillion roofs dry

, Build 90

For most skillion roofs the amount of air movement in the roof cavity, although limited, is sufficient to remove small amounts of vapour. The trick is to keep those amounts of water vapour small.

Figure 1: Air barrier over a boarded ceiling lining in a skillion roofs.

Skillion roofs are where the roof  cladding and the ceiling lining  are within 300 mm of each other  and both are usually directly attached  to the roof structure. Examples are:

• chapel or cathedral style roofs

• roofs with internally exposed  rafters

• roofs with the ceiling lining attached  to the underside of the rafters.

Skillion roofs differ from roofs with an  undeveloped attic or ceiling space  (close-couple roofs) in that:

• roof spaces or cavities are  inaccessible

• air spaces between the ceiling  lining and roof cladding are small

• natural ventilation in the roof  space is low

• they are less tolerant of poor  workmanship

• faults are difficult to locate and  expensive to repair.

All roofs must have a positive fall to  effectively drain water from the roof.  A significant number of ‘low slope’  roofs (less than 10° pitch) constructed  in New Zealand are skillion roofs.  When using a continuous impervious  membrane for ‘low slope’ roofs, the  minimum slope under E2/AS1 is 1.5°.  But the ponding risk is greater where  membrane roofs are built to the  minimum slope so, wherever possible,  increase the fall to 3° to ensure  satisfactory drainage. For other roofing  materials, the manufacturer’s  recommended lowest pitch must be  considered the absolute minimum.

Skillion roof ventilation

Skillion roofs are commonly thought  to pose more of a problem because  their tight construction and narrow framing cavities provide little oppor­ tunity for air entiy and movement. In  fact, research has shown that most  skillion roofs in New Zealand have no  problems, even those that are not  deliberately ventilated. Where problems  do exist they can usually be traced to  solar-driven moisture, leaks or  construction moisture (wet framing).

A surprising amount of air does  move through a skillion roof, except  perhaps for very airtight roofs, such as  those clad with long-run trough- section metal roofing, asphalt shingles,  timber shingles with an interleavement  (to prevent solar driven moisture), and  membrane on plywood roofs.

No amount of ventilation will dry  out skillion roofs that have moisture built into them as a result of enclosing  wet framing, or have moisture entering  from rain and plumbing leaks or from  interior spaces.

Keep skillion roof framing dry

Use kiln-dried timber for as much of  the skillion roof framing as possible  and make sure it is kept dry during  construction. Exposure of framing and  plywood to rain and dew during  construction will make them too wet  to allow application of the membrane  and the closing in of the framing –  where framing gets wet it must be  allowed to dry before work proceeds.

Figure 1: Air barrier over a boarded ceiling lining in a skillion roofs.

Internal linings should not be fixed  until the timber has dried to an  average moisture content of 20% or less – remember some lining manufac­ turers require a 12–16% framing  moisture content.

Prevent moisture entry from  spaces below

Once a skillion roof is completed and  the building occupied, moisture from  spaces below must not be allowed to  get into the roof framing cavities.

In an occupied building the air  inside is usually warmer and more  moist than the air outside because  human activities, such as breathing,  heating and cooking, generate  moisture and heat. Research has shown  that this moisture can be transported  by airflows through gaps, cracks,  penetrations and open downlights into  skillion roof cavities. It is therefore  important that a skillion roof design  incorporates ways of preventing this  moisture-carrying airflow.

Moist air not removed by air leak­ age, ventilation or diffusion, remains  within the skillion roof structure. On  contact with a cold part of the  structure, such as the underside of the roof covering, the moisture can  condense to form water, which can wet  insulation and damage materials and  finishes or, for membrane roofs, form  as bubbles under the membrane.

Stopping airflow by the use of a  vapour-permeable air barrier between  the interior and the roof spaces will  prevent moisture accumulation.  Options for an air barrier include  flush-stopped plasterboard or a wall  underlay material installed above an  air-leaky ceiling (such as T&G  boarding) that is suitable for use as an  air barrier (refer to Table 23 of  E2/AS1). Air barriers must be installed  without openings, i.e. there should be  no open downlights and all penetra­ tions for wires must be sealed.

Research and experience has shown  that in New Zealand vapour barriers  are not necessary in skillion roofs  except in very cold climates, e.g. ski  lodges, or where there is a wet process  in the room below, e.g. a spa pool. In  these cases a vapour barrier installed  immediately behind the internal lining  materials must be used.

Case studies have shown that the  installation of a vapour barrier (except  in the situations outlined above) can  be counterproductive because any  moisture that does find its way into  the roof spaces (from residual  construction moisture or some solar- driven moisture transfer) is prevented  from diffusing through the ceiling into  the spaces below. When the moisture  remains trapped, significant water  damage within the roof can occur.

4 key points to remember

1. Skillion roof materials must be dry  when closed in.

2. Providing ventilation is not a cure- all for wet skillion roofs irrespec­ tive of the cause of the moisture.

3. Airflow from interior spaces  (including subfloor spaces) into  skillion roofs must be prevented.

4. Vapour barriers are not generally  needed although an air barrier is.

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Figure 1: Air barrier over a boarded ceiling lining in a skillion roofs.