11 April 2018

What is BS 5250 and how does it relate to pitched roof design?


BS 5250 - currently the 2011 edition, with an amendment in 2016 - is the code of practice for control of condensation in buildings. It provides building designers with guidance on safeguarding the health of occupants by considering likely sources of moisture, avoiding the build up of excessive moisture, and preventing mould growth and condensation.

What are the sources of moisture?

Moisture can be generated internally by building occupants, or enter the building from external sources. It can also be present within the construction if the building has not been allowed to dry sufficiently.

The building fabric should provide a weather-tight envelope to minimise or prevent the ingress of moisture from external sources, and BS 5534 deals with the contribution of pitched roofs. The recent amendment to BS 5534, plus the introduction of BS 8612, has sought to improve the product quality and standard of installation of dry fix roofing systems.

Internally, moisture is generated by building occupants and their activities. Perspiration, respiration, cooking, cleaning and washing; all can generate significant quantities of moisture that needs to be removed from the building, usually by ventilation.

BS 5250 acknowledges that occupants rarely use a building in the manner intended by the designer. It recommends adopting “fail safe” solutions to account for the likely difference between theory and reality.

How is moisture controlled?

Traditionally, open fireplaces and chimneys achieved a property’s ventilation needs. Although the internal air was constantly refreshed, the downside of uncontrolled ventilation was to make it harder to heat the building and keep it warm.

Modern construction practice aims to achieve much higher levels of airtightness. The idea is to prevent the leakage of warm air through the building fabric, and instead refresh the air supply by controlled ventilation.

For any construction project, designers and contractors should be aware of the likely level of airtightness that will be achieved so that an appropriate ventilation system can be specified accordingly.

Why does condensation occur?

When warm air carrying moisture vapour leaks through the building fabric and bypasses the thermal insulation layer (or the ‘thermal envelope’), it risks coming into contact with cold surfaces. If the air cooled to the extent that it can no longer hold all of the moisture vapour it carried, that moisture is deposited as condensation.

Nearly all construction materials also allow moisture vapour to diffuse through them, to differing extents. For this reason, the airtight layer is often also vapour tight, again with the aim of using ventilation to control the movement of moisture in the building.

On a minority of projects, the materials used in the construction are all vapour permeable (or ‘breathable’). Their deliberate specification allows moisture to move through the construction freely, with appropriate levels of ventilation at the outermost layers of the building fabric to finally remove it to the external air.

How does BS 5250 relate to pitched roofs?

Due to the natural movement of air within a building, some moisture vapour is likely to reach the roof space - especially where insulation is laid along the horizontal ceiling. The question then is how to specify the roof to deal with that.

Generally speaking, the specification of ‘breathable’ underlays and pitched roof ventilation in combination is not what it could be. The extent to which the ceiling below is made airtight, and therefore how much moisture can reach the roof space, should also influence the choice of underlay and the provision of ventilation.

Among other things, BS 5250 gives guidance on general design principles for buildings, the role of air and vapour control layers, and the vapour permeability of common materials. Annex H deals specifically with design principles for roofs, including pitched roofs, and should be used to check a proposed roof design against good practice.

It also specifies minimum levels of roof space ventilation. Designers should confirm with the manufacturer of their chosen ‘breathable’ membrane that it provides the required air and vapour permeability, and be confident that the required level of ventilation has been designed for and can be achieved on site.

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