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

BS 5250 - currently the 2016 edition  - is the code of practice for controlling 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.

BS 5250 states that the designer should take account of the following moisture sources in buildings:

  • Water incorporated during the construction process (including precipitation)
  • Precipitation after construction
  • Water vapour arising from the occupants and their activities
  • Temporary condensation occurring when cold weather conditions are followed by warm, humid weather

The building fabric should provide a weathertight envelope to minimise or prevent the ingress of moisture from external sources, and BS 5534 deals with the construction of pitched roofs. The 2018 amendment to BS 5534 and the introduction of BS 8612 have 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 can all 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 refreshed continuously, the downside of uncontrolled ventilation was that it was 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 with 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 cools to the extent that it can no longer hold all of the moisture vapour it carries, that moisture is deposited as condensation.

Condensation in roof spaces has become more of a problem with the increase in highly insulated buildings. Moreover, changes in lifestyles have led to higher levels of water vapour in modern buildings. This water vapour naturally ascends to the roof space, where it condenses on contact with cooler surfaces. Further condensation is likely to be caused by climatic conditions, and may eventually result in timber rot, metal corrosion and damage to insulation and fittings.

Nearly all construction materials 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 structure freely, with appropriate levels of ventilation at the outermost layers of the building fabric to 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.
In a pitched roof application, the specification of a breathable or non-breathable underlay, along with the appropriate low (eaves) and high (ridge / tope edge abutment) level ventilation, will provide the necessary build-up to combat the risks of condensation. The extent to which the ceiling below is made airtight, and therefore how much moisture can reach the roof space, can 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.

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.

BS 5250 also emphasises the importance of well-sealed ceilings to curb the transfer of moisture into a roof space by means of moisture-laden air. This means:

  • The avoidance of gaps and holes in a ceiling
  • The sitting of access doors or hatches into the roof space away from moisture-producing areas such as bathrooms or kitchens
  • That hatch covers must be effectively sealed
  • High levels of workmanship

For more guidance relating to condensation, moisture and ventilation in roofs, refer to: 

  • Approved Document C: Site preparation and resistance to contaminants and moisture - Section 6
  • Approved Document F1: Ventilation requirements to maintain indoor air quality 

To discuss the implications of BS 5250 on your next roofing project, contact your local Marley specialist who will guide you every step of the way.

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