Building design

5 December 1997

Building design

can cut the risk

of pneumonia

Every year stock are lost and production stifled by poor

ventilation in cattle sheds. Royal Agricultural College

senior buildings lecturer Graeme Lochhead looks at how

to improve things

EVERY year livestock production is lost because of pneumonia caused by poor ventilation. Yet well-designed buildings with adequate ventilation, appropriate stocking rates and good management are effective ways of reducing the risk of viral pneumonia outbreaks. Poor ventilation doesnt just mean fatalities; lack of good air movement can cut livestock productivity by decreasing food intake and increasing stress levels.

Temperature

Farmers commonly underestimate the ability of livestock to tolerate low temperatures. Housed cattle and sheep, protected against the adverse effects of the UK winter, are unlikely to suffer cold stress. Well-ventilated buildings, which feel cold to a human, with good air movement often provide a better environment than warm, stuffy ones.

Though temperature is not critical, freedom from draughts is essential – problems often occur with animals catching a chill, which then develops into something more fatal. Solid walls and doors with draught excluders should ensure the elimination of draughts at stock level.

Wet lying conditions can cause stock to lose heat and become more susceptible to disease due to increased humidity in the building. This humidity can be reduced with attention to good ventilation and air movements, adequate stocking density and clean bedding.

Basic requirements of a housing system

The building should provide a dry bed for stock to lie on, freedom from draughts and as much fresh air as possible. Poor ventilation will not provide the essentials and will result in high humidity which can cause:

&#8226 Respiratory diseases and reduced animal performance.

&#8226 Condensation on the fabric of the building and deterioration of the structure.

&#8226 Wet floors and bedding.

&#8226 Unpleasant conditions for stock and stockperson alike.

The main cause of the failure to meet these basic requirements is underventilation of the building, often owing to:

&#8226 The misconception that stock need to be kept warm.

&#8226 The fear of rain or snow penetration.

&#8226 Lack of knowledge about how a building ventilates.

Natural ventilation

Air moves through a building aided by external wind effect, or the stack effect (the effect of hot air rising). This stack effect relies on the surrounding air being warmed by the stock and rising through convection (a similar principle to that of a domestic radiator). As this warm air rises, removing the stale contaminated air, fresher air fills the void left at stock level.

This cycle provides a continuous change of air throughout the building and ensures that it can "breathe" during still days when there is no wind effect. Calculations should be based on the assumption of the worst case scenario, ie windless, stale days, and buildings designed accordingly.

On windy days the wind blowing onto a building with ventilation openings will set up air movements within the building that are of unequal distribution. Side or lower openings will often act as both inlet and outlet and it is essential that these are above stock level to avoid draughts. Care must be taken not to induce a tunnel effect by leaving doors open, which can often carry stale, contaminated air to other locations in or outside the building.

Efficient ventilation

It is essential that the building encourages good ventilation and that openings allow the stack effect to operate efficiently. Attention should be drawn to the following: Size, position and design of openings, span and number of spans of the building, roof pitch, siting of the building and stocking density.

As a rule of thumb, an open ridge of width 25mm (1in) for every 3m (10ft) span of building, with an inlet area twice that of the outlet area, should be provided within a building. These should be evenly distributed within the building, above the height of the stock, and with a maximum possible height between inlet and outlet.

A variety of methods is adopted in the design of openings but the critical factor is the calculated area of the openings. Some typical opening designs range from continuous gap and spaced boarding inlets to open and protected ridge outlets.

The tradition of asbestos crown crank, commonly found in farm buildings, keeps out the elements but is an inadequate outlet and should be replaced. If spaced boarding is used then 100mm (4in) boards with 20mm (0.8in) gaps, or 125mm (5in) with 25mm (1in) gaps, are often sufficient but should be calculated to ensure efficiency.

Care must be taken when considering the air movement of large-span buildings to ensure that the air does not cool before leaving the outlet. In general this is not a problem with building spans less than 22-26m (72-85ft). The actual number of spans has a greater influence on air movements and care must be taken to ensure ventilation of the individual span without the need for shared air space or cross contamination.

This may be achieved by stepping the building at the eaves connection or valley, or by adopting a breathing roof. The standard roof pitch is now 15í and buildings should not drop below this as it will impair both the air flow and the stack effect.

The surroundings of the building play a major part on the ability of the building to ventilate efficiently and the proximity of buildings, trees, and even hay/straw stacks, will affect the air flow both inside and outside the building. Ideally the site should be level, with ridge running from north to south, not in a hollow or frost pocket, not closer than 5m (16ft) from a neighbouring building – although 10m (33ft) is the preferred distance – and away from high buildings and trees.

Stocking density

The number, size and age of stock will govern the need for the ventilation, together with the design of the building and the stack effect. Strict adherence to grouping by age, stocking density and the elimination of cross flows in the building to different age groups (and this can also be from building to building depending on prevailing wind direction) will aid ventilation.

A table showing stocking densities is given and these should be taken as minimum requirements and calculated on the estimated weight of animal at turnout in spring. Commonly, problems occur as the groups increase in size and weight over the winter period, which can be avoided through careful planning.

Building health check

By the time a problem in the building is brought to light, it is often too late and the result may be a considerable cost in time and money. Make improvements in good time. Poorly ventilated buildings are easy to spot – they often smell of ammonia, have cobwebs hanging from the rafters and purlins and have a black fungal growth in the underside of the roof.

At this stage a thorough inspection of the ventilation system should be undertaken, starting with the inlets and outlets. The outlet should be at least 25mm for every 3m span which, in most cases, will be around 250-300mm (10-12in) clear opening with inlets between wall head and eaves supplied by spaced boarding with 125mm boards and 25mm gaps.

Walls should be above stock level and free from overhead obstructions which may deflect incoming air downwards onto the stock. Gable ends should be enclosed to diffuse the incoming air by Galebreakers or equivalent and should not be relied on to ventilate the building, as this can often result in a tunnel effect through the building causing wind chill.

Doors should be fitted with draught excluders and gates sheeted to prevent draughts at stock level. Feed passage doors, shut to prevent the wind tunnel effect, will help ventilation from the sides, creating an even distribution of air through the building.

Check the proximity of the building to external influences such as embankments, trees, buildings and also hay/straw stacks, any of which may inhibit the air flow in and around the building.

To obtain a clear picture of air flows it may be necessary to carry out smoke testing. Look particularly closely at the rate and efficiency of the airs progress from inlet to outlet and try to judge what is influencing its behaviour, such as bad opening sizing, proximity of trees etc. The smoke test will only identify air movement not air capacity, though.

The common problems in poorly-ventilated buildings can be overcome by minor alternations, and these are:

&#8226 Inefficient crown crank ridge, improved by total removal or replacement with open ridge.

&#8226 Poorly-sized outlets, which can be adjusted by widening to clear width of the opening.

&#8226 Poorly-sized inlets, improved by increasing the gap in the spaced boarding.

&#8226 Draughts, reduced by the addition of draught excluders and Galebreakers.

&#8226 Poor inlets and outlets, which cannot be altered, but which can be improved with the modification of the roof to provide a breathing roof.

GOOD VENTILATION WILL:

&#8226 Remove waste air.

&#8226 Remove evaporation from muck, bedding etc.

&#8226 Minimise condensation.

&#8226 Maintain fresh air supply.

&#8226 Keep down micro-organisms.

&#8226 Lessen risk of disease and pneumonia.

Above: Stepped lean-to allowing stale air to escape upwards. Draught excluders on door bottoms, close-spaced boarding and Galebreaker reduce draughts. Left staggered space boarding allows

air-flow to be diffused before entering the building. The boards are fixed either side of the side purlins.

Multi-span buildings in close proximity to other (especially tall) buildings can increase risks of poor ventilation. Trees and stacks can have the same effect.

BUILDINGS CHECKLIST

&#8226 Outlet/size design: 25mm width for every 3m span.

&#8226 Inlet size/design: Twice the outlet size.

&#8226 Side walls: Above stock level.

&#8226 Doors: Below draught excluders, above gale breakers.

&#8226 External influences: Building, trees and straw stacks.

&#8226 Stocking density.

&#8226 Air flow: Smoke test.

An open ridge on this building allows stale air to escape, leaving the void to be filled with fresher, cleaner air. Poorly-ventilated buildings are easy to spot.

Left: An alternative to cutting slots in an asbestos roof is to lift the sheets and fix to 75mm (3in) battens, fixed in turn to the purlins.

The slot runs top-to-bottom of the ridge.

Stocking density*

animalincluding beddedkgarea and loafing/

feeding area (sq m)

2003.0

3003.4

4003.8

5004.2

6004.6

7005.0

*As taken from BS5502 Area allowance for cattle on straw bedded yards

A breathing roof allows multi-span livestock buildings to be ventilated effectively without cross flow or contamination between buildings.