Ventilation advice for a new cattle shed

An Irish Farmers Journal reader from Co Down contacted me with some queries on ventilation in a new cattle shed he is planning.

He wondered which option was preferable – a central opening in the roof ridge, with canopy, or spaced sheeting.

One of his queries was whether the spaced sheeting would have to extend over the feed passage side of the apex roof as well as the penned side.

Outlet openings

The openings at the top of a roof, whether a centre ridge opening, spaced sheeting or other, are not intended to let in fresh air. They are to let out the warm, moist and stale air cattle or other stock breathe out.

As well as being important for animal health, this protects the steel and timber components of the shed and prolongs their lifespan.

I have been in a number of relatively new sheds where white mould was visible on the timber purlins due to moisture.

Both ridge openings and spaced roof sheeting are effective when installed properly. Both are acceptable on the Department of Agriculture’s S101 general specification for agricultural buildings.

There are a few points to note. A ridge opening should run along the full length of a roof apex. Wider sheds require wider openings, as follows:

A ridge canopy is not recommended in S101, although it is not prohibited. Instead, the Department specification strongly recommends an angled upstand (see diagram). An upstand actually improves the ventilation (wind blowing over the upstand creates a suction effect) and it will keep out most rain.

Keeping the upstand 50mm (2in) back from the end of final roof sheet ensures any rain dripping down the inner face of the upstand is directed out onto the roof and onwards to the gutter.

A canopy does not create this suction effect and, in cases, may in fact hinder upflow of stale air through the ridge opening that it covers. To prevent it from blocking air escaping, the canopy must be raised high enough.

The gap under the canopy must be 275mm (11in), 350mm (14in) and 425mm (17in) respectively for the three widths of shed outlined.

Despite the Department’s reservations, most farmers choose to put a canopy over a ridge opening, often on the advice of the building contractor. The presumption is that a canopy will keep out rain.

In fact, a canopy will let in some rain, depending on wind. Keeping the canopy low, or wide, to keep out rain is likely to reduce the outflow of stale air.

In practice, most canopies I see are fitted 10in to 12in above the roof sheets.

Spaced

Spaced sheeting has become very popular with farmers, particularly over slats, and works well.

The gap must be wide enough. A 30mm gap is recommended for beef and sheep units, 20mm for cow units, and 12mm is recommended over the calf creep area in suckler units.

In most sheds I visit, the gap is at least 25mm and usually bigger.

With a spaced sheeting roof, the first two sheets at gable ends should be overlapped, but all other sheets should have a space between them.

Overlapping of sheets (ie on a conventional roof) boosts the strength of the shed roof. Therefore, it is important, if going for spaced sheeting, to follow manufacturers’ instructions on gauge and fixings.

The sheets are rolled so that when the sheet is placed with the external side facing out, the edge on both sides is turned upwards, effectively a miniature upstand.

Spaced sheeting is straightforward to fit and usually not accompanied by a ridge canopy. This saves on cost.

Spaced sheeting lets in a lot of daylight – certainly far more than a ridge opening covered with a wide canopy. This keeps a shed bright. In fact, under S101, translucent (clear) sheets can be omitted where spaced sheeting is used for the entire roof.

A ridge opening is a useful access route for birds – spaced sheeting is not.

The Co Down reader wondered if spaced sheeting would be required on the side of the building where there will be a feed passage but no pens.

My own view is that it would be best to have spaced sheeting on the whole roof, or else to install a ridge opening as well. Otherwise, one half of the shed would not have an effective outlet for any stale air that arises.

Single and round roofs

I have looked at many single (monopitch) roofed sheds built over the past few years which do not meet the S101 specification on ventilation.

This is where the roof sheeting is overlapped (ie not spaced) and there is a canopy fitted at the front over the feed face, sloping back down, to keep rain out. Warm, stale air will rise over the cattle but has no escape opening at the highest point in the roof.

Because these sheds are open at the front and generally don’t have a wide span, there are unlikely to be big problems.

However, S101 requires there to be an opening of at least 275mm (11in) running the length of the shed, near the highest point.

Spaced sheeting is required for new round-roofed sheds housing livestock. A former hayshed, with overlapping roof sheets, can be given ventilation outlets by raising two sheets in each bay by at least 275mm (11in).

Inlet openings

Stale air will only flow out properly if there are adequate openings to allow fresh air in to replace it.

The original, basic design of air inlet is an open gap running under the eaves for the full length of each side of the shed, or along the lower side of a single roof.

Note that the size (depth) of the opening is the same as the gap required at the ridge opening (see measurements above).

Vented sheets

Vented sheeting has now replaced clear openings for new sheds. It has the advantage of slowing wind and keeping out rain. Vented sheets are placed under the eave and should be at least 1.5m deep.

S101 strongly recommends using vented sheeting at the gable ends of cattle and sheep sheds, particularly where the span is wide.

Spaced (Yorkshire) boarding or plastic net, etc, can also be used in the side inlet gap. Again, they require a minimum depth of 1.5m under the full length of the roof. Boarding should be 25mm thick and of 75mm (3in) maximum width. The gaps between the boards should be at least 25mm. Spaced boarding can also be used on gable ends.

Under S101, where the inlet ventilation of an existing shed is reduced by new buildings alongside, then the inlet ventilation capacity in the new shed must be increased sufficiently to ventilate both sheds simultaneously.

This is a common scenario where a new shed is attached to the side of an existing one.

The ventilation spacings in the new shed must be sized for the combined structure and not just the new part.

This may involve removing cladding on the side/gable of the original structure that is now shared with the new building.

It may require increasing the inlet ventilation of the existing building on its unrestricted side.

A sliding door fitted on a shed sidewall should be made up with the same inlet ventilation as fitted on the sidewall, for example using vented sheets.

Under S101, spaced sheeting is not permitted for air inlet in a livestock shed. Neither is cladding which is angled outwards at the bottom to form a gap between the bottom of the sheet and the wall. These do not provide enough inlet openings.