A great deal of development has happened in the last two decades, with TAMS and its predecessors playing a large part in this. However, a lot of farmers have more modest developments in mind now, and lean-tos on to an existing A-frame shed are once again becoming common.

Grafting on

Grafting on is an acceptable practice under S.101 (Department of Agriculture farm building standards), meaning it is permitted for TAMS III builds. The stanchion (RSJ or upright girder) of the existing building, to which the stub stanchion of the new building is being grafted, should be of sufficient size for both the existing building and the new one.

Sheds with a span of 7.6m or less (25ft) must use H-frame steel of 200mm x 100mm (8in x 4in) for the stanchions and rafters where the eave height is no greater than 4m, but where the height is between 4m and 5m, this must be upgraded to 220m x 110mm for stanchions.

For spans between 7.6m and 9.4m (25ft and 30ft), the minimum specifications rise to 220mm x 110mm in the stanchions and rafters for 4.8m bays at 4m to the eaves.

With wider bays or those at 5m, eave height requires 240mm x 120mm stanchions and rafters. Sheds up to 12.2m (40ft) require 270mm x 135mm steel trusses and stanchions for both standard and larger bays at 4m eave height, with those between 4m and 5m eaves requiring 300mm x 150mm steel be used.

If the grafted-on girder is insufficient in capacity to take the required span of the new lean-to shed, then an internal girder may be required to reduce the load.

The stanchion and the base connection at ground level should only show signs of minimal corrosion, with the overlap of both stanchions being a minimum 300mm deep.

The connection shall be secured by six 16mm bolts, as opposed to being welded on.

A maximum of one stub (of the same size as the new stanchion) between the old and new stanchions is permitted. Grafting to a stanchion formed from a railway track is not permitted.

The key thing to remember with lean-to buildings is the effect they will have on the existing building; namely ventilation. Grafting on to the side of an existing shed will greatly affect the air inlet spacing if not properly conducted. Where inlet space has been removed by the new shed (space boarding or vented sheeting blocked) then additional air inlet spacing needs to be created.

This should preferably be done within the old shed itself where this is being used for livestock and the new shed used for storage. However, where the new lean-to is being used for livestock housing, then the ventilation should be placed here.

It’s also important to be mindful of sharing air space between the sheds where agitation points are in place. If there is an internal agitation point in the old shed, then grant aid will not be given for a new shed sharing this air space.

It is also recommended that sheep and cattle do not share the same air space due to the risk of air-borne pathogens causing the spread of diseases between the species.

Usage for these types of buildings is generally steered towards storage for straw, hay or machinery, or as temporary animal accommodation such as calving boxes. However, usage will largely be dictated by height of the shed. At a minimum, the roof pitch should be 12 degrees, but even at this, when grafting on to an older building, the offside of the lean-to can be too low for machinery access.

An alternative to this is to use an A-frame structure as a lean-to, as opposed to a straight roof joist, to give extra heights. Round trusses can also be used in this way, though triangular trusses will generally run to the same height as the eave and are not suitable to raise roof height in this case.