From the Welfare Codes for pigs, for outdoor pigs :
All arcs and huts used should be liberally provided with bedding and have a warm, draught- free lying area. This is especially important for the sow and litter at farrowing, during the suckling period and for newly weaned pigs.
You should properly maintain arcs and huts, especially to ensure that damage through handling or by the pigs does not produce sharp edges that may injure the animals.
Adequate shelter must be provided to protect the animals from extreme weather conditions. Arcs should be well fixed to the ground, particularly in cold windy conditions and should be sited so that the doorways can be adjusted to allow for changing weather circumstances. Wet conditions create greater welfare problems than the cold, as moisture is easily carried into the arcs on feet and bodies, causing chilling in piglets, and provides the ideal environment for micro-organisms to flourish.
The Welfare of Farmed Animals (England) Regulations 2000 (S.I. 2000 No. 1870), Schedule 1, paragraph 17 states that:
Animals not kept in buildings shall, where necessary and possible, be given protection from adverse weather conditions, predators and risks to their health and shall, at all times, have access to a well- drained lying area.
Adequate shelter must also be provided to protect the animals from the sun in summer. Wallows should be provided for breeding stock to allow them to cool themselves and to produce mud that can help prevent sunburn.