Sheds
A good shed is a safe, quiet and comfortable place for your guinea pigs to live in.
Safety and Security
There should be secure locks on the doors and windows and maybe even a shed alarm installed (available from DIY stores). A "beware of the dog" sign on the main door, even if you don't actually have a dog, is also a useful intruder deterrant. There should also be a fire extinguisher installed by the main entrance (also available from DIY stores), and this should be serviced at regular intervals.
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Comfort and Wellbeing
Guinea pigs should have access to natural light and so your shed should have windows. If the windows can be opened this will also provide fresh air, although if your windows cannot be opened then DIY air vents (again, available from DIY stores) installed into the doors will do the same job (a joiner should be able to do this for you). The shed should have methods for warming and cooling the temperature at various times of year. Electricity in the shed is not essential, but it allows the use of lighting, fans and heaters.
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Storage
Your shed should have storage space for your hay, feed and bedding. Somewhere to keep other items such as towels, grooming equipment and spare bowls and bottles is also useful. I recommend keeping your medical box inside the house with you.
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Dealing with furry intruders
Steps must be taken to ensure any holes in the skirting boards and walls are blocked to prevent mice and rats from gaining entry. Check regularly behind bins, hay bales and your storage areas for new holes. Mice are incontinent, which means they urinate as they move around and increase the risk of disease and infection.