You can claim the maintenance of existing fences against the income you are reporting. New fences are regarded as capital in nature and cannot be claimed as a current expense. You will show this as a cost of additions and claim CCA if you choose to do so.
Here is a list of CCA classes from the CRA : Capital cost allowance (CCA) classes - https://bit.ly/2J37ORB
Here is a list of Current versus Capital expenses from the CRA: https://bit.ly/2NRhY4B
And from TurboTax if needed - How do I enter my rental income and expenses in TurboTax? - https://turbotax.community.intuit.ca/replies/2647946
