Who is legally responsible for paying to build or repair a shared boundary fence in Ontario?

There's no automatic fixed rule saying neighbours simply split every boundary fence cost down the middle. In Ontario, disputes over who pays to build or repair a shared boundary, or "line," fence are governed by the Line Fences Act, which lets either neighbour ask the local municipality to appoint fence-viewers if the owners can't agree between themselves. The fence-viewers inspect the properties and issue a binding decision apportioning the cost between the two owners, and specifying the type of fence required.

The nuance many people miss is that cost-sharing isn't governed by a single hard-coded formula. Fence-viewers look at the actual circumstances and the benefit each property gets from the fence, so the split isn't automatically fifty-fifty in every case. Neighbours are also free to reach their own private agreement about cost-sharing at any time, and most fence disputes resolve informally without ever reaching the fence-viewer process.

If informal discussion doesn't work, the Line Fences Act process is the purpose-built route to get a binding answer rather than letting the disagreement drag on unresolved, and either neighbour can start it even if the other side is reluctant to participate.