Wis. Stat. § 90.12: Apportionment of cost of fence.

When, in any controversy that may arise between occupants of adjoining lands as to their respective rights in any partition fence, it shall appear to the fence viewers that either of the occupants had, before any complaint made to them, voluntarily erected the whole fence, or more than that occupant’s just share of the same, or otherwise become proprietor thereof, the other occupant shall pay for so much as may be assigned to him or her to repair or maintain; the just value thereof which the other occupant ought to pay shall be ascertained by proceeding as prescribed in s. 90.11.