Off-campus volunteer work for which a person would not normally be paid, which is part time and incidental to the main reason you are in Canada (to study), generally does not require a work permit. Examples of this type of work might be volunteering for a food bank or acting as a mentor to a child.
If the off-campus volunteer work is an activity for which a Canadian citizen or permanent resident might normally be hired, or if it is considered "competitive" within the Canadian labour market, it will generally be considered employment and you will require a work permit (or you will need to meet the eligibility requirements to work using your study permit).
For more information, please review the Immigration, Refugees and Citizenship Canada website. You may also contact an International Student Advisor, Immigration Specialist at International Services for Students if you have questions about work permit requirements for volunteer positions.
For volunteer opportunities, check out SFU Volunteer Services.