Large construction projects may be delayed across the province.
Twelve thousand Ontario plumbers and steamfitters, with the Industrial, Commerical and Institutional sector, are on strike.
The strike means workers like painters and drywallers won't be able to finish their jobs.
Residential plumbers aren't affected, so if a pipe bursts in your home, you can get someone to fix it.
The workers say the main sticking point in the negotiations, is the move to a five day, 40 hour work week.
Right now ICI plumbers work 36 hours over four days with Friday's off.