Steps to constructing a concrete roof slab


Imagine building a house and constructing the roof first. Hard to picture, eh? Well, on a handful of the Eglinton Crosstown’s stations, that’s exactly how we’re doing it.

Fairbank, Forest Hill, Chaplin and Mount Pleasant stations are being constructed from the top down –  the roof will be constructed first, and rest of the excavation and the station construction will follow. 

Forest Hill Roof Slab Cap Beam Pour

Pictured: The cap beam pour across the tops of the piles. 

This construction method brings a few great benefits. Once the roof slab is poured and hardened, the road re-opens while construction continues below, which ultimately mean less disruption. It also allows the underground utilities to be placed in their final configuration. Lastly, the concrete roof slab increases safety for workers underground.

Forest Hill Station, at Eglinton and Bathurst Road, was the Crosstown project’s first top-down station to install the roof slab. Preparations started in August, and the team poured the concrete roof slab on the night of September 25th, and into the morning of the 26th.

Forest Hill Roof Slab Prep - Mud Slab Poured

Pictured: An initial, thin concrete mud slab is poured and finished.

“Metrolinx, the City of Toronto, and our team developed a solution that will reduce the impact the project has to the travelling public, and provides a better working environment during station construction,” Spencer Cameron, Project Manager, Forest Hill Station. "I’m very proud of the Forest Hill team, our subcontractors and designers for working through the technical challenges encountered in implementing this method.”

Forest Hill Station Crosslinx Team

Pictured: Forest Hill Station team ready for roof slab construction.

Key steps to constructing a concrete roof slab

1. Cap beam pour: Once excavation and piles are complete, a concrete cap beam is poured across the top of the piles. The cap beam evenly distributes the load of the roof slab to the piles below.

2. Grading and Mud Slab: The earth around the excavation is graded and then an initial thin concrete mud slab is poured and finished.

3. Slab Structure: Reinforced steel is placed, and then formwork is installed for the concrete pour. The electrical conduits that will run through the structure and water stops are placed. Finally, a top mat of reinforcing steel is installed.

4. Concrete Pour: Concrete is poured, and then left to cure for approximately seven days.