Riach, too, said the new Forsyth Barr Stadium had shown the way for the South Island.
In the meantime the Crusaders will continue to base themselves at their new temporary stadium in Addington because any new stadium will take more than five years to build.
"Our view is the what, not the where," said Riach, adding that a capacity of 30,000 would suit the Crusaders.
"The recommendation in the draft plan is uncovered and 35,000 and we're saying we'd be perfectly happy to drop some capacity in order to achieve the roof, as a balancing factor or a compromise. We just think the opportunity to do that, particularly in the South Island, is the future proofing which would make it all worthwhile."
Council staff have listed six options for the new stadium, which will be on the former Lancaster Park site. Four are rectangular like the Addington layout, and unlike the Crusaders' old base at the condemned AMI Stadium, which had a capacity of 42,000.
Riach said he was reassured to see a rectangular model, rather than an oval-shaped one which did not suit rugby, as the preferred council option.
The deciding factors, as ever, are likely to come down to funds, with the Crusaders' preference costing $35 million more than the council's.
However, the predicted cost of a 30,000-capacity indoor facility - about $109 million - stacks up well compared to Dunedin's stadium, which cost about $200 million and has a similar capacity.
A roof would also make the stadium more attractive for concerts - as the Dunedin venue has proved - and other sports teams such as the Warriors and Phoenix.
THE OPTIONS
* Repair Deans Stand and replace Paul Kelly Stand. Capacity: 35,000. Cost after insurance: $49.6m. Build time: 4.5 years.
* Rebuild like for like. Capacity: 39,859. Cost after insurance: $66.6m. Build time: 4 years.
* New uncovered, rectangular stadium. Capacity: 30,000. Cost after insurance: $39.6m. Build time: 5 years.
* New covered, rectangular stadium. Capacity: 30,000. Cost after insurance: $109m. Build time: 5.5 years.
* (Council staff recommendation): New uncovered, rectangular stadium. Capacity: 35,000. Cost after insurance: $74.6m. Build time: 5 years.
* New covered, rectangular stadium. Capacity: 35,000. Cost after insurance: $144.6m. Build time: 5.5 years.