It's going to be a scorcher this coming weekend, and it's been a while since we've had a good rain. Typically, established ecoroofs can handle the extreme temperatures of Portland summers without much irrigation. If you have a new project (installed in the last 12-24 months), the plants are still getting established so you might consider irrigating them.
The Ecoroof Handbook recommends no more than 1/2 inch of water per 10 days during the first 2 years. This is a good rule of thumb for a simple sedum ecoroof. Amounts may vary based on plants and growing media, so consult your ecoroof installer for their recommendation.
Happy August and stay hydrated!