Thank you for your question, Brooks.  Are these to be planted in native soil, or containers?  That's important to selecting the right mature plant size.  It's also important to know how much sun they get, whether the soil drains, and whether there are condo restrictions on species and/or size.  I suggest that you use the following plant finder to select the appropriate species for this specific location:  http://pnwplants.wsu.edu/Search.aspx
Good luck!

Brooks:  I am somewhat leery of providing suggestions because I am a Master Gardener, and authorized only to provide suggestions on home horticulture.  I have tracked down a photo of your roof garden; the structures seem to be few and far between, and, from what I can see, have juniper which will, in time, become rootbound, and their roots will cease to have enough space to take up enough water and nutrients.

Perhaps you can answer a few more questions:  how often is water and fertilizer applied, and by whom (homeowners, committee members and/or landscape professionals)?  Does your HO committee want to stick with landscape plants, or are they interested in a "green roof" that can produce vegetables and fruits (in addition to cooling the surface to reduce heat on the upper floors)?  Here's an OSU article about some of the considerations for doing that:  https://extension.oregonstate.edu/catalog/pub/em-9202-vegetated-roofs-low-impact-development-fact-sheet  (Note that there are load and construction necessities in order to do this safely.)   The Portland Building has an eco-roof, but nowhere for tenants in the building to use the space for socializing, etc.  If you could please let me know of your goals, that would be helpful.  But, if you're just looking for replacements for the boxwood and the junipers, this searchable website can help you match your roof's environment with many native plant possibilities:  http://pnwplants.wsu.edu/Search.aspx

Brooks: your boxwoods may be suffering from several diseases,  effects such as underwatering and/or overwatering, or sunburn.  Ditto the junipers.  What do you mean that "they don't let residents up there"?  Who are "they"?  If this is a common space, it should be available to all owners.  You probably don't have a "green roof" because it isn't designed to be eco-friendly.  (Quote's aren't helpful.)

I suggest that you look through the "Shrubs" listed in the following, so you can see what they look like, how large they get (many at least as tall as treed), and what their sun and water needs are.  https://extension.oregonstate.edu/catalog/pub/ec-1577-gardening-oregon-native-plants-west-cascades#selecting-plants

If these plants appear to become too large for the space, you might consider some of the "Perennials and ferns" listed further into the article.  Groupings of them with similar needs--especially water--could be far more interesting than shrubs.

I hope this is helpful.

Welcome!