You can test how effective a particular filter is at blocking visible light. If you look through it at a bright light bulb, and it looks like a dim glow, while you can't see anything else in the room, then the filter is appropriate for looking looking at the sun
with respect to visible light. That says nothing about how good it is at blocking UV or IR, of course.
Since you can see things much dimmer than a lightbulb through the glass roof, it's nowhere near good enough at attenuating visible light. Which on its own rules out its use as a filter.
Apart from professional solar filters, there are a few other options.
1) Welding helmets / goggles are well recognized as being effective for viewing eclipses. They're designed specifically to protect your eyes from bright glows from hot incandescent plasma, which is what sunlight is. Of course, if you go outside wearing a welding helmet people will look at you like you're a weirdo.
2) Smoked glass (which passes the visible light test) also blocks sufficient IR and UV to be used; long ago, this was the primary means used for viewing eclipses. However, it's somewhat dangerous because the smoke coating is fragile and easy to damage, and it's hard to get the coating uniform.
3) CDs
can be effective filters, as determined in testing; the reflective aluminum coating also reflects UV and IR. However, there's a huge amount of variation in how much light transmits through CDs. If you test a CD first and it passes the light bulb test, then it should be fine... but really, are you really that cheap that you'd do that rather than order glasses?
A number of things suggested as filters should not be used. For example, floppy disk cores are suboptimal at blocking IR, and they distort the sun anyway. Most photography filters should not be used; they transmit way too much IR. Photographic film should not be used either.
