What are roofing filters? Are there different kinds of things that are called roofing filter? Why would a circuit or component be called a roofing filter instead of just a IF bandpass filter? Why is the word “roofing” used? (as it seems to have nothing to do with keeping the rain out of the chassis.)
The roofing filter is the filter through which the first IF must pass. The roofing filter is an IF filter, specifically the first one. AB4OJ provides a block diagram:
As for why it's called a "roofing" filter, Elecraft suggests it's about protecting the subsequent stages from spurious signals:
The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers. Such receivers have an IF above the highest RF band covered; it's typically something in the range of 30 to 70 MHz or higher. But "roofing" as a term should be interpreted as "protective," not "high in frequency." A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. It still provides the same protective function.
Not long ago, it was uncommon to find a high-quality radio that did not have 2 or more IFs. This was largely a consequence of the design constraints imposed by an all-analog design. In this world, a roofing filter would be unambiguous and ubiquitous.
But today, many high quality HF receivers are direct-sampling, so they have no IF at all. Higher frequency receivers are often direct-conversion, so you could say they have one IF of 0 Hz. These designs have been enabled by the recent lower cost and higher performance of digital hardware.
So do these modern radio topologies still have roofing filters? Even a direct-conversion receiver will have some filter, is it a roofing filter?
I would say no: any SDR is probably going to have a preselector and an anti-aliasing filter, but that may be the only analog filter it has. Talking about a roofing filter in an SDR is like talking about a carburetor in a fuel-injected car. It simply doesn't exist, although there are other parts (fuel injectors) which serve a similar function.
But reading the Elecraft article and taking a cursory look through some search results from various amateur forums, I get the impression that manufacturers are broadening the definition of "roofing filter" to include just about any analog filter which is near the antenna, simply because a radio which does not boast an "excellent roofing filter" is not as marketable.