Don't do it! Sure, it’s a quick fix and it works for a while. But now your repair is costlier because that slathered on tar must be removed before the original leak can be repaired! It also just looks bad. So, if you see what looks like tar on your roof it’s because there was (or still is) a leak that was not properly repaired the first time.
A pretty green moss may give a lakeside cottage a quaint look, however plants growing on a roof's surface will eat into the roofing material. This causes premature failure of the material and also slows the drainage of water. Products like 'Wet & Forget' can be sprayed onto a roof to temporarily remove moss, mold, mildew, and algae stains.
It’s always a good idea to find out if the building you are interested in purchasing was or is being heated by using fuel oil. Many parts of New Jersey do not provide for natural gas as a fuel source. Tanks used to store fuel oil for the homes using this source were/are either buried in the ground or are installed above the ground outside of or inside the building. Underground, buried fuel oil tanks have a history of leaking causing significant environmental problems which require significant dollars to clean-up.
It’s also a good idea to have a discussion with the building owner, the building department or the gas utility to determine if a fuel oil tank or fuel oil heating system was ever installed. Having the property scanned for a possible buried fuel oil tank if unknown or having a known buried tank and soil tested is recommended.