The story is from Procopius, who specifies why the roof was valuable. From Book III of On The Wars: "[Geiseric] also plundered the temple of Jupiter Capitolinus and tore off half of the roof. Now this roof was of bronze of the finest quality, and since gold was laid over it exceedingly thick, it shone as a magnificent and wonderful spectacle."

That agrees with Plutarch, who writes that Domitian spent a fortune gilding the place over 300 years earlier: "the greatest wealth now attributed to any private citizen of Rome would not pay the cost of the gilding alone of the present temple, which was more than twelve thousand talents." Zosimus says that Theodosius I cancelled all government funding for non-Christian religious rites shortly before he died in 395, so it's possible that the roof was maintained until that point. If Zosimus is to be believed, Rome's temples were still in use at that point, if a little faded from their former glory (he writes that there was only one elderly Vestal Virgin left, who was powerless to stop Stilichos' wife stealing some necklaces off a statue of Rhea).

In any case, it seems that Vandals stole the valuable metal from the church roof, which remains a problem to this day.