Origins
The word 'roof' is one of the most ancient and etymologically isolated terms in the English architectural vocabulary. It descends from Old English 'hrōf,' which meant roof, ceiling, or summit, from Proto-Germanic *hrōfą. Unlike most major architectural terms in English — 'temple,' 'tower,' 'palace,' 'pillar,' and 'arch' are all Latin or Greek borrowings — 'roof' belongs to the native Germanic word stock, with no certain etymology beyond the Proto-Germanic level.
The Old English form 'hrōf' began with the consonant cluster 'hr-,' which was still pronounced in early Old English as a voiceless 'r' preceded by an aspirate 'h.' This cluster was gradually simplified to plain 'r-' during the Middle English period, a regular sound change that also affected 'hring' (ring), 'hræfn' (raven), and 'hnecca' (neck). By the fourteenth century, the word was spelled and pronounced much as it is today.
Within Germanic, the closest cognate is Old Norse 'hróf,' meaning a boat shed or the roof of a building. Middle Dutch 'roef' (a deckhouse or cabin on a ship) also appears to be related. The nautical associations of these cognates have led some scholars to suggest that the original *hrōfą may have referred specifically to the curved or arched covering of a boat — a structure that, turned upside down, resembles the pitched roof of a building. This connection between boat hulls and building roofs is well attested in Norse culture, where upturned boats were sometimes literally used as roofing.
Old English Period
The word may also be connected to Old English 'hrōst' (a perch for roosting, a framework or rafter), which survives as modern English 'roost.' If this connection is valid, the semantic core of the root would be 'an overhead framework or structure' — applicable to both the roof over one's head and the beam on which a bird perches.
Attempts to trace the word beyond Proto-Germanic have been inconclusive. Some scholars have proposed a connection to a PIE root meaning 'to cover,' but no widely accepted cognates exist in Latin, Greek, Celtic, or Indo-Iranian that would confirm this. The word may be one of the Germanic-only innovations that entered Proto-Germanic from a substrate language — though this too is speculative.
In its long history in English, 'roof' has remained remarkably stable in meaning. From Old English to the present, its core sense — the upper covering of an enclosed space — has not shifted. What has expanded is the figurative and compound vocabulary built upon it. 'Rooftop,' 'roofline,' and 'roofing' are straightforward extensions. 'Sunroof' (an opening in a vehicle's roof) dates from the twentieth century. The phrase 'to raise the roof' (to make a great noise, to cause an uproar) dates from the nineteenth century. 'Under one roof' (within the same building, under the same authority) is attested from the fifteenth century.
Later History
The idiom 'to hit the roof' (to become extremely angry) appeared in the early twentieth century, personifying the roof as a ceiling of emotional containment. 'A roof over one's head,' meaning basic shelter, captures the word's most fundamental function — the absolute minimum of architectural protection against the elements.
The plural of 'roof' is 'roofs' in standard English, though the variant 'rooves' (by analogy with 'hoof/hooves' and 'proof/prooves') appears occasionally and was historically more common. The standard plural reflects a regularization that occurred in early Modern English.