Caesar Roose lived his life on the Waikato River. The elder son of English-born Mary Ashley and her German husband, Ceasar (Caesar) Henry Roose, he was born on 29 July 1886 at Mercer, on the 67-acre Tuoro Island, which was owned and farmed by his family. He spent much of his early life milking cows and delivering vegetables, milk and cream to Mercer customers. He also spent time whitebaiting, eeling, duck shooting, rafting logs, and canoeing – he had learned to paddle a canoe at the age of five, and later rowed competitively. Caesar earned some money working as a commercial photographer and borrowed £100 to buy his first boat in 1902. Two years later he ordered the Rawhiti from the shipbuilding firm of Bailey and Lowe.
In 1906 Roose bought one of the numerous flax mills along the Waikato River. He earned his river steamer master's certificate in 1909 and his engineer's certificate in 1911. Two years later, as his shipping business flourished, he had a five-bedroomed kauri homestead built on Tuoro Island facing Mercer township. In 1915 Roose inaugurated a regular river service between Port Waikato and Cambridge, but the following year he sold his vessels – two steamers, three launches and seven barges – to the Waikato Shipping Company. In 1918 he built and launched the Aurora, a charter vessel which catered for duck-shooting parties and picnickers. After the First World War, during which he served at Trentham and Featherston military camps, Roose bought timber mills at Mercer and Katikati; he became a major exporter of timber to Australia and later supplied a box factory in Tauranga with kahikatea.
Caesar Roose realised the vital importance of the river trade to both the Maori and Pakeha communities of the Waikato region. He had a lifelong association with Waikato tribal leader Te Puea Herangi, with whom he had attended Mercer School, and in 1921 he helped transport her and her people from Mangatawhiri, near Mercer, to the new Turangawaewae marae at Ngaruawahia. The following year he founded the Roose Shipping Company. With six of the Waikato Shipping Company's former vessels, he established regular services on the Waikato River and its tributaries, such as the Waipa River, and encouraged trade through Port Waikato. In 1924 he visited Britain, where he ordered the construction of a 210-foot paddle steamer, also named Rawhiti. The largest vessel to operate on the Waikato, it entered service between Port Waikato and Hamilton the following year.
On 3 March 1931 Roose married Australian-born Gladys Ethel Fortescue Wiseman (née Hoare) at Glendale, California; their only child, a daughter, was born in 1934. He spent his honeymoon observing oil wells and shipping in the United States and Europe, and while in Germany purchased the Argus (later the Holmglen ) on behalf of the Holm Shipping Company, in which he was a major shareholder. He began a ferry service at Mercer in 1932 and the following year patented the Roose–Atkins Grab, used for coaling ships, loading and unloading barges, and salvage work; Roose Shipping Company manufactured the grabs in a workshop on Tuoro Island.
The Roose Shipping Company became involved in a range of other activities: it purchased a coalmine in 1922 and established one of the country's first open-cast mines in 1945; it built Hamilton's Fairfield Bridge (1933–36) and helped to build the Ngaruawahia traffic bridge (1953–54); and it established its own trucking fleet. Caesar and Gladys divorced in 1946, and on 8 April 1947 he married Fanny Hill in Auckland; there were no children of this marriage.
In 1947 Roose travelled to Hawaii to purchase an American tank-landing ship, which became the third Rawhiti. Probably the first roll-on-roll-off vessel in New Zealand, it was capable of carrying up to 3,000 tons of cargo, and traded throughout Australasia and the Pacific islands. The following year he founded C. Roose (Fiji) Limited, but after the 1951 waterfront dispute he was forced to sell the Rawhiti. From 1947 to 1961 Roose served on the Auckland Harbour Board. In the 1950s he became interested in oil exploration in Taranaki, and helped to launch Egmont Oil Wells Limited.
Regarded as an independent thinker, a man of vision and high ideals, Roose was generous to the causes he believed in. Throughout the 1920s and 1930s he had worked with Hilda Ross and W. H. Paul to set up a children's health camp at Port Waikato. He gifted land to the Mercer Rowing Club for a new pavilion, and in 1965 offered £50,000 towards a bridge to replace the Mercer ferry. Named the Caesar Roose Bridge, it was opened in 1972.
Roose remained a strong advocate of water transport as a viable alternative to road and rail: during the 1960s he campaigned vigorously for the dredging of the Waikato River and the building of the Waiuku–Waikato River canal for shipping and flood control. He died in Epsom, Auckland, on 6 July 1967, survived by his daughter; Fanny had died in 1956. He is buried at Mercer public cemetery, beside his beloved Waikato River.