 In 1847 – 8 several of the old houses associated with the cathedral on the west side were demolished , and some minor changes were made to the boundary wall . Structural reinforcement of the ruin and some reconstruction work began in the early 20th century , including restoration of the east gable rose window in 1904 and the replacement of the missing form pieces , <unk> , and decorative ribs in the window in the north @-@ east wall of the chapterhouse ( Fig . 15 ) . By 1913 , <unk> the walls and additional waterproofing of the wall tops were completed . In 1924 the ground level was lowered and the 17th century tomb of the Earl of <unk> was <unk> . Further repairs and restoration ensued during the 1930s , including the partial <unk> of some 19th century <unk> ( Fig . 16 ) , the reconstruction of sections of the nave piers using recovered pieces ( Fig . 17 ) , and the addition of external roofing to the vault in the south choir in 1939 ( Fig . 18 ) . From 1960 to 2000 , masons restored the cathedral 's crumbling stonework ( Fig . 19 ) and between 1976 and 1988 , the window tracery of the chapterhouse was gradually replaced , and its re @-@ roofing was completed ( Fig . 20 ) . <unk> , glazing , and a new roof were added to the south @-@ west tower between 1988 and 1998 and comparable restoration work was completed on the north @-@ west tower between 1998 and 2000 ( Fig . 21 ) . 
