This is a term of the thatcher’s trade. A yealm — in older works spelled yelm — is one of the individual bundles of straw, a sort of straw tile, that’s laid on the roof. A writer in East Anglia in 1825 explained that a yealm was the largest quantity of straw that could be carried under the arm at one time.
As befits such an ancient trade, yealm is Old English, spelled then as gielm, gylm and in other ways. Its first sense was of a sheaf of reaped corn (wheat or barley) and only later changed to mean the long straw that remained after threshing. It has often been confused with halm or helm in the same sense, and with haulm for the stems or stalks of peas, beans, potatoes and other crops that remain after harvesting. However, these last three are from a different Germanic source which comes from an Indo-European root that appears in Latin culmus, a stalk, and Greek kalamos, a reed.
Yealm doesn’t often appear outside technical descriptions of thatching. This is a rare example, from a novel:
Luxuriously full, the cat slept on the window-ledge. Meantime a roadman was cleaning a gutter, a thatcher pegged down his yelm.
In a Green Shade, by Maurice Hewlett, 1920.
The yealms are fixed in place by hazel sticks called brotches, a word that was once commonly spelled broach or broche and which could mean a pointed device of several kinds. It’s the same word as brooch for the ornamental pin. The Oxford English Dictionary, in an entry that was written rather more than a century ago, says of brooch that the differentiation of spelling from broach was recent “and hardly yet established”.