Value of sales fell from £66.9m to £53.9m, while volumes fell from 3.4 million to 2.8 million.
Average prices fell from £19.67 to £19.25.
GfK consultant Paul Mitchell said:
"This year was a challenging year for barbecues, with consistent rainfall through the summer leading to a 20 per cent decline in the market. The fall in sales was equal for both charcoal and gas products."