Can you explain clearly this please

Mrs. Rusholme engaged the firm of Saunders and Watts Ltd. to refinish floors in certain rooms of a home owned by her and her husband in Red Deer, Alberta. She told Mr. Saunders that she and her husband would be away on vacation and that she would leave a key for the workers with her next door neighbor. She asked Saunders to leave the windows partially open for ventilation and to give the key back to her neighbour who would close the windows when the floors were dry. The Rusholmes' house was serviced by gas. Among the appliances was a gas stove in the kitchen with a pilot light (a small flame permanently burning which will ignite the burners or oven when the burners or oven are turned on) which could be turned off manually. The downstairs floors of the house, including that of the dining room, were to be sanded; the dining room was next to the kitchen with no door between the two rooms. After sanding the floors, Saunders and Watts applied a sealer that they had bought from Standard Household Products Ltd. The label on each can of sealer contained the following warning:

DANGER-FLAMMABLE

Do not smoke. Adequate ventilation to the outside must be provided. All spark-producing devices and open flames (furnaces, all pilot lights, spark producing switches, etc) must be eliminated in or near working areas. Avoid prolonged breathing of vapours. Wear a vapour cartridge mask. Replace cartridge before saturation.

On completing their work Saunders and Watts discussed what they should know about the doors and windows they had opened to allow the vapour to escape. Saunders recalled a reference to a recent case he had read about in the newspaper in which repairers who had left doors open after they had finished their work had been held liable in damages when thieves had entered and stolen valuable contents. The two men then closed all the doors and windows and left the key on the kitchen table beside the stove.

About 8 hours after Saunders and Watts left, an explosion occurred in the house and it was seriously damaged. On returning home from their vacation the Rusholmes found their home in ruins, and were dismayed to discover that they had neglected to renew the fire insurance policy, which had expired only a few days before they had left on vacation.

The Rusholmes brought an action for damages against the firm of Saunders and Watts Ltd. and against Standard Household Products Ltd.

As you have been retained by the Rusholmes as their lawyer - advise them on what unintentional tort would most likely be the basis of their lawsuit and what has be established by them in order to be successful. Please anticipate any defences that either defendant could raise and whether vicarious or primary liability applies.