Learn Polish: Jestem wściekły, bo te majstry nie mogły naprawić dachu. - I am furious because those tradesmen couldn't fix the roof.

This is the depreciative form in action. Because the speaker is furious, they deliberately use non-masculine-personal (non-virile) grammar for a group of men to show contempt and frustration. They swap out the respectful masculine forms for the non-virile demonstrative te, the non-virile noun majstry, and the non-virile verb mogły.

What is the normal, respectful plural for majster?

The standard, respectful masculine personal plural is majstrowie (or sometimes majstrzy). The form majstry used here is specifically the depreciative plural, which strips away the grammatical "personhood" of the tradesmen to make them sound incompetent.

Why is the roof in the genitive case (dachu) instead of the accusative (dach) when it's the object of naprawić?

This is the "genitive of negation" at work. Even though the action verb naprawić (to fix) is in the infinitive, the main verb governing it (mogły) is negated (nie mogły). In Polish, when the main verb is negated, the direct object of its attached infinitive generally shifts from the accusative to the genitive case.

If a woman were speaking, would the depreciative forms in the second half of the sentence change?

No. A female speaker would change the first part to Jestem wściekła, but the second half would remain exactly the same: bo te majstry nie mogły naprawić dachu. The depreciative forms strictly reflect the speaker's negative attitude toward the tradesmen, regardless of the speaker's own gender.