Complex sentence: Sparks from paper or burned rubbish can ignite dry thatch on the surface around a chimeny, and fires can also begin when sparks or flame work their way through a degraded chimney and ignite the surrounding semi-charred thatch.
Split into simpler sentences: Sparks from paper or burned rubbish can ignite dry thatch on the surface around a chimney. Fires can also begin when sparks or flames work their way through a degraded chimney and ignite the surrounding semi-charred thatch.