Final answer:
Sills and sleepers on a grounded concrete slab require decay-resistant or pressure-treated lumber to prevent rot and insect damage, especially without a moisture barrier.
Step-by-step explanation:
When constructing sills and sleepers on a concrete or masonry slab that is in direct contact with the ground, the kind of lumber required must be either naturally resistant to decay or should be pressure-treated lumber. This is because wood in direct contact with concrete, which can absorb moisture from the ground, risks rot and insect damage.
To prevent this, either a moisture-impervious barrier should be placed between the wood and the concrete or a decay-resistant lumber like cedar, redwood, or pressure-treated wood must be used. Using pressure-treated lumber is a common practice where an impervious barrier is not practical.