When we see movies of medieval times the siege engines are often dark stained gritty planks. But what would be the actual status of these planks especially when the siege just begon. Wouldn't they new fresh planks?
Also most planks shown are nice and straight, would this be a requirement or would they just cut up slabs and nail them together leaving a lot of gaps. And would they strip all the planks or leave bark etc just to go quicker?
Whilst some smaller sieges engines (ballistae, smaller catapults) were technically portable and redeployable, such an integrated role was abandoned after the Roman era. Therefore, siege engines were purpose-built in their emplacement by highly specialised engineers. Therefore, the wood would have been fresh and the engines assembled with great care, especially considering the weight and tension the wood would have to endure.
(Note: Siege engines were also extremely expensive, therefore their deployment was well planned and somewhat limited. Movies are unsurprisingly ignorant of medieval economics.)