Does the sub floor have to be perfectly level, before installation?
Most timber floors can be laid on floors that are not perfect, you can get away with a sub floor that has a hollow or high spot of no greater than 2-3mm over a two square meter area, example, if you place a straight edge on the sub floor, and there are slight variations, such as hollows, then you should be able to get away with it, as underlay will take up most of the fill, but if you have a situation where the floor is level and say a nail or stone is sticking out the floor by 2mm, then this has to be removed, the variation must be spread over an area of at least two square meters. There are good leveling compounds that are cement based available at most hardware outlets, these are mixed with water and spread over the low points and are self leveling, meaning that they find a straight level and fill that area, its worth going to the small expense of using a leveling compound on a poor sub floor to bring it up to standard.
