One of the most common subfloor issues is lack of rigidity. The subfloor could be lacking proper support from the framing underneath, wrong thickness, or both. This weakness in the subfloor will cause sanding machines to vibrate during sanding, and could lead to chatter marks. Other issues might develop over time as the floor is walked on, creating micro deflection with use. The finish might stretch and develop tear lines on the seams, or just flake off from lack of adhesion.
Working predominantly over wooden subfloor systems, we often deal with solidness/fastening issues. Poor subfloor fastening leads to excessive movement of the subfloor and flooring. This movement can allow for machine wheels to dip, creating side cuts and digs, which take extra attention to flatten. It also can mean filler may not hold well in gaps, and works its way out, often before sanding is completed. We try to recognize and avoid filling seams in these floors. Movement also often means squeaks, which are a nuisance to most anyone. The best time to address them is prior to refinishing the floor.
A common subfloor condition that can cause sanding issues is excessive movement due to deflection or loose subfloor boards. Excessive movement can cause chatter or wave. It’s always advisable to ask questions about how the subfloor is constructed or look at the subfloor construction (when you are able to do so). A good question to ask either the builder or homeowner is, “How old is the home?” Older homes may have a solid board subfloor. These subfloors were usually laid on a diagonal and fastened using common nails without the use of adhesive. The wood flooring that was laid over top of these subfloors was laid perpendicular to the floor joists and nailed using common or flooring nails driven in by hand. When this type of floor was hand nailed, they drove the nails directly into the floor joists (16” on center) to avoid the “bounce back” that would occur if you tried to drive the nail in between the floor joists. These boards may become loose over time and if the wood floor has been sanded a few times, the amount of deflection or vibration increases.