In this video

Analyze the existing stair structure. The primary issue is that the old treads (the horizontal part) hung over the risers (the vertical part), creating a gap. The underlying wood is often 'crummy' and unsuitable for direct finishing. The solution proposed is to install new, primed 1x8 pine boards as risers to cover the gap and provide a clean, flush surface for the new treads. Small blocks are shown being used as temporary spacers or permanent supports behind the risers to prevent them from pushing back.