Most of the time, you shouldn't reuse old underlay. We recommend that you buy and fit new underlay at the same time as your new flooring. Old underlay can disintegrate as it ages, as well as harbour lots of dust and dirt. It will have compressed over time where you've walked, so reusing old underlay will affect the performance of the new flooring laid on top. Your underlay might look in reasonable condition if you lift up the edge of your flooring to check it, but remember that’s an area where there's been no footfall! At Tapi, we can come and have a look at your existing underlay when we measure up to assess whether new underlay is required. If it’s relatively new (laid in the last couple of years) then it should be ok to reuse.