<li>Perforated pipes should be continuously perforated, smooth interior HDPE (or equivalent material) with a minimum inside diameter of 100 mm. Where freezing is a concern, horizontal underdrain pipes should be over-sized to a minimum 200 mm diameter. All underdrains should be capped on the upstream end.</li> | <li>Perforated pipes should be continuously perforated, smooth interior HDPE (or equivalent material) with a minimum inside diameter of 100 mm. Where freezing is a concern, horizontal underdrain pipes should be over-sized to a minimum 200 mm diameter. All underdrains should be capped on the upstream end.</li> |