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Before permanent installation, dry-fit each new tread and riser combination. Check everywhere for a good, tight fit and minimal gaps. While tiny gaps can be filled later, strive for as few as possible to avoid relying on excessive filler. To ensure a gap-free fit at the bottom of the tread against the previous riser, place cardboard shims at the top of the newly installed riser. Apply a generous amount of strong construction adhesive (like PL Premium) to the underside of the new tread, then press it firmly down onto the existing tread and against the riser. Use a nail gun to secure the tread in place by firing nails into the tread and riser.