An exterior door threshold is more than just a strip of material at the bottom of your entry door. It plays a vital role in weatherproofing, insulation, structural support, and aesthetics. One of the most common homeowner questions is: “What should I put under an exterior door threshold?” This guide answers that question in detail, exploring materials, installation techniques, mistakes to avoid, and professional recommendations. Designed as a 5,000-word in-depth blog article, it ensures you understand every aspect of threshold installation and maintenance. Quick Answer The best material to put under an exterior door threshold is a weather-resistant, supportive base such as: Pressure-treated wood (traditional but prone to rot over time). PVC or composite sill support (long-lasting, rot-resistant). Concrete or mortar bed (stable for masonry openings). Flashing tape and waterproof caulk (moisture barrier). The exact choice depends on your door type, home design, and climate. Why the Base of a Door Threshold Matters Moisture Resistance – Prevents water intrusion that leads to rot and mold. Structural Support – Provides a stable, level foundation for the door frame. Energy Efficiency – Helps seal gaps, keeping conditioned air inside. Longevity – Extends the life of the threshold and door. Pest Prevention – Blocks insects and rodents from entering. Common Materials Used Under Exterior Door Thresholds 1. Pressure-Treated Lumber Traditional choice. Readily available and affordable. Must be sealed properly to avoid moisture damage. Can warp over time if exposed to water. 2. PVC/Composite Sill Support Won’t rot, warp, or attract termites. Excellent for humid or wet climates. Often used in modern entry systems. 3. Concrete or Mortar Bed Best for masonry installations. Extremely durable and moisture-resistant. Requires skill to level properly. 4. Metal Threshold Plates Sometimes installed directly on concrete. Must be sealed with caulk to prevent water intrusion. 5. Flashing Tape & Sealant Essential moisture barrier. Applied under and around the threshold. Works with other base materials. Step-by-Step Installation Guide Step 1: Prepare the Subfloor or Concrete Ensure the surface is level and clean. Remove old caulk, adhesive, and rotten wood. Step 2: Apply Flashing Tape Cover the sill area with waterproof flashing tape. Extend it slightly up the door jambs. Step 3: Choose the Right Base Material Wood (sealed), PVC, or composite. For concrete openings, apply a mortar bed if needed. Step 4: Apply Bead of Sealant Use high-quality exterior caulk. Create continuous beads under and around threshold edges. Step 5: Install Threshold Set threshold on the prepared base. Check level before fastening. Step 6: Secure and Seal Screw into place (pre-drill if necessary). Seal screw holes and edges with caulk. Mistakes to Avoid Skipping flashing tape – invites leaks. Using untreated wood – leads to rot. Uneven base surface – causes door misalignment. Not sealing edges – allows water infiltration. Overtightening screws – warps threshold. Professional Tips Always slope threshold slightly outward (1–2°) for drainage. Use composite sill supports in wet climates. Apply expanding foam sparingly under jambs (never under threshold). Re-caulk joints every few years for maximum waterproofing. Consider pre-hung entry systems with integral sill pans. FAQs Q: Can I put a threshold directly on concrete?Yes, but you must use flashing tape and caulk to prevent leaks. Q: Should I use pressure-treated wood under my threshold?It’s acceptable but less durable than PVC/composite options. Q: Do I need to insulate under a threshold?Insulating foam or sill pans can improve energy efficiency. Q: How long does a properly installed threshold last?With correct materials, 15–30 years. Conclusion What you put under an exterior door threshold determines whether your entryway lasts for decades or fails in a few years. The safest choice is a PVC/composite sill support with flashing tape and high-quality caulk, though pressure-treated wood or concrete are also viable. Proper preparation, sealing, and maintenance are the keys to a long-lasting, weatherproof threshold. Post navigation How Long Should Drywall Mud Dry Before Priming? | Complete Guide How Much Thinset Do I Need to Remove?