How to install flat roofing on commercial buildings

Flat roofing is a popular choice for commercial buildings due to its cost-effectiveness, ease of installation, and modern appearance. Whether you’re a professional roofer or a DIY enthusiast, understanding the process of installing flat roofing is crucial for ensuring a durable and reliable roof. Here’s a step-by-step guide to help you install flat roofing on commercial buildings effectively.

1. Planning and Preparation

Before starting the installation, thorough planning and preparation are essential. Here are the key steps to consider:

1.1 Assess the Roof Structure:

  • Inspect the existing roof structure to ensure it can support the weight of the new roofing material.
  • Check for any damage or weaknesses that need to be repaired before installation.

1.2 Choose the Right Roofing Material:

  • There are various flat roofing materials to choose from, including built-up roofing (BUR), modified bitumen, and single-ply membranes like EPDM, TPO, and PVC.
  • Select the material that best suits your budget, climate, and building requirements.

1.3 Gather Tools and Materials:

  • Essential tools include a utility knife, roofing adhesive, a roofing roller, a heat gun, and a measuring tape.
  • Ensure you have all necessary materials, such as roofing membranes, insulation boards, and fasteners.

2. Removing the Old Roof

If you’re replacing an existing roof, the old roofing material must be removed completely. This process involves:

2.1 Safety First:

  • Wear appropriate safety gear, including gloves, safety glasses, and a hard hat.
  • Ensure the work area is clear of debris and obstacles.

2.2 Strip the Old Roofing:

  • Use a roofing shovel or a similar tool to strip away the old roofing material.
  • Remove any nails, screws, or fasteners from the roof deck.

2.3 Clean the Roof Deck:

  • Sweep the roof deck to remove any remaining debris, dirt, or loose material.
  • Repair any damage to the roof deck, such as replacing rotten wood or fixing structural issues.

3. Installing the Insulation

Proper insulation is crucial for energy efficiency and temperature regulation. Follow these steps to install insulation:

3.1 Measure and Cut Insulation Boards:

  • Measure the dimensions of the roof and cut the insulation boards accordingly.
  • Use a utility knife or a saw to make precise cuts.

3.2 Lay the Insulation Boards:

  • Start at one corner of the roof and lay the insulation boards in a staggered pattern to prevent gaps.
  • Secure the boards to the roof deck using appropriate fasteners or adhesive.

4. Installing the Roofing Membrane

The roofing membrane is the primary barrier against water and weather. The installation process varies depending on the type of membrane chosen.

4.1 Single-Ply Membrane Installation (EPDM, TPO, PVC):

  • Position the Membrane: Unroll the membrane and position it over the insulation boards. Ensure there are no wrinkles or air pockets.
  • Cut to Fit: Trim the membrane to fit the roof dimensions, allowing for a few extra inches around the edges.
  • Adhere the Membrane: Apply roofing adhesive to the roof deck and press the membrane firmly into place. Use a roofing roller to ensure proper adhesion.
  • Seal the Seams: Overlap the membrane seams by at least 2-3 inches. Use a heat gun to weld the seams together for a watertight seal.

4.2 Built-Up Roofing (BUR) Installation:

  • Apply Base Layers: Apply multiple layers of bitumen (asphalt) and reinforcing fabrics in alternating layers. Use a mop or sprayer to evenly distribute the bitumen.
  • Add Surfacing Layer: Finish with a top layer of gravel or mineral granules to protect the underlying layers from UV radiation and weather.

4.3 Modified Bitumen Installation:

  • Position the Membrane: Unroll the modified bitumen membrane and position it over the insulation boards.
  • Heat and Adhere: Use a torch to heat the membrane as you roll it out, ensuring it adheres to the roof deck.
  • Seal the Seams: Overlap the seams by 3-4 inches and heat-weld them for a secure bond.

5. Flashing and Edge Details

Proper flashing and edge details are crucial to prevent water infiltration and ensure a finished appearance.

5.1 Install Roof Flashing:

  • Install metal flashing around roof penetrations, such as vents, pipes, and chimneys. Use roofing nails or screws to secure the flashing in place.
  • Apply a layer of roofing cement to seal the flashing edges and prevent leaks.

5.2 Secure Roof Edges:

  • Install metal drip edges along the roof perimeter to direct water away from the roof and into the gutters.
  • Attach the drip edges using roofing nails or screws and apply roofing cement to seal any gaps.

6. Final Inspection and Maintenance

After completing the installation, perform a thorough inspection to ensure everything is properly installed and sealed.

6.1 Inspect for Defects:

  • Check for any wrinkles, air pockets, or loose seams in the roofing membrane.
  • Ensure all flashing and edge details are securely fastened and sealed.

6.2 Regular Maintenance:

  • Schedule regular inspections and maintenance to identify and address any potential issues before they become major problems.
  • Keep the roof clean and free of debris to prevent water pooling and damage.


Installing flat roofing on commercial buildings requires careful planning, proper materials, and meticulous execution. By following these steps, you can ensure a durable and long-lasting flat roof that provides reliable protection for your commercial property. Whether you choose single-ply membranes, built-up roofing, or modified bitumen, attention to detail and regular maintenance are key to a successful flat roofing installation.