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How to replace felt on flat roof

Updated February 21, 2017

Replacing the felt on your flat roof is not a job for everyone, but anyone can do the job by following a few simple directions. The most important concept in replacing your roof felt is making sure the base is in great condition. If your decking material isn't of the highest quality, no amount of felt is going to make your roof waterproof and strong. Follow the instructions carefully and you will soon have a roof you'll be proud to show off.

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  1. Prepare the foundation. Before laying 13.6kg felt on a flat roof make sure the plywood base is in good condition. Any rotten decking should be replaced before laying the felt. All new boards should be coated first with a preservative. Once your roof decking is suitable, you are ready to begin replacing the felt.

  2. Remove excess felt, nails, and adhesive before replacing the felt on your flat roof. Allow any wet plywood to dry out. There should be no extra debri of any kind on the decking before you begin.

  3. Nail the first layer of felt to the plywood using 20mm galvanised clout nails with large heads spaced every 150mm along the planks. Make sure the felt is straight and flat. Always start nailing in the centre of the felt sheet working your way outward towards the edge of the roof. Do not nail on the edge of the roof where the second layer of felt will overlap the first.

  4. Paint a layer of adhesive, such as bitumen mastic, over the top of the first felt layer, a length at a time. The second layer of felt is then placed over the top of the first. Do not let the ends join together. There must be an overlap to prevent leaking. Either use a broom, or if the roof is strong enough, walk over the top of the roof felt to get rid of any air bubbles.

  5. Cover the roof edges with strips of felt before laying the final layer of roof felt. Remember to allow 50mm of overlap with each piece of felt layed. Paint bitumen mastic over the top of the entire final layer of felt to provide the most waterproofing. To strengthen the surface of your new felt roof cover the surface with gravel, granite, or limestone.

  6. Tip

    When laying down your layers of felt roll out about 1 metre so that you are only working on a small area at a time.

    After laying down a top layer of roof felt make sure that the overlap is secure by painting more adhesive over the top.

    Adding loose chipping material to your final felt roof prevents the surface from absorbing the sun.


    Make sure your shoes are free from sharp edges that would cut or damage the just laid felt.

    Only use nails on your first layer of roof felt. The subsequent layers of felt are secured by adhesive only.

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Things You'll Need

  • Hammer
  • Tape Measure
  • Strong Cutting Knife
  • Nails
  • Felt
  • Adhesive
  • Chipping Material
  • Broom

About the Author

Lacy Enderson is an Addictions and Recovery Counselor. She is Certified with the American Association of Christian Therapists and holds a Master's Degree in Biblical Counseling. She is currently enrolled in Liberty University's Master of Divinity Degree program with Chaplaincy. Lacy is a graduate of Rhema Correspondent Bible School and has completed the first section of Berean School of the Bible. Lacy is the author of, "Addiction: A Personal Story" and "So You Want to Lose Weight But You Can't Stop Eating." Her newest novel is a teenage Christian fiction titled, "Honey Sweetheart."

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