How To Felt a Flat Roof or Refelt a Roof

How to felt a flat roof? If you’re looking to protect the flat roof of your shed or other outdoor structure from the natural elements, there’s no better way than by laying down felt on the roof.


While felting a flat roof is not a particularly complex project, it does require some patience and perseverance to make sure that the felt goes on smoothly and evenly. This is a good DIY weekend project, requiring only a few basic materials.

Materials for felting a flat roof

In order to lay felt on a flat roof, you will need to gather the following materials and tools:


In terms of materials, please note that there are several different weights for the roofing felt. This guide will use the more durable #30 roofing felt, although you can use #15 felt.

Step-by-step guide for felting a flat roof

Follow the 10 steps below in how to felt a flat roof.

Step 1: Measure the roof

You can either measure the roof from the ground or by standing on a ladder and actually going up on the roof. You will then need to measure the length and width of the roof. Multiplying these two together will give you the square footage of the roof.

As part of this project you are using three different layers of felt, so be sure that you then multiply this square footage by three. This will tell you how much roofing felt you need to lay.

Step 2: Prepare the roof

You must remove any rotted pieces of wood on the roof and replace them with your plywood planks. Also, you will need to remove any old felt on the roof, as well as any nails or roof staples. This is done to ensure a flat surface for the roof felt.

Step 3: Lay down felt and secure to roof

Roll your felt out on the roof and cut it to fit with a utility knife. Roll the felt so that it overhangs the edge by 2 inches. Let it stand for about 30 minutes and then, using galvanized nails and your hammer, nail down the first layer.

In terms of spacing, leave about 6 inches between nails along both of the short sides and the side lying next to the edge of the roof. As you secure the roof felt to the roof, make sure that it lies even and straight.

One way to do this is by starting your nailing in the center and working out – this allows you to pull the felt snug as needed.

Step 4: Repeat until roof is covered

If your roof is larger than just a single layer of felt, you will repeat the process in Step 3 until the entire roof is covered. As you roll out each additional layer, make sure that there are at least 2 inches of overlap.

This means that you will be nailing both the original layer and the overlap layer along the edge of the felt that is parallel to the roof edge. When the final piece of felt is added, you will need to make sure that all 4 sides (not 3, as in the earlier rolls) are secured to the roof.

Step 5: Prepare the second layer

The key to weatherproofing your flat roof is adding a total of 3 layers; thus you will repeat the earlier steps. Before adding the second layer, though, spread a thin layer of adhesive on the first layer using your trowel. This will help the two layers adhere to each other.

Step 6: Add the second layer

You are now ready to place the second layer of felt. Cut it with your utility knife so that it lies correctly and fits the roof, AND so that it will overlap the first layer by 2 inches. This overlap is needed to ensure a tight, weatherproof seal.

You can either walk over the roof to remove any air bubbles between the two layers or use your trowel to ensure a smooth, exact fit.

Step 7: Prepare the third layer

You are now ready to lay another roll of felt on the roof. As before, you will need to place adhesive on top of the felt.

Step 8: Add the third layer

Now you are ready to roll out the third layer of felt. Cut it to size with your utility knife and place it over the second layer.

Be sure that there are 2 inches of overlap with the previous layer, and that some felt covers the edge of the roof. Now place the third layer on top. As before, you will need to make sure that there are no air bubbles between layers.

Either walk over the felt with your feet to ensure a tight, secure fit or pat it down with your trowel. At this point, you will now have three different layers of felt secured to the roof.

Step 9: Final roof treatments

Now, spread some adhesive on top for maximum weatherproofing effect. You can now cover the surface of felt with gravel for added strength and durability.

Step 10: Clean up

You’ve now completed the felt roof installation! All that’s left to do is a final cleanup of materials from the roof. Dispose of any waste – such as old felt or used nails – in an environmentally-friendly way.

Safety tips for flat roof felt installation

Any time you are working on a roof and using a ladder, you must practice good safety. This includes taking extra steps to ensure that you always have at least one hand free as you go up and down your ladder to the roof.

If you are thinking about refelting a roof or replacing a layer of felt, take care that you use the appropriate type of roof felt and adhesive.

Conclusions on felting a flat roof

Laying down felt on a flat roof is an easy and effective way to improve the strength and durability of an outdoor structure like a garage or shed. By placing three even layers of felt and taking great care that there is enough overlap and that any seams don’t create a straight line, you can ensure that your flat roof will be optimally weatherproofed.

Last update on 2023-07-04 / Affiliate links / Images from Amazon Product Advertising API

Founder & Editor

Hi, I’m Jim. I was a roofing constructor for 20 years, before deciding to start and gather the best content about roofing. I love woodworking and construction and it was only natural for me to start this passion project of mine. Thank you for visitng.