Felt roofing is a very common material used on flat roofs such as garages, garden sheds and on extensions.  Generally, felt underlay roof works by combining two or three layers of material together to make an impenetrable barrier that is then coated with a waterproof finishing material to further protect the felt.


Why use a felt underlay roof?

There are a number of reasons for felt underlay roof on top of the fact that it is a proven material that has been in use for many years. First, the size of the job is immaterial – it can be large or small and the felt can cover it easily, so whether for a home or garage roof felt, you'll find this to be a versatile product. Roofing felt is both durable and able to deal with the worse of the weather such as wind and rain. It can also be applied to both flat roofs and also pitched and curved roofs meaning there are many jobs where it is the perfect roofing material.  There are also a range of different ways that it can be applied to the roof.

The main downside of a felt underlay roof is that it doesn’t last forever.  However, modern techniques have been developed that have lengthened the lifespan and durability of the felt underlay roof even further than before.  One of these is using oil and other specific chemical additives to the waterproofing layer to prevent it from drying out and cracking, which is one of the chief reasons roofing felt needs replacing.  Roof felt does also need to be check periodically to make sure no rips or tears have appeared, particularly after high winds or storms.  But this applies to all roofing materials and with garage roof felt, this is visually easier to do.

DIY Felting a Flat Roof

If you wish to know how to felt a flat shed roof, or complete any DIY felt underlay roof project, it is within the scope of many DIY enthusiasts and can be simplified into six easy steps.  The basics of these steps are:

  1. Preparing the roof for the felt to be laid
  2. Laying the first underlay felt
  3. Laying the second boded underlay felt and adding any gutter drips
  4. Laying the top bonded layer of felt
  5. Installing any verge edge and edging to any flashings and vertical walls
  6. Complete the finishing touches


Comprehensive preparation is a key factor in how to felt a flat shed roof. The deck that the felt underlay roof material is to be laid on should be moisture tolerant wood such as 18mm WBP plywood or a suitable chipboard that rests on treated timber joints.  If the roof is new, make sure it is dry and clean and that all the nail heads are flush on the deck and not protruding.  For an existing roof, remove all the old house, shed or garage roof felt as well as adhesives and any nails so that there are no protrusions and the deck is dry.  Also, check the wood to make sure none of it is wet or rotten, in which case it will need to dry out or be replaced.

If there is any mortar to chip out or other preparation work, this should be done before the felting a flat roof and any debris from this cleared away.

First layer

The first layer introduced is the felt underlay roof material, which needs to be nailed to the deck without using any adhesive, popular product Icopal 3B is perfect for this use.  This is because wood will move when walked on as well as with temperature and moisture so the felt needs to be able to move slightly with it.

felting a flat roof

Start at the lowest edge of the roof and roll out the felt at right angles to the direction of the full.  Once you have aligned it, measure and trim to the correct size with a sharp knife.  Once it is cut to the correct size, start at the centre of the sheet and work towards one of the edges placing nails at random intervals.  Usually, the recommended nails are 150mm galvanised clout nails.  Keep away from the very edge of the roof, as this is where the second layer will be added.

Second layer

When considering how to felt a roof torch on felt, the second layer is also an underlay felt but is bonded to the first layer with adhesive.  When laying this layer, avoid having joints in the felt at the same place as on the first layer, so it can be best to start sizing with a half width of a roll to achieve this. This applies whatever garage roof felt product you choose.

Once the garage roof felt has been cut to size, start once again in the centre and follow the manufacturer’s instructions on how to felt a flat shed roof with the adhesive.  It will then need to be left for the designated period of time before the felt is added.  This needs to be done carefully to avoid any air being trapped between the layers so applying a smaller area at a time can be the best method.  No nails are used.

Gutter drips are now formed by overlapping the last sheet of the felt with the guttering.  This means that water will fall straight off the roof into the guttering and won’t come into contact with the deck beneath.

Top layer

The top layer is applied with the same method as the second layer, using an adhesive.  When this layer is started, lay with the selvedge (the edge of the sheet that doesn’t have a mineral covering) away from the roof edge.

Brush off loose mineral chippings once it is in place and apply the gutter drips so they are around 25mm away from the edge of the roof.

Verge edges

Verges are put in place to stop rain driving over the edge of the roof and are made when the top layer has been locked into place.  Typically, a fascia board upstands the roof deck by around 50mm, usually with a 50x40mm triangular fillet (arris rail) and is placed between the junction of the deck and the fascia board.

Similarly, with vertical walls, a triangular fillet is used but fascia boards aren’t required to perform a smooth transition from wall to roof.  A length of top layer felt is used to create the overlap.

Finishing touches

To ensure the job is finished, check for any gaps in the seams where the layers meet and if any look like they need a little further attention, apply more adhesive.  Some people also consider adding solar paint after the garage roof felt is laid to reflect the solar energy and stop the felt from getting too hot.  This can further extend the lifespan of the felt so may be worth consideration.


There are a few basic things to remember when felting a flat roof – don’t do it in wet or cold conditions; unroll the garage roof felt half an hour before use; make sure you use the right nails and don’t cut the felt straight onto the roof or any layer already in place but make use of a cutting board.  Follow these guidelines for how to felt a flat shed roof and those provided by the manufacturer and the new roof should be in place in no time.