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How to Replace your Hot Felt Roof

Learn How to Replace your Hot Felt Roof using our step by step guide. This guide talks about 3 layer hot felt systems for domestic installations only.  If works are to be carried out on site or on industrial projects, then you will need advice from a site manager for relevant credentials and training before work is carried out. You must take health and safety very seriously when installing hot felt roofing systems. Ensure your wear Personal Protection Equipment (PPE) at all times, and ensure you have someone help you.

Safety Wear Required

When following this how to replace your hot felt roof guide, it is essential you use the safety wear listed below. Also, read this Safe System of Work for Gas Torches by the NFRC, as it provides excellent safety guidelines for gas torches. The NFRC is the UK’s largest roofing trade association.

Heat resistant gloves

heat resistant gloves

Heat resistant footwear

safety boots for hot felt roofing

Knee pads

knee pads for hot felt roofing

Heat resistant clothing

heat resistant clothes for hot felt roof

Safety Glasses

safety glasses for hot felt roofing

Tools Required

Below are the most common tools you need to replace your hot felt roof.

Long Lance Roofing Torch

Torch-on felt needs a wide column of flame. This means it is best to buy a torch with a 50mm bell.

hot torch to replace your hot felt roof

Short Lance Roofing Torch

hot torch to replace your hot felt roof

Felt Rolling Bar

Image below shows a rolling by manufactured by R-Bar. Buying a rolling bar similar to this makes rolling felt a much easier task.

rolling bar for how to replace your hot felt roof

Stanley Knife

stanley knife for hot felt cutting

Claw Hammer

hammer for clout nail

Masonry Chisel

hammer and chisel for chase

Lead Snips Set

lead snips for how to replace your hot felt roof guide

Lead Dresser

lead dresser tool for replacing your hot felt roof

Masonry Brush

masonry brush for bitumen

Hand Saw

hand saw

Tape Measure

tape measure

Spirit Level

Battery Drill

battery drill

Materials Required

Below are the materials you need to replace your hot felt roof.

25mm Marine Plywood Board

plywood for how to replace your hot felt roof guide

32mm Arris Rail

arris rail for how to replace your hot felt roof guide

Bitumen Primer

can of bituemn

Breathable Felt Layer (1st Layer)

Also known as perforated underlay or a venting layer. You can buy the breathable felt layer below from The Roofing Superstore.

roll of breathable felt

Torch-on Felt Underlay (2nd Layer)

This is 2mm thick torch-on underlay. It bonds to the 1st layer (venting layer).

roll of underlay felt

Cap sheet (3rd/Top Layer)

roll of cap sheet felt

Drip Former

One below is from Roofing Superstore

drip former for how to replace your hot felt roof guide

30mm Long Clout Nails

30mm clout nail

50mm Long Wood Screws

50mm wood screw

Flashing Tape

Below is a lead flashing substitute. However, lead flashing is usually considered the best.

black jack flashing tape

Propane Gas Cylinder

You can buy propane gas cylinders from BOC.

BOC propane gas cylinder

How To Install Flat Roof Felting Systems Guide

Follow the steps below to learn how to replace your hot felt roof.

Step 1: Learn What a 3 Layer Felt Roof is

3 layer hot felt roof systems are one of the most popular and reliable roof systems in the UK. They are also very cost effective. The infographic below summarises each of the 3 layers, and their position on the flat roof. You will also notice that we have labelled up certain components of the roof, such as Arris rails and upstands.

Traditionally roofing felt has been manufactured from modified bitumen, which has been mixed with fibreglass or polyester. These days we are seeing more being mixed with polyester, as this material is very difficult to tear. Therefore, such a material is able to deal with flat roof movement. In general, hot roofing felt is reliable with a life span between 15 to 25 years. You will notice that the top layer (known as the cap sheet) has minerals/chipping imbedded. This helps to reflect the heat from the sun, helping to prevent the felt from becoming too brittle over time.

Typical Components of a flat roof:

Deck: This is a flat plywood base which is usually 25mm thick. It is always best to by an external grade such as marine plywood.

Arris rails: These are two triangular pieces of wood which help channel the rain water down the flat slope.

Drip former: This is located at the lowest end of the flat deck. It helps to ensure water drips into the guttering.

Upstands: Attached to the outer side of an Arris rail (if there is no building joining). Upstands improve the appearance of the roof. It also provides a surface for the felt to be torched on to.

Flashing: This is basically a thin piece of waterproof material which prevents the passage of water into a joint. Many people in the roofing trade prefer to use lead flashing, however there are alternatives.

Drip Former: A piece of felt edge which is attached to the lowest end of the deck using clout nails. You can even make your own drip former which can be torched on, but it is easier to buy one.

Figure 1: Quick guide to the 3 felt layers

Step2: Calculating How much Material you Need

Calculate Roof Surface Area for plywood and felt:

Measure the length and the width of the flat roof area. Multiply the length and width to get the area of your roof (Area in meter square (M2) = Width in meters X Length in meters).  This will give you an idea of much marine plywood you need to order.   And also give you the measurement for the breathable felt layer.

  Step 3: Removal of Old Felt, Timber Fixtures and Flashing 

Remove Old Felt: If you still have your old felt on your roof, then this will need to be completely removed. It will be much easier if you remove the guttering around your flat roof to do this. This will then allow access to the drip former (which helps drips go into your guttering) at your roofs lowest point. This is a good starting point, where you can cut out the drip former using a Stanley knife.

Removing Old Flashing: If your garage is adjacent to your house. Then there will probably be lead flashing at the side of your flat roof, butted up against your homes exterior wall. If so, then you will need to remove all this flashing detail. Wear gloves if you suspect the flashing to be lead, as lead is toxic. If you have trouble removing the flashing, then a 6″ angle grinder may help, but be careful! Make sure you don’t cut the joint too deep or wide.

Removal of Timbers: If there is an Arris rail (triangular wooden beam) on the existing system, that will also need to be removed. Make sure all the up stands are removed too. Upstairs are the pieces of wood which secure the felt to the outer sides of the garage.

Ensure Roof is Free of Debris: Use a brush to remove the remaining dust from all surfaces.

Step 4: Replacing or Installing the Plywood Base

Your roof slope must be constructed from a moisture resistant board such as 25mm marine plywood. This marine plywood should sit on your flat roofs treated timber joists. If there are any signs of rotting wood, then this must be corrected before further work can be carried out.

If you need to install new plywood, then you will need a tape measure, skill saw, battery drill and screws for this part. Make sure you stagger your 2400mm x 1200mm marine plywood boards, so the ends do not line up with the board next to it.

Once you have fitted all your marine plywood boards, you can then fit the Arris rails. Arris rails are basically triangular wooden rail. They run along the sides of the roof (see infographic). Take care not to split the Arris rails as you screw them into the marine plywood. Drive all screws so that there head are below the surface of the wood. We recommended a hand saw for cutting the Arris rails. Make sure the cuts are nice and flush.

Now use bitumen primer and a masonry brush and coat all of the plywood, arris rails and upstands. This will help seal the porous wooden structure, creating a better bond between your felt and wood.

Fix the plywood sheet to the roof joists using nails. ensuring the board joints are staggered. No fix your triangular arris rail each side of the roof, also using nails. These rails help to channel all rain water down the flat roof slope.

Ensure the roof is clean and dry, and all nail heads are below the surface of the marine plywood and arris rails. Use a spirit level to check that the roof is flat, and sloping downwards towards the guttering.  There should be no dips or low spots in your flat roof.

Step 5: Installation of the Breathable layer (1st Layer)

You will need a long and short lance roofing torch, an adjustable spanner, a bottle of propane gas , rolling bar, Stanley knife, knee pads, tape measure and a chalk line for these next few steps.

Put your gas bottle on top of your flat roof. Now place the roll of breathable felt layer (1st layer) onto the roof.  Starting from the highest point of the roof, use your roll bar to push the roll back towards the lowest part. You only need to cover the flat part of the roof (see image in Step 1). Unroll your felt very slowly as you simultaneously heat it using the long lance torch.  With the deck primed, this will NOT need a lot of heat. The heat will make the felt layer sticky, and it will bond to the plywood. Work you way to the lower part of the roof, heating each part up as you go.

Once you have completed one length, you can then start the process again. This time but the new layer up against the one you just laid. No need to overlap the 1st felt layer on this step.

You may hear that some people prefer to nail this first layer on. We don’t think this is the best approach, as there is more chance of vapour getting trapped under the air pockets. When torching it to the deck, you area bitumen sealing the whole felt surface to the deck. Which we believe is the best way of doing it. Bitumen is extremally flexible.

Figure 2: Drip former, at lowest point of roof.

Figure 3: Quick guide to the breathable felt layer

Step 6: Installation of Underlay (2nd Layer)

Place your underlay felt roll at the highest point of your flat roof. In this step you will be rolling felt over the entire roof, including your Arris rails.

First, place some of the felt so it is in the corner of your roof, over-hanging the corner on both sides by about an inch.

You will need to use more heat than before to tack the under layer to the deck/breathable layer, concentrating the heat into the felt and deck moving the torch from left to right of the roll.

When you get to the end of the roll, take the bar out before you reach the end to prevent it falling over. Then hold one end of the felt, and torch the last part in by hand.  Ensure the felt is stuck right to the edge. Apply the same method the other end, you need to make sure the felt is pushed in nice and tight to the Arris and to the up stand.  Make sure there are no creases or voids, ensuring it is stuck to every part.

Before you start the next run, you will need to mark your overlap. Every new layer must overlap the previous one to ensure a water tight seal. The ideal overlap is 4 inches. Before laying your new layer, draw a chalk line 4 inches into your previous felt. Set the next run of felt up so the overlap runs nice and straight on the chalk line, and roll the felt up with the bar.


When heating up the felt try and keep a lot of pressure on the roll. You will see the felt absorb the heat as you put the flame to it. You should see the bitumen pushing out the side of the roll as you are torching it, this is called the bleed. The bleed is vital for sealing the joints of the felt. This is what you should be looking for throughout.

Now all the under layer is complete, make sure you cut any over hanging felt flush to the edge and make sure all the under layer is cut flush to the chase.

Figure 4: Torching the felt onto the external sides of the arris rail.

Figure 5: Quick guide to the underlay felt layer.

Step 7: Installation of Cap Sheet

Before you torch on the flat, you need to work out how many drips formers you need. Give them a coating with primer, once dry you need to work out the measurement of the flashing for the drip former. You do this by measuring 4 inches past the bottom of the drip former and 5 inches on to the flat roof.

Using a hammer and clout nails, nail the flashing so they are on the edge, flush to the flat. Make sure you place them lapping the salvage (black strip) the right way round, so there is no salvage  showing. The drip needs to be hanging down with the cap sheet facing the fascia side.  Then you need to nail the drip former over the top.

Now all this is set in place, you need to use a short lance torch and hold the flashing and apply heat. Pull it tight against the drip former and firmly slide your hand across the cap sheet making sure it is firmly stuck to the drip former Then use your filling knife to hold up the flashing to apply heat, and then make sure you slide your hand firmly across the flat pushing the bleed out the edges. When moving on to the next flashing, you want to point the heat inwards towards the next flashing, only applying necessary heat.  Too much heat will risk it protruding through to the face of the cap sheet.

Once all the drip edge is complete, measure from the edge of the roof, 2inches on to the flat and strike another line. This is where you want to terminate the flat to the cap sheet.

Repeat the process the same with the underlay, ensuring you are careful not to scratch and mark the top layer. And not to use to much heat and risk scorching the cap sheet. Once down it cannot be altered!

Make sure there is a continuous bleed throughout the system.

You have now learnt how to replace your hot felt roof. If your garage is stand alone, then you are done! if next to your home, then you will need to install flashing to bridge the gap between your garage and home.

Figure 6: Finish roof, with all 3 layers.

Figure 7: Diagram of cap sheet.

Step 8: Installation of Lead Flashing

This step is only necessary if your garage is next to your home. Flashing helps to bridge the gap between your home and flat garage roof. With all 3 felt layers complete.

First you will need to understand if you have a chase present or not. A chase is simply a cut out channel in your homes exterior wall, which faces the flat roof. In the diagram below you will not that the flashing and lead bead are inside the chase, then extend onto the felt. The purpose of the chase is to create a water tight seal.

I you do not have a chase, then you will need to cut a channel into the exterior wall of your home which runs 150mm above the flat (or pitch) of your roof (refer to diagram below). You can do this using a 6 inch angle grinder. But only grind the channel about an 25mm ( 1 inch) deep.

Now you will need a piece of lead that is L-shaped. Being 150mm wide one side, and about 25mm (1 inch) wide on the other side. The side that is 25mm will insert into your chase. You can secure this lead in place by stuffing off cut pieces of lead into the case, forcing a tight fit. Make sure there is about 150mm (6 inch) overlap when inserting another strip of lead.

Now use your lead dressing tool to shape the flashing, so it lead follows the profile of your arris rail.

Use mortar to fill up the chase hole.

Figure 8: Quick flashing guide.

Alternatively, you could tuck the felt into the chase, as in the image below. It all depends on the shape and layout of your roof what method would be best.

Figure 9: Image of felt underlay and cap sheet tucked into external wall chase.


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Posted on 24th September 2021 at 11:58 am

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