Learn how to install a flat roof felting System 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
It is crucial to prioritize safety when using this guide for installing a flat roof felting system. Additionally, we recommend referring to the Safe System of Work for Gas Torches by the NFRC, which offers comprehensive safety protocols for gas torches. The NFRC is the largest trade association for roofing in the UK.
Below are the most common tools you need to replace your hot felt roof.
Torch-on felt needs a wide column of flame. This means it is best to buy a torch with a 50mm bell.
Image below shows a rolling by manufactured by R-Bar. Buying a rolling bar similar to this makes rolling felt a much easier task.
Below are the materials you need to replace your hot felt roof.
Also known as perforated underlay or a venting layer. You can buy the breathable felt layer below from The Roofing Superstore.
This is 2mm thick torch-on underlay. It bonds to the 1st layer (venting layer).
One below is from Roofing Superstore
Below is a lead flashing substitute. However, lead flashing is usually considered the best.
You can buy propane gas cylinders from BOC.
How To Install a Flat Roof Felting System Guide
Follow the steps below to learn how to install a flat roof felting system with ease and efficiency. It is important to note that this guide assumes you have a basic understanding of roofing principles and techniques. If you are unsure about any steps or techniques, it is recommended that you seek professional assistance to ensure proper installation and safety.
Step 1: Learn What a 3 Layer Felt Roof is
Opting to install a flat roof felting system for your garage is a smart decision. These systems boast a 3-layer design that is widely trusted and cost-effective throughout the UK. To simplify the understanding of this system, an accompanying infographic illustrates the placement of the three essential layers on a flat roof, with key components such as Arris rails and upstands clearly labelled for your reference.
Traditionally, roofing felt has been made from modified bitumen mixed with fibreglass or polyester. Nowadays, more felting systems are being made with polyester, as it is incredibly tear-resistant and able to handle flat roof movement. Typically, hot roofing felt has a lifespan of 15 to 25 years and the top layer, known as the cap sheet, has minerals or chipping embedded in it to reflect heat from the sun. This feature helps 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 start to install a flat roof felting system, determining the required number of drip formers is a crucial step. Begin by applying a primer coat to these formers, allowing it to dry. Subsequently, calculate the dimensions of the flashing for each drip former. This is achieved by measuring 4 inches beyond the bottom of the drip former and extending 5 inches onto 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
If you are planning to install a flat roof felting system for your garage and it is adjacent to your home, you may need to incorporate flashing to bridge the gap between your home and the flat garage roof. Once all three felt layers are in place, the next step is to determine if you have a chase present. A chase is a channel cut out of the exterior wall of your home that faces the flat roof. In the diagram below, you will see that the flashing and lead bead are inside the chase and extend onto the felt. The purpose of the chase is to create a watertight seal to protect against leaks. Please note that this step is only necessary if your garage is next to your home.
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.
Anyone who wants to install a flat roof felting system is undergoing a substantial project, but one that can significantly enhance the durability and protection of your home. By following the steps outlined in this guide, you can ensure a successful and long-lasting roof replacement. Remember to prioritise safety, gather the necessary materials and tools, and take your time to complete each stage carefully. If you’re uncertain about any aspect of this process, it’s always advisable to seek professional assistance. Your investment in a new hot felt roof not only secures your home but also contributes to its overall value and longevity. With proper maintenance and timely replacements, your roof will continue to shelter you for years to come.