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What Type of Chimney Flue Is Safe for a Thatched Roof?

What Type of Chimney Flue Is Safe for a Thatched Roof?

A thatched roof is a quintessentially rustic feature that adds an enormous amount of character to a home. However, there’s no getting away from the fact that they can pose a fire risk - especially if they are matched with the wrong kind of chimney flue. However, it is possible to have an open fire and chimney flue in a property with a thatched roof. You just need to use the right equipment and installation. Keep reading to find out more…

Thatched roof fires

There are a number of reasons why a thatched roof may catch fire. However, in recent years there has been a suggestion that the increased use of wood-burning and multi-fuel stoves may make a thatched roof fire more likely because they burn hotter than traditional open fireplaces.

This became known as the ‘heat transfer theory’ - the idea that heat from the chimney stack was causing thatch to combust.

However, an extensive three-year study by the Fire Protection Association (FPA) disproved the heat transfer theory. Instead, the study found that the more likely cause of thatched roof fires is the leaking of hot flue gases from defects in chimney stacks.

In other words, the most important thing you can do to prevent a thatched roof fire is to use a high-quality, insulated chimney flue that’ll prevent any hot gases from leaking.

The safest type of chimney flue for a thatched roof

Which brings us onto the central subject of this article - what is the safest type of chimney flue for a thatched roof?

The short answer is - a twin wall flue system.

Shop Now - Twin Wall Flue Kit - 6” Stainless Steel Single Storey Straight Up Internal System

The longer answer is that the type of flue you can use is governed by a set of government regulations. These regulations governing the types of chimneys and flues that can be used with a thatched roof are set out in Approved Document J: Combustion Appliances and Fuel Storage Systems, Building Regulations 2010 which can be viewed here.

The document sets out on page 30, paragraph 2.12, the exact stipulations regarding chimney flues and thatched roofs:

“Thatched roofs can sometimes be vulnerable to spontaneous combustion caused by heat transferred from flues building up in thick layers of thatch in contact with the chimney. To reduce the risk it is recommended that rigid twin-walled insulated metal flue liners be used within a ventilated (top and bottom) masonry chimney void provided they are adequately supported and not in direct contact with the masonry.”

“Non-metallic chimneys and cast in-situ flue liners can also be used provided the heat transfer to the thatch is assessed in relation to the depth of thatch and risk of spontaneous combustion.”

So, with all of those caveats and provisos in mind, a twin wall flue is regarded by the Building Regulations as the safest form of chimney flue for a thatched roof.

How should a twin wall flue be installed in a thatched roof property?

A twin wall flue is the safest type of flue to use in a thatched roof property, but how do the regulations stipulate that one should be installed?

When twin wall systems are installed in a single brick chimney stack, they should be assembled and fed carefully into the chimney stack. Once in place, the flue should be inspected to ensure that every joint is locked tight and will not allow hot flue gases to escape. 

When putting the flue in place, it’s highly likely that you’ll need to break into the chimney stack and/or partially disassemble it so you can attach support brackets to the flue system.

Should you have a chimney stack in your property that has bends or offsets, you may need to use a combination of a flexible flue liner and a cast in-situ concrete or pumice lining to ensure that the whole of the chimney is properly insulated instead.

In addition to looking at the way in which the twin wall flue will need to be installed, there are other factors that you need to take into consideration.

Compliance with Approved Document J of the Building Regulations

As we saw earlier, any work on the chimney of your thatched roof property must comply with the stipulations set out within the Approved Document J of the Building Regulations. But, what exactly does this mean?

Firstly, any work on your chimney is notifiable to your local authority. If you use an appropriately qualified person such as a HETAS installer, they will take care of this for you and provide the local authority with a HETAS compliance certificate.

Note - any material change to any part of the combustion system (appliance or chimney) will require you to bring the entire system into compliance with the current Building Regulations.

Distance to combustibles

In addition to this, the Approved Document J of the Building Regulations sets out detailed guidelines on the ‘distance to combustibles’. Paragraph 2.19 states:

“Combustible material should not be located where it could be ignited by the heat dissipated through the walls of fireplaces or flues. A way of meeting this requirement would be (to ensure that) combustible material is at least:

> 200mm from the inside surface of a flue or fireplace recess; or

> 40mm from the outer surface of a masonry chimney or fireplace recess unless it is a floorboard, skirting board, dado or picture rail, mantel-shelf or architrave.

Also - metal fixing in contact with combustible materials should be at least 50mm from the inside of the flue.”

Whether you have a small single brick chimney or a larger chimney will determine how you comply with these ‘distance to combustibles’ rules. The size and dimensions of your chimney will also influence what type of chimney flue set-up is most appropriate.

Tip - a HETAS-qualified installer will be able to provide you with further information on the exact details of chimney flue installation on thatched roof properties.

Providing extra ventilation

Extra ventilation will go a long way towards keeping your twin wall flue system cool and stopping your thatch from overheating.

There a number of things you can do to increase ventilation in your chimney:

> If your chimney flue is completely vertical, you can back ventilate the void between the twin wall flue and the chimney brickwork. Ideally ventilation for this void should be vented in from outside (rather than coming from inside your property).

> You can install a high-level weatherproof vent in the chimney stack above the thatch. This will allow heated air to escape and prevent the build up of heat within the chimney stack.

What is a twin wall flue?

At this point, if you’re not an expert on chimneys then you may be wondering what a twin wall flue is (we don’t blame you for wondering! Chimneys can be complicated).

A twin wall flue (also known as a double skin flue), is basically a steel flue pipe that sits within another. The two flue pipes are kept separate by a layer of high grade insulation.

Hence the name twin wall flue. The insulation between the two flues means that the exterior of the flue pipe never gets too hot - certainly never hot enough to cause combustion. That’s why the Building Regulations recommend this type of flue for use in thatched roof properties.

Twin wall flues are normally provided with a shiny stainless steel finish - however, with a thatched roof this doesn’t matter as you can only use a twin wall flue within a traditional masonry chimney stack - so the flue won’t be visible.

Other ways to reduce the fire risk of thatched roofs

In addition to using a twin wall flue that has been professionally installed in-line with the Building Regulations, there are other things you can do to reduce the fire risk of your thatched roof. We have listed these additional actions below.

Check there is enough distance between the top of the thatch and the chimney pot

Occasionally a fire may give off sparks or glowing embers. These can find their way out of the top of your chimney. As you can imagine, you don’t want these sparks of embers landing on dry thatch.

Luckily, these embers and sparks are generally low in energy and short lived. So, the further they have to travel from the chimney pot to the thatch the less likely they are to cause a fire. The Approved Document J of the Building Regulations gives guidance on the minimum distance you should have between your chimney pot and the top of the thatch.

Currently the regulations state that the top of the chimney pot should be at least 1.83m above the thatch ridge.

If your chimney stack doesn’t currently meet this distance requirement, then it’ll either need to be rebuilt to be taller, have a taller chimney pot fitted or some combination of the two.

Fit a bird guard to prevent birds nesting in your chimney

Should a bird ever find its way into your chimney, the resulting nest could cause both your chimney and thatched roof to catch fire.

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When your wood burning or multi-fuel stove is lit, a bird’s nest made of dry twigs and leaves could easily catch fire and be expelled from the top of the chimney as a bundle of burning embers. These types of embers are generally high energy and are much more likely to land on the thatch and start a fire.

Play it safe and install a bird guard on the top of your chimney.

Install a stove pipe temperature gauge

It’s important that you always operate your stove within its stipulated operating temperatures.

Why? Because if your stove is burning at too low a temperature, this increases the likelihood that creosote (tar) will be deposited in your chimney. Over time this creosote can build up and cause a blockage which can lead to a fire. Once a fire has started, the creosote acts as the perfect fuel, causing the fire to grow quickly. 

Shop Now - Stove Pipe Thermometer

A stove pipe temperature gauge will help you to accurately read the temperature of your stove and ensure that it is burning at the correct temperature.

Be careful when fuelling your stove

When many people refuel their stove, they set the stove controls to increase ventilation and boost the fire - to help the newly added wood/solid fuel to start burning.

However, it’s vital that you then return these controls to their normal settings as soon as you’ve finished fuelling the stove. This is because, if left in their ‘boosted’ state, these controls can cause your stove to run out of control, with the fire burning far too fiercely and inducing high flue gas velocities.

It doesn’t take a genius to guess that a stove burning like this can quickly lead to the eruption of burning embers and hot gases from the top of the chimney making a thatch fire almost inevitable.

Note - this also applies to lighting the stove. Once your stove is lit ensure you return the controls back to their standard settings.

Have your chimney inspected and swept frequently

Build ups of creosote or birds nests are a leading cause of chimney and thatched roof fires. You can avoid this by having your chimney both inspected and swept on a regular basis.

How often you should have your chimney inspected and swept depends on the type of fuel you are burning and how often you are using your stove. For example, a chimney connected to a stove that is used everyday throughout autumn, winter and spring will need to be swept at least four times a year.

Guide - to find out more about chimney sweeping and inspections, read our complete guide here.

Always use professionals

Whether you want to install a new chimney flue, have your chimney cleaned or inspected or you want advice on your thatched roof, always use a qualified professional.

Although it may be tempting to skimp and save some money, a thatch fire can be devastating and result in the comprehensive loss of property (and in the worst case, life). Use a qualified professional in all instances and you’ll be reassured that you’ve done everything you can to keep you, your family and home safe.

To help you find expert advice from the relevant professionals, we’ve listed the appropriate professional bodies:

> HETAS - for the installation of wood burning or solid fuel appliances and their chimney flues, speak to HETAS (Heating Equipment Testing and Approvals Scheme). HETAS maintains a database so that you can find a qualified installer near you.

> National Association of Chimney Sweeps - ensure that you use a suitably qualified and experienced chimney sweep. You can find one near to you by using the National Association of Chimney Sweeps’ database here.

> National Society of Master Thatchers - if you need advice about your thatched roof or need to locate a qualified and experienced thatcher near you, contact the National Society of Master Thatchers.

Notify your insurance

Finally, always be sure to notify your insurance when you are modifying or updating the chimney on your thatched roof property. This is also why it’s important to always use qualified professionals, as work carried out by amateurs could completely invalidate your home insurance policy.

Twin wall flue kits for thatched roof homes

As we’ve seen, twin wall flue kits are the ideal (if not the only) option for installing or updating a chimney in a property with a thatched roof.

Here at Trade Price Flues we stock a huge range of twin wall flue kits in sizes and specifications to fit all types of wood burning and multi-fuel stoves.

Not only do we have twin wall kits to fit nearly all appliances, but our kits are of the highest quality. In fact, we’re so confident in the quality of our twin wall chimney flue kits that they come with a 30-year no quibble warranty and customer satisfaction guarantee.

Plus, if you’re not too sure what you need, feel free to contact us and one of our expert team will be happy to offer you some free advice and guidance.

Shop twin wall chimney flue kits at Trade Price Flues now

Read more chimney flue and stove advice on the Trade Price Flues blog

Chimney Sweeping: A Complete Guide | What Different Types of Chimney Are There? |
What Size Flue Do I Need for a Wood Stove?

11th Aug 2021 Trade Price Flues

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