How to Build a Fire Bunker
Preparing for Fire Season
When the government issued a catastrophic fire warning for our area, we packed the car and headed for the nearest town, but first I put water and first aid supplies in my fire bunker just in case our neighbours or the local volunteer fire brigade were caught in the area and needed to take refuge.
There was no immediate fire danger but with winds 100km/hr and temperatures over 40°C there was little chance of controlling any outbreak.
In the car, we took passports and birth certificates, laptop computers with all our photographs, our family pets, a few special items plus water, woolen blankets, and a chainsaw. The blankets were to cover ourselves and offer protection from the heat in the event that we were caught in a fire on the way and forced to huddle below the level of glass windows, and the chainsaw might be needed to clear a path if fallen trees blocked the road.
It was only as we were driving in that we noticed just how big some of the older trees are. Huge trees can topple in such winds, so next time, we will remember to take wire cutters in case the quicker and easier option is to cut through a wire fence and travel across adjoining land to rejoin the road past fallen trees that are too big to clear.
In the event we encountered a fire along the way, it would be safer to leave the tree-lined road and drive the car into a relatively clear area where the fire was racing through grass instead of trees. Very few properties have livestock on them and we could always go back and fix the fence after the fire danger passed.
The catastrophic fire warning was sent to phones in the area so residents could implement their fire plan. Our plan is to evacuate if safe, or climb into the fire bunker in an emergency.
What Will You Do in a Wild Bushfire?
I began building our fire bunker soon after purchasing this land. Whichever way you look at it, our home is vulnerable to fires. This region did not have a history of catastrophic bushfires, but nor did other areas in Australia that have been devastated by unexpected fires in the past 20 years with extensive loss of life.
There have been similar wildfires overseas, including California. I believe every home that's vulnerable to fire needs a safe refuge. One of the safest places when a bushfire is raging is an effective hole in the ground.
My husband agreed a fire bunker would be wise but insisted we should wait until we could afford to build a professionally designed fire bunker constructed from concrete with a proper fire door and all the trimmings.Meanwhile our money was better spent on more immediate needs like sleeping and cooking and showering and solar power and hot water and all the other necessities that make a home instead of spending it on an expensive hole in the ground that we may or may not ever have to use.
Of course he was right about where the money was best spent, but I insisted I'd rather have something than nothing.Trees in our part of the world are notorious for quickly catching alight and spreading embers and burning bark great distances in high wind.
My husband thought it was hilarious when I began digging a hole in the ground.
What Makes a Fire Bunker?
Fire Bunker Design
I wanted the fire bunker to be deep enough to receive protection from the surrounding earth and also deep enough to allow my tall husband to stand within if he needs to stretch his legs.
I designed an extra deep point so that carbon dioxide, which is heavier than oxygenated air we breathe, could fall to the bottom well below the level at which I was creating built-in seating.
It also needed to hold a plastic garbage bin in the bottom that I could fill with water so we could wet woollen blankets or other items we might need to protect ourselves from smoke in the event my fire bunker was less effective against keeping smoke out than I hoped.
I chose an area alongside our dwelling. If we wake in the night to the smell of smoke and the bushland around us is alight, the fire bunker needs to be close and easy to access.
The entry door faces away from our home because there's every chance our home would burn sooner or later in a severe bushfire and I wanted the door to be clear of any falling debris. It would need an overhang to protect the door and the fire bunker door must open inwards just in case something blew against it. We wouldn't want to be trapped inside.
Another reason for the door opening inwards was I like the idea of being able to pile some kind of additional insulation against the heat immediately outside the door before closing it.
Constructing the Fire Bunker
The ground at the site I chose was red clay, different to the rich dark soil elsewhere on the property. It was like digging into rock so I waited for rain and dug when the ground was softer.
All the while I was hatching a plan for an efficient design that would not cost too much money.
I considered building a structure from rendered straw bales because the clay I was digging was ideal for the render stage, but that would still involve some expense because I wanted it strong enough to have a living roof.
My fire bunker was destined to be temporary not permanent. I wanted a fire bunker that was strong and effective if needed, but I had insisted it would be inexpensive and I had implied I'd be using recycled materials.
As word spread of a newcomer building a fire bunker one friendly couple offered me a Cinva Ram to make my own bricks. They hadn't used it for years and offered it to me as a gift.
I tested my clay, established it was perfect for the press and made a few bricks but decided I'd need far too many bricks to provide the level of protection I was looking for, so the Cinva Ram waits for another project at another time.
Here's what I did instead.
As the hole deepened I built steps down from my planned entry area. As soon as it was deep enough for me to stand inside and wield my shovel without the need for much headroom, I reclaimed the largest of the old rusting corrugated iron water tanks that were on the property when we bought it.
We had a number of corrugated iron water tanks sliced into sections around the garden being used as raised garden beds and animal shelters so I asked my husband to cut the large ones in half, making domes.
Each section overlapped the next and then I placed a second layer of tank pieces over the first for extra strength.
The base sections of tank form the far end of the fire bunker.
The edges of all pieces have been inserted into a shallow trough I dug around the perimeter of the bunker to ensure they retained their dome shape while I covered them.
Water and Heat Insulation
We had some left over RhinoWrap – silver insulation with a blue plastic covering generally used to insulate walls and ceilings in a traditional building, and I thought it might be better to cover the structure with the insulating material instead of just waterproofing with black PVC we also had in the shed.
I phoned the telephone number printed on the RhinoWrap and spoke to a very friendly man in the technical section who listened to my fire bunker construction dilemma and was very encouraging. He stressed that the company doesn't recommend their products for fire bunker construction but said that it should last for a good ten years underground. He was extremely helpful, reminding me to put the blue plastic on the outside because it should help water that penetrates the earth to run off the surface of the water tanks and it melts in extreme heat, leaving the silver to provide the final layer of heat protection. We discussed the appropriate amount of overlap needed and he suggested I tape the joins, which I did.
I ran the RhinoWrap from one side of the fire bunker up and over the dome and down the other side, making sure there was sufficient on the ground to run a skirt onto the surrounding ground, which I immediately covered with clay.
I put a double layer of RhinoWrap at the end of the fire bunker closest to the potential greatest fire risk. I believe this product is available in many parts of the world.
Additional Protection From Radiant Heat
By this stage of the fire bunker construction my husband was sufficiently convinced of my determination to work with what we had available that he became actively involved and helped me build the thick earth walls.
We sliced big sods from the ground in a different part of the garden destined to become a vegetable garden. Nowhere near the same amount of clay in the soil, but enough to hold the soil together in what was very much like a big slab of mudbrick.
For the first batch I trimmed the grass with scissors before placing the sod in place, leaving the roots of the grass in place to help hold the natural mudbrick together. Before chopping further slabs from the ground my husband had the genius idea to mow the grass first.
I used the block-like sods to hold down the ends of the RhinoWrap and form a base extending out at least three feet from the fire bunker's corrugated iron frame on all sides except the front, then packed earth in between and on top of each sod in the way one puts mortar between bricks.
The earthen protection is even deeper as the dome rises because I wanted the outside face of the earth 'wall' to be steep against the fire and thick enough to provide plenty of insulation against the heat irrespective of how strongly the fire wind blows.
At each level my daughter and any visiting friends would stomp the ground down and most evenings I sprinkled loads of water from a watering can over the new work to help pack the earth down in and around the natural mudbricks.
Whenever we had a spare moment my husband would slice the blocks of earth and I would build.
For the past couple of years since its construction, including during the winter months, I have been climbing into the bunker and digging deeper with a small trowel, removing the extra earth in a bucket. The door is tiny and requires my tall husband to be a contortionist when he enters but he now has plenty of head room once he's inside.
Covering the Top of the Fire Bunker
The very top of the fire bunker ended up with about 18 inches of soil and clay layered and mixed, significantly less than the walls but with the RhinoWrap and two layers of corrugated iron beneath it I'm confident there's enough protection from the heat.
I believe the walls will have to endure much more of an onslaught from the wind and wind-blown burning debris than the top, and I don't want to push my luck with the weight of wet earth despite it being a dome shape.
During most of the year our fire bunker simply looks like a big mound of earth covered with grass and herbs, lush and green ... but having been hammered by extraordinary heat and ferocious winds in recent weeks the growth was looking was looking dead and sad.
We have removed the dead grass from the top and added a little more soil covering in the past few evenings. I will give it an inch or two of good garden soil then sprinkle camomile seeds and plant strawberries over it within days.
I will feel happier watering strawberries than grass with our precious water, and because strawberries have nice thick relatively shallow roots they should help bind the soil together without compromising the integrity of my waterproofing.
Position of the Fire Bunker
Our fire bunker is positioned close enough to our home to be able to reach it easily if we wake in the middle of the night to discover the bush around us is on fire.
We have placed a large water tank nearby and changed some of the guttering to collect rainwater in it. With a hose it can gravity feed water into the fire bunker to fill up the water storage there, and to fill the channel I dug into the clay across the entry way, hoping to catch embers before they reach the door.
With a Shurflo diaphragm pump (made in the USA) connected to a car battery we can pump water over the fire bunker to dampen the earth. If the water tank melts or explodes due to the heat in a fire, I expect any remaining water to run downhill without compromising the integrity of the fire bunker.
Between the fire bunker and the trees and bush beyond is the little tin shed that houses all our solar batteries plus the inverter and charger. I am concerned about the flammability of deep cell batteries but the earth wall is even thicker on that side of the fire bunker and I like being able to run power into the bunker year round to occasionally run large fans to freshen the air. I hate the idea of our fire bunker being filled with stale air if we are forced to take refuge there.
My most recent thought was to place my Envirogrow CFL energy saving horticultural lamp in the bunker to encourage plant growth inside the fire bunker. What better way to ensure plenty of oxygen in such a confined space? By growing culinary herbs in pots in the deepest hole in the fire bunker I can get rid of carbon dioxide and put the plants to good use in the kitchen. Running the horticultural lamp requires an extension cord to the solar shed. I have put the lamp in there but have yet to position it correctly and connect it to power.
I don't need the main solar power system for general lighting especially during a fire because I have a little Nature Power solar lighting system in the fire bunker with its own small solar panel that keeps it charged for its own lights plus any USB connections.
When the Fire Is Burning...
There is no way to predict the exact circumstances of any future fire that reaches our home. For this reason, I am taking steps to ensure we are adequately equipped to safely fight the early stages of fire if necessary, and to safely extinguish spot fires after the main fire front has passed.
In addition to cotton and woollen clothing, we have invested in a number of specific items to increase our effectiveness when fighting fires, and give us the best possible chance of avoiding injury.
If you live in a high fire danger area, I encourage you to do the same.
Ventilation in the Fire Bunker
I researched and considered my options for ventilation from the very beginning of building the fire bunker.
I liked the idea of cross ventilation but considered the risks too great in trying to incorporate a ventilation source on the side of the fire bunker where our home and all its potentially toxic smoke might be generated in a severe or catastrophic bushfire.
The idea to grow plants within the fire bunker using the horticultural lamp is only recent but it is not the first time I considered growing plants to oxygenate the air. I considered a lot of options in the process including installing a small double-glazed window to let sufficient light in to grow plants, however the risk and the expense of building an appropriate frame and an effective shutter to block the heat during an extreme fire seemed too great.
At the moment here are my ventilation measures in our fire bunker.
An old-fashioned metal ventilation insert with a knob on the front to open and close from the outside dependent on the weather, and a hook on the inside that allows me to open and close it without leaving the safety of the fire bunker once inside. The ventilation insert has been framed with wood and the frame painted white before being used.
The RhinoWrap is caught between part of the frame and the ventilation insert to help seal the area. The wooden frame on the inside extends beyond the actual ventilation source and can be sealed airtight and fire tight if the air outside is too smokey.
I have two tiny fans removed from old computers ready with large torch batteries to power them. Turned one way, they suck air into the bunker. Turned the other way, they push air out. My early experiments with the computer fans included inserting them into lengths of PVC pipe with a view to expelling carbon dioxide from the depth of the hole. I intend to use them to increase air flow through the metal ventilation when appropriate.
A tyre insert filled with air, ready to let small amounts out when needed. Admittedly the air will smell of rubber, but if we need it that won't matter much. A local fire brigade chief suggested it and explained that sometimes you need to increase the air pressure inside the fire bunker to stop the infiltration of smoke. I'd prefer the smell of air from an air mattress, but sadly my fire bunker is not large enough to hold one. The fire chief, by the way, loves my bunker. :)
The wooden door to the fire bunker is painted white. It has small gaps in it that also contribute in small part to ventilation when there's no fire. If we have to use the bunker we will tack extra RhinoWrap to the outside of the door plus probably build a structure of bricks or cement blocks or cinva ram blocks in the sheltered area outside the door. On the inside of the door we will attach wet fabric.
Interesting to note, my research indicated that wood painted white has greater strength during a fire than metal. Of course a properly endorsed fire door would do the job better but in the absence of an official fire door, I opted for white paint on wood. Worth mentioning, you can buy a powdery substance to add to paint that increases its insulation value and its ability to resist fire. My husband threw the packet away before I could photograph it or take note of its name but every paint store should be aware of this type of product.
Pots of herbs ready for their new home in the fire bunker to help oxygenate the air.
The design of the fire bunker allows me to drape an additional fire blanket or wet woollen blanket over the entire entry area from the living roof across the clay channel to the ground beyond although I'm not sure how it would resist the wind. I am looking out for an appropriate screen door from a large glass sliding door to cover with a fire blanket then prop there. With a bit of fine-tuning, I suspect I could capture 'good air' in the area outside the bunker.
Inside the Fire Bunker
Inside my fire bunker I keep the bare essentials.
- A plastic garbage bin contains woollen blankets for storage, although at the moment given the recent catastrophic fire warning I have filled the bin with water and taken the woollen blankets out.
- All blankets but one are stored in thick plastic bags because I'm leaving the door open during the days at the moment to let plenty of fresh air in and there's a slight possibility venomous snakes will enter the bunker. I check there's no snakes in there before locking the door at night, ready in case we need to climb into it in an emergency.
- I have a plastic box filled with first aid supplies and basic tools. I keep a long hose rolled up and the fittings for the tap in the hope that at least one water tank and tap would survive the onslaught of fire. If our home has survived the initial fire storm I would like to be able to put out any spot fires.
- I keep another plastic box with food items - enough to sustain us after the fire, if needed. Plus a generous supply of drinking water.
- There's room for our family pets but sadly the farm animals will have to fend for themselves. We plan to make a hen house and pig shed from rendered straw bales one day in the future, but that hasn't happened yet.
- There's room for laptop computers, mobile telephones, ipods, cameras and important certificates and paperwork to be stored in a large plastic tub with a lid. We might decide to spray water throughout the entire bunker and it would be a shame to ruin the electronics and paper work in our enthusiasm for dampening everything.
- My one regret when sitting inside the fire bunker was my decision to place the rustiest sections of water tank beneath the stronger nicer ones. I dislike looking at the rusty surface, but from an effectiveness perspective I'm sure I made the right decision because strength is important.
- The nice smooth surface close to the earth above is good for protecting the RhinoWrap from tearing. Perhaps I didn't need to use the additional pieces at all, but the project is finished now and I am certainly not tempted to pull it all apart and start again.
- I have considered painting the interior to seal the rust, but I see no advantage to contaminating the air with paint. Perhaps I will reconsider it in the winter. Meanwhile I keep dust masks in the top of the large first aid box. We can use them to filter the air if we are in there during a fire and worried about the rust.
- We'll have music, light and a few books to read to help us relax and not breathe too heavily while waiting for the fire to pass. I also have a small shovel and trowel on hand, ready to fill any problem air leaks particularly in the front entry side of the bunker with wet clay should the need arise.
- I am in two minds whether or not to include a tent in my fire bunker in case we need it after the fire passes and before any insurance claim comes through. At the moment I am thinking I'd rather have the extra space filled with air instead of a rolled up tent, but I might regret that decision if our entire area is devastated by fire.
There may be a few essential items I have overlooked. What would you put in your fire bunker?
Drinking Water ... Essential during a Fire
I began with a soft collapsible container to hold drinking water but replaced it with a strong water container like this one. Why? Because there's a risk we'd accidentally break a less sturdy container in the process of quickly entering and moving around the fire bunker during an emergency. I suggest you also purchase the waterbrick spigot if you choose this option. If you stack other items on top of the water brick, you'll want easy access to the water without having to remove the lid.
Inside Our Fire Bunker
Update to my fire bunker story ...
- Building rammed earth walls to improve my underground bunker
After receiving positive feedback from everyone who sees our fire bunker, (including professional fire fighters!) we are adding rammed earth walls to create a new entry. After voting and commenting here, please come and take a look. Thank you!
Questions & Answers
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