Build an outdoor fireplace

Fire up your outdoors area by building yourself an outdoor fireplace
By Robin Overall

Leaf through home improvement magazines and you will often find, somewhere, a picture of a resplendent outside fireplace. They are fast becoming desirable for the outside living area, whether monolithic structures costing an arm and a leg or the modest fire-pit or gas heater. Like all good shed-dies, I eventually thought the fatal thought. “I can build one.” Where to start? First, decide on what type of fireplace would be best. We know the average wood or coal-burning fire in a house is inefficient. There’s endless re-search on to how to economise on fuel, get more heat into the room and less heat up the chimney. All this without getting smoke into the room. And that’s what the “Rumford fireplace” fire is designed to.
An outside fire can be larger than the usual indoor version, though that consumes more wood. But it should have visual appeal and a certain “wow!” factor. I opted to make the opening 700 mm x 700 mm, so I can put on a reasonably chunky piece of ti tree without constant refuelling but also leave a few trees standing on the property. I made the hearth about 600 mm above ground level, as most people will be standing in front of it.

The base

Construction
This size construction needs a good foundation 100 mm thick and extending 100 mm out from the fireplace perimeter. Reinforcing mesh is placed in the foundation for added strength. A standard concrete mix of 8:1, eight parts of builder’s mix to one part cement, is ideal for this part. For the amount of concrete, a trailer-load of build-er’s mix is more economical than premixed concrete in bags.
The main construction surrounding the fireplace is best made with concrete blocks for a solid structure built accurately and quickly with average skills. The actual fireplace is built from firebrick.
I used a combination of standard bricks and a brick that is thinner than a full-sized brick and placed them on edge. The space between the firebrick and the concrete block outer wall is filled with a scoria and cement mix. It is stabilisation for the firebrick and insulation to stop precious heat being conducted into the mass behind the fire. Some of the space was also filled with empty wine bottles to create larger air pockets.
The concrete blocks are laid dry, that is to say, without mortar between them. This is a tad unconventional but removes a part of the process that requires a reasonable amount of skill to do correctly. They are going to be back-filled with concrete so no structural loss is created, they go up more quickly and are easier to arrange. I am also assuming that many people building this will have never laid a concrete block before so it needs to be hassle-free from the outset.
The bottom two courses are laid as in the diagram and then filled with concrete and reinforcing rods (re-bar) inserted. I have inserted short lengths that form a continuous tie between three to four layers. This is so that additional blocks can be “threaded” over the rods without the need of long ladders and scaffolding. Again, the lack of a continuous reinforcing from top to bottom is no major loss on a small structure like this. Just ensure that each new length of rod overlaps the previous one by at least one layer. Once the bottom two courses are set, the space in between them is filled with rubble, rocks, bottles or whatever will create an inorganic mass that will not compress easily. The third layer is then set in place and this is made up of corner blocks called pier blocks. These are like a standard block with a half-block joined on the side. They create a more stable corner. Each side a half lintel block is laid on its side, as in the diagram. This allows room for the side bricks of the actual fireplace to fit in neatly.

Up to third course

Fireplace hearth with scoria-cement mix layer between bricks and outer wall

The gap at the back is filled with a narrow full block (I use narrow 15 Series and full-sized 20 Series from Stevenson’s). Other manufacturers have a range of similar sizes which are generally intermixable. For rebar I used both 10 mm and 12 mm diameter as I had assorted lengths of both – don’t fret too much if you can’t get one or the other.
This course is repeated for another eleven layers. (Take a break every three layers if you suffer from altitude sickness…)
The middle blocks at the back of my fire were are not installed until later. This is because I had a scaffold plank resting between the third layer and an adjoining deck. If you have clear space all around you will need to construct a platform of sorts. This is the part where you become a truly creative sheddie. Much to the horror and amazement of my professional builder acquaintances, I built this whole structure without once donning an orange jacket, hard hat or using rows of plastic cones.

Clamp ply form at bottom to bend…

…then at the top.

Wet paper as release agent allows the form to be pulled out when the concrete has set

Front ply tied

Hearth
The fire hearth is constructed in firebrick of the standard size i.e. 230 mm x 115 mm x 75 mm. The front row bricks are laid on edge and the remainder laid flat. The back wall is then constructed from the same standard-sized firebrick laid in conventional bond, so each brick spans two underneath it. They do not need to go full width as long as you have a continuous back wall that is two-bricks wide, so half-bricks can be used at each end on alternate layers. Firebricks are not laid with conventional mortar but with a thin bed of specially formulated fire cement. This is best bought premixed and you will need approximately 4 kg for this job. It can be dispatched throughout the country by Certec (certec.co.nz) in Auckland. Failing that, a traditional bond can be made using fireclay but it is not as good in the long run.
The angled, corner bricks are half the thickness (actually 40 mm) of standard bricks and are called “splits.” These need to be cut at 45° with a diamond blade. I use a slot-ted or ripple blade dry in a cheap angle grinder, as copious abrasive dust will kill an angle grinder fairly quickly. That said, I am on my third project using the same grinder. Full-sized firebricks cut to length are then placed at each side to complete each fireplace layer. Back-fill the joints on each lay-er with a handful of refractory concrete rammed gently in place.

Filling in the throat.

A wine bottle…

…and bricks bulk out the core.

Ordinary cement against front ply (top) and refractory concrete against the curved form (bottom)

When you use refractory concrete, it is essential not to over-wet it. It is mixed with a minimum amount of water so that a squeezed handful just holds together. Then you tamp it into position and allow it to dry slowly.
Continue building each layer, back-filling with a mixture of fine scoria or pumice and sand and cement. Use about proportions of 4:2:1, four parts scoria to two parts sand to one part ordinary cement. Continue until you have completed six layers. The back wall and angled walls are continued for an-other three layers beyond this and will form the back of the throat. At this stage you install the arch bar and construct the throat which is the very heart of a Rumford fire-place.

Next day...pulling out the ply form.

Throat and arch bar
The arch bar is a piece of mild steel 100 mm x 10 mm x 600 mm. It needs to span the front of the fire and leave a 15 mm gap at either end. This will allow for expansion when the fire is going. The ends of the bar are wrapped in galvanised sheet 1 mm thick and measuring 100 mm x 200 mm. The middle of the bar is wrapped in tinfoil temporarily so any brickwork or cement will not adhere to it. Full-sized firebricks are laid on edge along the bar and mortared in place as best as you can (they are going to be back-filled with refractory concrete soon). Mortar can be plopped around the tinplate covers at either end and this holds the bar in place and yet allows the bar to expand and contract.
The throat is cast in situ against a plywood form. Cut a piece of 3 mm ply. You may have to shave the edges when positioning it so have a small plane or rasp handy. I used an abrasive disk in the angle grinder for this job. A bevelled strip of wood approximately 50 mm x 50 mm x 450 mm is nailed to the bottom edge and a couple of blocks 100 mm x 50 mm x 150 mm are pinned to the top edge. Pin these blocks with small panel pins as you only need to hold them in position. Once the throat has set, they have to be removed before the form can be taken out. Clamp the bottom of the form to the arch bar with a couple of G-cramps. Then, bending the ply gently lower it down to the level of the back bricks and clamp it in position using another two G-cramps.
It helps considerably if you soak the ply for an hour or so prior to doing this and then it will bend more easily. Also ensure that the outside grains of the ply are running horizontally.
This stage is fiddly rather than difficult and there is no rush to get it right so you can take your time. Gaps between the ply and the wall, when it comes time to cast, can be stopped with clay or wet newspaper to stop any refractory mix dropping through, so as they used to say on Dad’s Army…“Dooo--n’t panic.”

Prepare galvanised sheet…

…to place around arch bar…

…and add temporary foil.

Sloping ply
You should eventually have a gently sloping form of ply which will be the face of the throat. To stop refractory concrete sticking to it, you need to have some form of release agent. This can be silicone spray, oil, grease or simply wet newspaper. I prefer wet newspaper as it is the least messy of the lot, even if a little more fiddly. The front of the throat is formed against a sheet of ply spanning the concrete blocks and held in place by a tie-down. The throat will be cast in refractory concrete against the sloping ply and ordinary concrete against the front sheet of ply.
Mix the refractory concrete as per instructions above and tamp it in place against the form. Do the same with the ordinary concrete against the front ply. This is best achieved if you work slowly up in layers. The space in-between is filled with some wire mesh or off-cuts of rebar and the odd bottle or two to create a bulked-out core. Then the voids are filled with a sloppy concrete mix. Level off the concrete until a platform is formed at the height of the top of the throat. Leave this to set for at least 24 hours before doing anything else to it. Once it is set and cured, prise out the blocks at the top and carefully remove the plywood form.

Ply sloped to create smoke chamber opening

Ply spacer in place.

Looking down into smoke chamber. Tied wood pieces break apart for pulling the ply spacer out after the concrete has set.

Smoke chamber
The throat reduces the flue to a narrow strip 100 mm wide and this needs to be converted to a square outlet which in turn is changed to a round flue of 200 mm diameter. The original theories surrounding this conversion area or smoke chamber as Rumford called it are contradictory. We are concerned more with radiating heat and not so much with smoke, as after all we are outside and part of the fun of entertaining alfresco is going to bed reeking like a kipper.
I have constructed the smoke chamber as per plans that appear to be tried and true and it seems to work very well. It is constructed in part with firebrick and in part with castable refractory concrete. A cast slab tops this structure and has a 250 mm diameter opening in which a 200 mm stainless steel flue section sits. An expansion joint is formed with fibreglass rope.
Two firebricks are laid one either side. The back wall is extended a further three layers and a corresponding front wall built up three layers. Two pieces of ply are cut at 230 mm wide and sloped inwards to create an opening 230 mm x 230 mm. Castable refractory concrete is laid against the ply to a thickness of about 30 mm and then overlaid with ordinary concrete. When this is set, the plywood strips are removed.
The capping slab is made totally from castable refractory concrete and is cast on the ground and then placed in position once it has cured.

Top of the smoke chamber. Note top of ply forming opening.

Casting stepped hole with wood core.

It measures 350 mm x 350 mm with a stepped hole in the middle.
The shape of this hole is formed by making a polystyrene or wood core and casting around it. The smaller hole is 195 mm diameter and the larger hole is 250 mm diameter. This allows the stainless steel flue section to sit on a ledge and then be held in position by an expansion joint of fibreglass rope as described above. Once this slab has cured, it is cemented in position with the firebrick cement.
At this stage stick your head into the fireplace and see if there are any creases or voids that need patching with the rest of the firebrick cement. This not only makes for a more professional finish but aids in the flow pattern of the burning gases and smoke

Cast stepped hole in place

Chimney
You can either leave the stainless steel flue exposed, finish it with a cap quaintly known as a China-man’s hat or surround it with a large field tile, as I have. The flue expands a little with heat and if the field tile is too close a fit it will crack. I have packed the gap be-tween stainless steel flue and field tile with a sandy vermiculite mix. You could also use a refractory blanket available from a pottery sup-ply firm. Don’t pack it tightly. The field tile I used is 300 mm diameter and weighs a reasonable amount so I welded a support frame from 30 mm x 30 mm angle iron for the field tile to sit on

Flue stepped in

Fibreglass rope expansion joint.

Welded angle-iron frame supports field tile around flue.

.

Grate
While creative sheddies may like to make fire dogs, I have used a standard 600 mm x 300 mm hard-ware store grate. I raised it above the hearth base by situating cut bricks either side and at the back. If you want a larger fire, then I suggest you buy a grate first and scale the fireplace to fit that. Trying to hunt down an odd size could be a recipe for disaster.

Nail holds mesh for rendering.

Spread render mix by hand.

Rendering
The chimney , once in position, is held there by a framework of re-bar cemented into the top of the blocks. The gap at the front of the fire between the concrete blocks is spanned with a sheet of fibrolite or similar and the whole structure covered in steel lath or chicken mesh. This can be tied tightly in position and pinned every now and then with nails driven into the gaps between the blocks and then bent over.
A spatter coat of cement and sand is flicked onto the structure to form a key. The second coat is a 5:1:2 mix, made up of five parts of coarse sand or plasterer’s sand to one part of cement and two parts of reinforced exterior plaster. This latter compound contains fibre-glass strands and keys the whole mix beautifully. Dampen all areas before you apply plaster, otherwise the dry surface sucks out the moisture from the render mix and it will not adhere very well. If you have access to chopped fibreglass strands then these could be used instead of the plaster.
This mix is spread on with the hands to about 30 mm thick. As it cures, there is a small window of opportunity when you can smooth it with a damp sponge and achieve the perfect adobe look. If you need to apply more to sunken areas, do it after the first layer has dried. In hot conditions, cover all work with wet sacks or similar until it is cured. The final coat is a mixture of silica sand, lime and white cement. If you want a coloured finish then you can add oxides to this layer and depending on depth of colour you may be able to use ordinary cement instead of white cement. The recipe for this mix is 6:1:1, six parts silica sand to one part of white cement and one part lime mix. This can be applied with a sponge or stiff brush or even sprayed on. The advantage of these finishes over paint is that they age elegantly and mature rather than merely look neglected and unpainted in a few years time.

Outdoor fire finished with gargoyle.

Other finishes
Other finishes can be applied to structures such as fireplaces or pizza ovens. The whole could be clad in recycled brick to complement a period villa, for example. Or how about surrounding it in a cage of galvanised mesh and filling this with stones like those fantastic South Island river stones.
Or, cladding the whole structure in tile or mosaic à la Hundertwasser or Gaudi?
Whatever you decide to do, you will have a magnificent addition to your property and one that will undoubtedly add value to it. And you will have built it for a fraction of the cost of those advertised in the glossy magazines.
Have fun.


THE IDEAL FLUE
The ideal flue size is related to the area of the fireplace opening, with the flue area approximately one-seventeenth of the fireplace opening area. But as flues are designated by diameter, you need to work out the diameter of flue needed.
Calculate the area of the fireplace opening by multiplying the width of the fire by the height of the fire and to get the ideal flue area, divide this area by 17. Then to get the flue diameter (using the formula that radius squared = area divided by π) divide again by 3.142, find the square root of the result for the radius and double it for the diameter. The diameters of flue given in the table are those of readily available flues which are nearest to the calculated diameter.

Count Rumford

THE RUMFORD FIREPLACE
The object of a chimney is to create a draw for the flames and therefore drag in more air (oxygen) into the burning fuel. During the 18th-century, American inventor Benjamin Thompson developed a better way of burning wood in an open fire.
His design enabled the fire to draw sufficiently so that smoke did not enter the room but it also consumed the fuel more completely. Thompson was born in Woburn, Massachusetts in 1753 and, because he was a loyalist, left with the British in 1776. As an employee of the Bavarian government he received the title “Count of the Holy Roman Empire” and so became known as Count Rumford.
Back in England, Rumford applied his knowledge of heat to the improvement of fireplaces. His “Rumford fireplace” was smaller and shallower than normal with widely angled covings so they would radiate heat better. He streamlined the throat junction between the fire and the chimney. Unlike an ordinary fireplace sloping towards the front, a Rumford fireplace has a vertical back.
The distance from front to back is also less than in a conventional fireplace. The opening at the front is usually wider and indeed approaches square. This enables the fire to reflect more heat into the room and cuts down on the speed at which the smoke rises.