Just as pallets are incredibly useful, so too is 'pallet wrap' - a very thin, transparent and incredibly strong plastic film supplied on rolls which is both stretchy and self adhesive. Other than a pallet itself - surely there is no other product with such useful qualities? The number of possible uses is endless, but helping stack firewood must be somewhere near the top of the list.
Why do we need pallet wrap and why do we need to stack firewood at all?
First of all, firewood needs to be stored somewhere dry, preferably for about 2 years, so that most of the moisture within the wood is evaporated or else much of the precious carbon energy locked in the wood is wasted by evaporating large quantities of water in the fire itself. Check out 'The latent heat of evaporation of water'.
Secondly, stacking wood saves space and thirdly, pallet wrap stops our stack from collapsing and provides extra heat from the 'Greenhouse' effect of using translucent film.
We could just throw our precious logs onto the ground and cover them with a tarpaulin, but this has many problems:
- The logs in contact with the soil will stay damp
- It takes up a lot of space
- Tarpaulins are expensive
- There won't be much ventilation for the logs to dry effectively
I stack my logs on pallets to keep them off the ground and help with ventilation - some air can circulate under the log stack. I also create special vents in the pallet wrap to help evaporate the water..
But let's just go back in time a bit to last year when I wanted to stack my firewood on a pallet to dry. The middle of Summer is the best time to do this job as it gives the wood, which was cut the previous winter, enough time to dry for the forthcoming winters.
So, I was dutifully stacking my wood thinking what a nice pleasant and easy job it was and after getting to about 1 metre (3 feet) high ........ One of the corners of the stack spontaneously collapsed and I literally had to rebuild the whole thing from scratch - DOH!
Stacking wood is a lot easier against a wall or in a shed where you have upright surfaces to lean it against, but my stacks need to be free standing like in the photo above. Anyway, after rebuilding the structure 3 more times, I realised that I had to give the whole task a great deal more thought and consideration before the next build.
Step 1: How It Works
The log stack is ventilated by a combination of natural ambient wind and hot air convection currents induced by the 'Greenhouse Effect' within the stack.
When the sun is shining, convection currents occur as expanding hot air requires an escape route. If the sun is in the west, then the west facing sides of the stack get warm - up to 40 degrees C so far. The hot air expands and rises and this movement sets up a circular current inside the stack which results in air moving downwards on the colder East side of the stack and out of the vents.
The simulation above assumes that the wind and sunshine is coming from the same direction. The air comes in from the vents and under the pallet on the left hand side and rises up into the top of the stack by convection currents and comes down again and out of the vents again. The graphs at the 'Graphs' step confirm that air is moving fairly freely, even deep within the stack, by showing constant temperature and humidity changes on the probe located therein.
If the humidity in the centre of the stack was a constant 100% and the temperature never changed much, then the conclusion would be that there was no air circulation and inevitably our logs would turn to compost.
Step 2: Key Considerations
- Protection against the rain.
- Having enough ventilation to remove moisture in the wood.
- The stack does not fall down or fall on top of anybody.
- Easy to build without too much skill.
In theory, we could build the log stack without pallet wrap if we had the skill or were just plain lucky, but to make sure we are successful, pallet wrap is the answer. But why risk the stack falling over at all? What if a child was playing nearby or started to fiddle with some of the logs at the bottom or even climb up onto the top?
Other than using pallet wrap, I have also attempted to stack the logs with the best possible technique. Looking at the first photo above, the blue line shows the vertical and the red lines show where some of the corner logs are sloping downwards slightly - this is to stop them spontaneously 'popping' out of the wood stack and causing total disaster!
The photo shows the 'XY' plane, but we also need to have logs doing the same thing in the 'YZ' plane. If this is beginning to sound complicated, then looking at the computer simulated log stack in the video or next photo should help. Who would have thought that we'd need computer simulation to learn how to stack firewood?
A big problem with creating a stack of wood is that everybody, when asked, will reply 'yes' if they are asked: 'Do you know how to stack wood?'. What could possibly go wrong with something so simple?
If firewood was the shape of regular bricks, it would be much easier, but we'd still need interlocking corners.
Summary: Try and get the corner logs to interlock and slope downwards slightly into the stack in both the XY and YZ planes.
Step 3: The 'Magic Log'
The computer simulated log stack shows much more clearly how to actually stack firewood without the whole stack falling over. In purple, we have our interlocking corners, which are fairly strong and self supporting, BUT, we have also got 'pressures' or 'forces' from within the stack trying to push these corners outwards, especially if all the logs are round. Fusion 360 was used for the modelling and animations in the video.
If we wanted to go to the next stage of sophistication with our log stacking we'd want to interlock the corners themselves with the rest of the pile of logs.
One of the 'magic logs' is shown in the green circle and these are used to interlock the corners with the rest of the stack. We can also see how the logs in the corners are sloping downwards in the YZ and XY planes, which are effectively 'pushing back' against the forces within the centre of the stack.
Please note that it is not necessary to machine all your firewood into perfect cylinders! The best firewood for stacking is the stuff that has a square-ish profile and has been chopped from the rounds. To make life easier, select the square logs as much as possible for the corners and interlocks and use the round logs for the sides.
Any 'irregular' logs that are difficult to stack due to side branches etc are put into the centre of the stack, out of harm's way. They can, themselves, be stacked or just thrown in randomly.
Summary: Create interlocks between the corners and the rest of the stack and use the best shaped logs for the most critical parts of the structure.
Step 4: Tools and Materials
- Large roll of clear pallet wrap
- Roll of duct / gaffer tape
Step 5: Start With the Corners
Carefully select logs that create good foundations for the corners. Square profiled logs are better. Try to get the logs sloping back into the stack a bit.
Step 6: Fill in Between Corners
It's important to build up the stack in a uniform manner, without building the corners too high too early or else they won't interlock with the rest of the pile.
Step 7: Build Up to Fourth Layer High
Step 8: Fill in Along the Sides
Step 9: Build Up Corners to Six High
Now we are getting to a more critical stage and it's important to try and keep the corners and sides as upright as possible.
Step 10: First Layer of Pallet Wrap
Wrapping up the first layer should stop the corners from popping out as long as they have been built reasonably well. The pallet wrap will help correct and alleviate any mistakes in the stacking.
Step 11: Fill in the Middle
The middle of the stack can be filled in with stacked logs, or if they are too irregular, just chuck them in randomly.
Step 12: Another Wrap at 12 High
The pallet wrap is one continuous run - it is not cut at all between layers.
Step 13: Another Wrap at 18 High
Get the wrap nice and tight without actually damaging it on any protruding logs.
Step 14: Random Fill the Top
A nice domed top is created at the end for spilling away rainwater.
Step 15: Wrap Over the Top
The top is wrapped over as shown and then another horizontal wrap is made to tie in all the vertical wraps. The final edge is tapped up with duct tape.
Step 16: Insert Ventilation Logs
The horizontal wraps can be pulled open slightly at the bottom of each section to create 'louvre / louver' vents. A suitably sized log is stuffed into the gap to hold it open.
If these louvres are not created, the moisture from the wood will not escape properly and condensation will form on the inside of the wrapping. Rain can't get into the vents as the gap is at the bottom.
Step 17: Graphs and Measurements
I've only had the sensors working for a few days so it's too early to see the long term humidity trends, but there is already some useful information to see.
- Channel 1 (BLUE) : Sensor located in free air in shade under pallet.
- Channel 2 (GREEN) : Sensor at top of wood pile just under the polythene.
- Channel 3 (ORANGE) : Sensor right in the middle of log pile.
- Channel 'IN' (RED) : Sensor on the west side of wood pile just under the polythene (gets afternoon sun).
The 'Greenhouse effect' is causing large temperatures to develop in the top and sunny side of the pile and there is enough air circulation for the hot air to find it's way into the middle and cause temperatures to rise there as well, especially during the middle of a sunny day, even though hot air does not normally want to go downwards. The humidity in the centre of the pile is also fluctuating which also suggests that air is moving around nicely.
If the logs in the centre were still wet (they were stacked after a rainy period) the humidity would be 100%, so obviously the water on the surface of the logs has gone, which I confirmed by manual inspection. Moisture inside the logs will come out much more slowly and I'd expect the humidity on channel 3 to slowly drop over the next 18 months. Watch this space for the next exciting update!
Just to prove that the logs are actually drying out, one freshly cut 'green' log was cut, weighed, labelled and placed in the centre of the stack and weighed again 6 days later. It had changed from 1.638 to 1.596 kg which is 2.6 % weight loss. I will weigh it again in about 2 weeks time, when it should have lost about 5% - It's going to be a parabolic curve when displayed in a graph as weight loss will be quick to start with and then progressively slow down to zero.
- 0 days: 1.638 kg --------------
- 6 days: 1.596 2.6% loss
- 26 days: 1.480 9.6%
Have a look at the last graph above!
Step 18: Finished
Tape up any loose edges in the pallet wrap with gaffer tape or else the wind will catch hold of the edges and start to undo all our good work!
- Create interlocking corners with logs sloping downwards into the pile slightly.
- Select the best shaped logs for the most critical parts of the structure.
- Create louvre vents in the pallet wrap.
A tree fell down in a neighbour's garden so I sawed it up into rounds and stacked it with a chimney as an alternative to the louvre vents.
Please feel free to add suggestions for improving this technique in the comments section below. This instructable will be updated if I have missed anything.
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