Understanding Timber Movement


Why it is imperative you have a clear understanding of what timber is and what it does.


As a qualified carpenter and joiner and; flooring layer and finisher, I have been working with timber for over 35 years now and one thing that has become abundantly clear is that there is a lack of knowledge and understanding of what timber is and what it does. So this article is for those who wish to gain a greater understanding so they can avoid common mistakes or prevent potential issues in the future.

What is Timber or Wood? A quick search of google will produce on Wikipedia; Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material – a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression.

Put into simpler terms timber is made up of cells that are like straws packed together, the straws carry moisture and nutrients through a tree throughout its life and when the tree is felled, the cells are full of water. When the tree is cut up, it is considered to be wet, so the timber is air dried for a period of time to let the moisture evaporate from the cells , this process can take some time and some timber mills will let the timber air dry for a couple of years prior to then putting it into a kiln to dry the timber further. Some timbers will never be kiln dried, for examble sleepers are only ever air dried, typically they are used in an outdoor envoironment, so the moisture content can remain higher.

A piece of furniture or a timber floor however, will need additional drying in a kiln to bring the moisture content more inline with the internal envoironment of a home. Typically this is around 9 to 10 percent, this equates to a relative humidity of about 55 percent and and average temprature of 20 degrees. Timber that has been dried to 9 to 10 percent in this environment will stay reasonably stable and shouldn’t move a great deal however, if you were to drop the humidy to 40 percent and increase the temp to 25 degrees, the moisture content of the timber would need to be 7.6 percent for it to remain stable. Timber will always try to reach an equalibrium with its environment, so timber that is at 10 percent put into this environment will lose moisture until it reaches 7.6 percent, think of this in terms of filling a balloon with water, if you reduce the amount of water in the balloon – the balloon skrinks in size, add water and it expands, well timber is made up of millions of little balloons and responds the same way. You can find a more in-depth description of this here https://www.connollys.com.au/acclimatising-timber-flooring/

You need to understand this because a number of things you do in your home will affect how the timber reacts. Things to be aware of but not afraid of, is heating and cooling. Different types of heating and cooling systems will affect the relative humidity within the home, reverse cycle systems will strip moisture from the air, this is why they have a drain hose, ducted heating systems can dry the air as well as introduce heat, evaporative cooling systems will introduce moisture into the air, so understanding the type of heating and cooling you have can help with preventative actions to counteract them. For example with evaporative cooling the biggest mistake I see people make is not opening up doors or windows enough to let the air being pumped into the home to escape and this makes the moisture build up within the home, when the correct amount of air is able to flow, there will still be an increase in humidity but not to the extent that it dampens surfaces. A good idea is to keep a small weather station inside the home which will tell you at a glance what the humidity and temperature is within the home (click here to view), and what is good for the timber is also good for you is humidity somewhere between 50 and 60 percent and temp around 20. If you find that this is the average that you see in your home, you shouldn’t see large movements in any timber within the home. If you find it varies between 45 and 65 percent you could expect some additional movement in the timber but not so much that should be of major concern, however if your humidity is 40 percent or lower for extended periods of time you should expect to see shrinkage accross the floor or in furniture. If the humidity is 70 percent or higher for extended periods of time, then you should expect the timber to grow and cause cupping. Knowing what is happening inside the home is the best way to counter conditions that aren’t ideal. I had a phone call from a customer at the begining of winter who had a contained wood fire with a fan that was blowing accross the floor causing the floor to shrink, he was wondering if there was anything he could do to reduce the impact on the floor. As a possible solution I suggested perhaps placing a flat tray of water in the line of the hot air flow, this will do two things, firstly it will take the brunt of the hot dry air off the floor surface and secondly as the water evaporates it will replace the humidity that was being lost as a result of burning a fire inside.  He rang back about 6 weeks later and thanked me, he had done exactly what I suggested and it had worked a treat, and the added benefit was a more enjoyable environment. Ducted heating is a common cause of shrinkage in flooring especially around and close to vents, a few things to beware of, first make sure the boot inside the opening comes all the way to the top of the floor, second make sure the air is being directed up and not accross the floor surface, thirdly don’t put anything over the top of a duct like a couch or cupboard, this will trap hot dry air on the floors similar to putting it in an oven.  All of these things you would think are obvious, but you would be surprised how many times I encounter some or all of these things happening in houses. Other things that can affect your timber are direct sun on the floor through windows, the heat from the sun will activate the moisture within the timber and cause it to come out resulting in shrinkage, simple fixes are curtains, blinds, and awning etc anything that will diffuse or deflect the direct sun on the floor. 

Understanding how timber reacts to external forces will either give you the tools to prevent problems or to troubleshoot them. For instance if you had a floor and in a single section of the floor it started to grow and cup or tent but the rest of the floor was fine, you can be pretty confident that there is moisture ingress localised to that location, so start looking for the cause, because trust me there is always a cause. As I’ve tried to establish, timber only grows when it takes on moisture and only shrinks when it loses it, the first thing to look for is any water sources ie pipes to sinks, shower, washing machines etc, or drainage points – sinks, baths, dishwashers anything that carries water to and from the house and check all of these for leaks, also check showers and tiles in bathrooms for potential leaks. Once all of these are checked and if no leak exists, check under the house for wet spots, water gathering under your house can basically steam the floor from underneath as it evaporates. Another common point of moisture ingress and probably the least known is from outside the home, where pathways or garden beds have been raised up too high and the moiture works its way into the home laterally. So, if your garden beds have been built up against the house you may want to check there irrespective of whether you have timber, carpet or tiles, moisture ingress like this can cause mould to form and also rot in timber frames and cause expansion of skirting boards. I once had a floor on the bottom storey of a two storey house, the floor was in a theatre room that was located in the middle of the house and it was growing substantially and the home owner and the builder had no idea why, the rest of the floor was perfect. They called me out to investigate and initially I was a little stumped until I noticed a tiny amount of swelling in a skirting board, we removed the skirt and found the frame was saturated, to make a long story short, it turned out a flashing on the roof had come off, it probably was never secured properly from the beginning, but rain water was getting in there and making its way down two floors and coming out into the theatre room. What makes this one so memorable for me was that the builder was adamant that it was either a timber problem or an installation problem right up until I found the problem with his house. In my 35 years plus I have only ever encountered one floor where it was a timber problem and not as the result of some external force being exerted on the timber and the timber just doing what timber does.

The power of timber – People don’t seem to understand just how strong timber really is and what it can do if mistreated. In medieval times if you wanted to crack a large piece of granite in two how would you do it with the tools available to you back then? The answer might surprise you, some smart cookies worked out that if they drilled into the rock then hammered in a dried peice of timber and placed a drip feed of water onto it,  the timber would soak up the water and expand and split the rock in two, pretty clever right? They understood that and so do we. This is why you have a 8mm gap between 135mm decking boards and why you should always coat the entirety of decking timber prior to installing it, the bottom, the sides and the tops and ends. You would be amazed to know how many times I have had arguments with people calling themselves carpenters who lay decks with 3mm gaps and only oil the top, build over ground with no drainage, build to close to the ground, use nails instead of screws, nail or screw in the wrong place, don’t know which way is the best way to face the grain of the timber and I could go on; and then they wonder why they have problems and the first thing they do is blame the timber. It is not the timber that is the problem, it is the complete lack of understanding of how to treat the timber to get the most out of it. Therefore, knowing beforehand that the timber is going to expand and contract, especially outdoors and, knowing that a 135mm board that is saturated could expand 6mm or 7mm , it does not make sense why anyone would think that a 3mm gap would be sufficient, believe me it won’t be! If they swell 7mm they will push against each other and tear out whatever nail or screw you have used to hold it in place. Not coating the boards all the way around will result in cupping, you feed moisture into the underside of the timber and lose moisture from the top, so the bottom grows and the top shrinks causing the boards to curl up at the edges. It’s all cause and effect, if your floors are shrinking there will be a cause, if they are growing there will be a cause and therefore there will be a solution and that may be as simple as opening a window. 

To summarise, timber is not a man made product that is impervious to its surroundings, it is one of natures most wonderful inventions and as a natural product it is susceptible to the laws of nature, we can machine it, mould it, stain it, screw it, glue it, and turn it into the most incredible things and if we look after it, it can perform amazingly for hundreds of years, so understand this amazing product and treat it the way nature intended and you won’t have problems.

Below are some videos from around the world, you will notice they all have the same message and that is because the rules of timber don’t change wherever you are, please enjoy.