# does external wood shrink or expand in the winter?



## fobos8 (18 Jan 2013)

Hi all 

I'd really like to understand better how weather/seasons effects the width of wood that is outside. 

I know inside a house wood decreases across its width in the winter due to drier air/less humidity and increases across its width in the summer due to more humidity in the atmosphere. This explains why doors catch in the summer but close fine in the winter. 

Isn't external wood effected in the same way? 

Would a piece of external painted timber be narrower in the winter than in the summer? 

Cheers, Andrew


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## eribaMotters (18 Jan 2013)

In theory yes, [I can think of something else very important that shrinks in the cold], but its the moisture content that counts with wood. 

Colin


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## nanscombe (19 Jan 2013)

If wood shrinks / expands based on the amount of water in it then wouldn't the coat of paint reduce the amount of change in moisture content and hence wood movement?


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## eribaMotters (19 Jan 2013)

Traditional oil based paints are a barrier to moisture. Modern water based paints are to a certain extent breathable and allow the passage of moisture. The number of coats applied can and will however inhibit this breatability and I do recall some manufacturers specifying an ideal number.
When the Dulux Weathershield system for wood first came out 20+ years ago it was marketed as flexible and resistant to cracking. I cannot recall if this was due to its breathable nature.

Colin


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## Jacob (19 Jan 2013)

Internal woodwork will shrink _if in a heated environment_ as this will reduce the _relative_ humidity by a large amount, as the _absolute_ humidity of the local atmosphere will already be low due to low temperatures.
External woodwork won't dry out as the external air has high _relative_ humidity at external temperatures. Also the relative humidity will regularly reach 100% and cause condensation (fog, mist, frost etc) which will wet the wood.
So an external door with a face in each environment can go either way!


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## fobos8 (21 Jan 2013)

thanks for the replies guys.

I found out over the weekend that where I live in the Channel Islands the relative humidity is pretty constant (85%) throughout the year and so external timber shouldn't exhibit any significant dimensional changes due to the seasons.

Kind regards, Andrew


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## AndyT (21 Jan 2013)

fobos8":2sr0s41s said:


> thanks for the replies guys.
> 
> I found out over the weekend that where I live in the Channel Islands the relative humidity is pretty constant (85%) throughout the year and so external timber shouldn't exhibit any significant dimensional changes due to the seasons.
> 
> Kind regards, Andrew



That's quite the opposite of what applies in the USA with its continental climate - so don't be put off if you read advice in American books or websites which says different. I think they have a far bigger range of seasonal humidity to cope with than we do so tend to get a bit more anxious about it.


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## custard (21 Jan 2013)

AndyT":2c46g7op said:


> fobos8":2c46g7op said:
> 
> 
> > thanks for the replies guys.
> ...



Not only that, in much of the USA external moisture levels are highest in the summer, where as in the UK it's the other way round. Our garden gates stick in the winter because they expand with the higher winter moisture levels. In much of the US garden gates stick in the summer months.


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## andersonec (21 Jan 2013)

Timber will expand when the humidity / moisture content is high and contract when the humidity / moisture content is low, so during the winter when your central heating is blasting away inside the house it could be drier but the prevailing weather conditions outside during winter are predominantly wet so exterior timber will expand.
Breathable finishes are designed to allow moisture to evaporate / escape outwards from within the timber, if it did not then the finish would separate and or blister, make sure when applying finishes that you timber is as dry as possible.

Andy


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## adzeman (21 Jan 2013)

Timber does move due to moisture content and some timbers move at larger or lesser rates than others dependant on their type (hardwood or softwood) their seasoning (air dried or kiln dried) and in the case of softwood its growing rate which affects the width of the growth rings. It is essential to select timber based on the location where the project is positioned. Selection of joints is important. To use a joint designed for cabinet work would be useless externally. Joints extermally do not use glue it will use timber wedges and dowels, these will expand and contract at the same rate as the main structure. Nails allow expansion and contraction but in themselves deteriate. I prefer reclaimed softwood for projects when I can obtain it. You will find the growth rings closer together, it has been air seasoned, It can be cheaper, its moisture content will be low due to being in a building for a long time, and it has a nice pattina. Dissadvantages, look out for nails, woodworm and you have to regularise it your self if you have the means. If you dont it costs more thsn timber yard regularised.


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