Pre-treated Wood

Another week and another blog from PCA trainer & honorary member Gervais Sawyer which this time, is on the matter of pre-treated wood. How much do you really know about pre-treated wood? How long can you expect it to last and is it up to standard? Well, read on for more of Gervais’ words of wisdom…..

What exactly is pre-treated wood?

Now listen up all you surveyors and specifiers. What I am going to tell you is very important and if you stick with this short homily, you will know a lot more than your suppliers.

You will all remember that exam question on the specification for dealing with wet/dry rot? You answered, quite correctly, that any replacements should be using pre-treated wood. But what is pre-treated wood? Have a search through the websites of timber suppliers and under ‘treated wood’ they will use emotive sexy words like ‘high pressure’ and ‘deep penetration’ but nowhere do they tell you what the treated timber is suitable for. The nearest I could get is one company that has a little foot note “not suitable for ground contact”.

Well, that is a start. It is fair to assume that if you buy a fence post it will be suitable for ground contact, but how many years of service life can you expect?

What do the Standards say about pre-treated wood?

The relevant British Standard is 8417 that lists five use classes according to the environment in which it is installed, and these are:

  • Use Class 1 – interior, covered and dry. Possible insect attack. Upper floors, roof, floor boards etc.
  • Use Class 2 – interior or covered. Possible insect and fungi attack. Slight risk of getting wet. Ground floor joists.
  • Use Class 3 – exterior, frequently wet. Risk of fungal attack. Anything outside NOT in ground contact.
  • Use Class 4 – exterior in ground contact or in fresh water. High risk of fungal attack. Fence posts, gravel boards.
  • Use Class 5 – in salt water. Risk of marine borers. Marine piling, sea defences. No preservatives are available for UC5. You must use durable hardwoods only.

With that in mind, you will now discover that most treated timber offered to you by timber merchants is Use Class 2. If you were buying treated fence posts, I think that you would argue under Trades Descriptions that it should be Use Class 4 (should last 15 years unless otherwise stated). Just go down to your friendly timber/builders merchants and ask them about Use Classes and treated timber and they won’t have a clue. This has been demonstrated by survey.

Consider the risk and cost of the timber failing

BUT, and a very BIG but, the story doesn’t end here. You must also consider the risk and cost of the timber failing in service. The cost of a single joist is nothing, but add in the cost of lifting expensive hardwood floors, disturbances, redecoration to say nothing of loss of reputation.

Suppose you were repairing a ground floor damaged by dry rot. There are spores around and drying of the structure may take sometime. If dry rot re-established itself and somebody fell through the floor, the cost to life and limb might be even more serious. Clearly a Use Class 4 situation.

In order to meet these Use Classes, the amount of preservative impregnated into the timber is specified AND the depth to which it must penetrate. Imagine if you will that fungi are constantly exploring and challenging your treated zone, and eventually they will succeed and get through to the untreated wood below. There must therefore be enough chemical to give you the desired service life.

Unfortunately, much of the building timber offered to you is spruce which is notoriously difficult to treat. Indeed ‘pressure painted’ might be more appropriate than ‘pressure treated’. Just one or two millimetres may be all that can be achieved.

Make sure you know what timber you are buying!

The Wood Protection Association and the Timber Trade Federation are starting a big advertising and education programme and you will soon get used to the “Make sure it’s 4” slogan. In support there is a quality control scheme for UK manufacturers, but much timber is imported as ‘treated’ and they may or may not be up to standard.

Before 2004, the green preservative was copper chrome arsenic. This was excellent and quite forgiving. The replacements do work, but only IF the specified retention and penetration is achieved.

In conclusion, just be aware of what you are buying and ensure that you follow the relevant Code of Practice for remedial works. It could well be that applying preservatives by brush on-site could achieve the required result.

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