Thread: i thought treated sleepers would not rot?

If they are treated, the treatment makes them more resistant to rotting and insect attack than untreated timbers. Soil conditions may determine the effectiveness of the treatment.
This article has a lot of information. Timber in the landscape
It appears HW posts in the ground have about a 10 yr life span, as you have discovered.

Hi,

Everything rots, eventually. I had a series of treated pine log posts in concreted holes, and after 15 yrs they were rotted.

Cheers

So what timber is used on bridges and piers/ jettys then? Surely they last longer than 10yrs.....

Originally Posted by Apples

So what timber is used on bridges and piers/ jettys then? Surely they last longer than 10yrs.....

typically, in NSW at least, it's Class 1 North Coast hardwood -- which is increasingly difficult to source in the required dimensions -- and turpentine for piles and jetties.

to quote http://www.dpi.nsw.gov.au/__data/ass...ernal-Uses.pdf

The failure of timber to perform to user expectations when used externally in situations such as decking, pergolas and landscaping, is probably the greatest cause for complaint by users and owners alike. Generally though, the fault rarely lies with the timber itself. Rather, the cause is more often the result of a lack of understanding of the material and its inappropriate use by specifiers and users. ... when specifying or ordering timber for external use it would be unwise to simply request hardwood. (my emphasis)

I'm doing a great deal of work with TP sleepers and posts at the moment. Many of the posts and sleepers are still full of moisture (so much for kiln drying) and the post faces that face north are cracking like crazy (up to 3mm wide, 300mm+ long, and lots of them). These cracks are a very large contributor to future rot because when they are underground they will let water back into the posts, thus creating rot.

The chemicals in the "treating" process usually only penetrate perhaps 2-3mm below the surface, so when cracks open up it lets the water in way below the treatment.

I have gone over the posts and filled all the cracks with Sikaflex to mitigate this as much as possible. However, because the posts are yet to be fully dry (two years from now???) I know that the cracks will reappear. Even in the two or so weeks since I filled them they have opened up some more in many places. The posts and sleepers are now painted but that won't stop the cracking - moisture has to find a way out.

The difference in weight between some of the posts and sleepers was huge. I'd say some of them were a good 30% heavier and this is attributable to much higher moisture content more than variations in the Pine. Some of the posts squished out moisture when batten screws were put into them.

When one of the 150x150mm posts was delivered a few weeks ago the truck driver decided to not use the Hiab and just roll it off the top of the pile, hoping it would come to rest on the truck tray......'cept it kept on a'rollin' right off the side. Landed on one end first and snapped it in the middle (a 5-6m long 150x150 is heavy beast, even in pine if it''s green enough). He actually did us a favour because in the snap we could see that the treatment had penetrated right to the middle of the post but on the other newly exposed face the timber looked completely white and untreated. Clearly there was a very deep crack in this post even before it was treated, which allowed the treatment to get so deep into it. A replacement post was delivered a few days later.

The moral of this tale is to use timber that is properly dry and without cracks as much as possible. Fill any cracks with Sikaflex (an expensive but excellent product). Any cut faces should be retreated, especially if they are going in the ground. If you can somehow get hold of genuine creosote then that will assist greatly with rot prevention, but it's a very nasty product (wear a P2 mask).

I thought that treatment only penetrated sapwood, so the heartwood didn't get properly protected anyway

There are also different grades of treated lumber (here in the northwest /Washington) as lumber designed for piers and wood foundations with a higher psi treating process for deeper injection of the chemicals. Also providing drainage with gravel at the bottom of a post as well as making sure the surface around the post is graded to eliminate standing water will extent the life of the wood posts. E aware that some pressure treated product are rated for "ground contact"as opposed to under ground burial as well.

Any timber used for retaining walls, posts etc should be minimum H4 if not H5 treated pine. These are pressure treated where the preservative is forced into the end of the timber and out the other end so the whole cross section of the timber is treated. Being softwood this is not difficult to achieve. Hardwood on the other hand is nigh on impossible to do this to so surface treatment only is the only option. The downside of this of course is that being so hard, the treatment will only penetrate a few mm into the timber. Once the timber cracks as it inevitably will, the preservative becomes almost irrelevant as moisture, algae etc will be able to get right into the centre of the timber in question. As a feature wall for a retaining wall though, hardwood can look a whole lot better than the green tinted pine. I saw one done quite a few years ago where recycled redgum railway sleepers were used that had been roughly planed and belt sanded then coated with a clear coating. It looked very swish. If it was me I'd also use a plastic moisture barrier behind the wall to keep the timber as dry as possible seeing as there would be plants behind it that would get watered. You couldn't do much about the posts in the ground though but maybe creosote that part as well for a bit of extra protection.

Stewie