Preserving roof timbers

We're finally going to replace the roof at the farm!

The current timbers are probably about 40 years old and not too bad, but they are a bit saggy and have rather a lot of tiny critter-holes in. We think they were treated with diesel oil. We're going to replace them, but what, if anything, should we treat them with? The locals say that borax and diesel are the traditional treatments. How good/bad are they? Is there anything better?

Borax is definitely good. As to the oil, around here it used to be waste motor oil cut with old fuel. It would make for a BIG fire FAST if the barn ever caught.

Does anyone know the correct dilution for borax for treating wood? And how often will have to repeat the treatment?

We are using one and on half cups 20 mule team Borax to one gallon hot (to dissolve the borax) water in a pump sprayer under our floor. Everything we read said it wont work over finishes though so I dont know if it will penetrate something with diesel oil. We've used it on front porch timbers and then put on linseed oil . We were careful to not leave puddles anywhere because of our honey bees and other insects.
Our information said to apply three times Until it runs off and let dry in between applications. The "bookmark" I found was "safe control of termites". Maybe you could search for that. I am unable to link. I had read in the past if you apply to wet wood it will penetrate more deeply. We love following your roof progress.
I think our information said to treat every three years.

Thanks for that, Judith!

The borax we finally found, after much searching, was indeed 20 mule team. It must have a global monopoly!

We decided against the 'diesel oil', especially as the more we tried to find out the less we were sure if they meant 'diesel' or 'used sump oil'. Neither of those things really appeals to me, and borax seems relatively harmless, so long as we keep it out of the garden. I think we're going to treat the ends of the main beams with bitumen as extra protection against any damp that might try to get in during wet weather.

Glad you're enjoying the roof progress! I'll update in a day or two when I've gone through all the photos - but you'll never guess what the special lifting device was that the guy brought with him when he delivered the wood this morning

I dont understand treating roof timbers with borax. Is it for the exposed ends? Borax tends to get washed away with repeated wetting events.

You might want to look into elimanting exposed rafters to help reduce thermal bridging.

Look at wood doctor website for CPES and epoxies for the longest lasting wood preservation.

And this is where it becomes apparent that I really don't have much clue about what I'm doing...

The ends *shouldn't* be exposed for the most part. Those that are at any risk of getting damp are going to be painted with bitumen. In theory, all the rest of the woodwork should stay bone-dry. We're looking for a treatment to repel insects and fungus, but ideally one that I'm happy to spend the rest of my life sleeping under.

Epoxy is ringing alarm bells in my head - sounds unnecessary, unnatural, and not really my kind of thing.

I had to google 'thermal bridge' and, from the brief bit I read, I don't think it's going to be much of a problem. The plan is to leave all the beams and rafters exposed to view from underneath, but have them covered on top with a complete layer of sterling board (OSB?), then a layer of insulation, then the roof tiles. The sterling board isn't exactly my first choice, but we had to balance up budget, availability, and making things easy to do.

Here's a photo of the old timbers before we removed them, so you have a better idea what we're working with.

That looks like a fun project. I wouldnt worry too much about what I say here as things can get needlessly complicated in a hurry. Looks like you are in a west coast climate which is pretty dry compared to mine. I probably wouldnt worry about the borax but preserving the ends that are exposed to the exterior is a good idea.

I dont know much about doing this with Bitumen but I would question its appearance and longevity. True, Epoxy can be nasty stuff but it could have a lower environmental impact than numerous bitumen applications and would mean lower maintenance too. If they are completely covered by the roof, youre probably right that it may be overkill to use CPES and epoxy for this application.

The thermal bridging is probably only something to worry about if you use fossil fuels for heating and cooling. However, reducing thermal bridging would also mean using less firewood if thats your fuel source. By having your insulation above your rafters, you would be surprised at how much energy the rafters transport outside the building envelope if they are carry through to the outside.

We used borax to kill any existing termites , fungus and mildew and any other insects lurking in the wood but now that we have everything dry there is probably no need to treat again. Our house had been sitting empty uncared for for about ten years when we bought it and had some serious termite damage that was due to water getting into the walls and floor sills. We dried it out and are gradually treating under the floor with borax to make sure the chewing has stopped.

Is borax the poison that is used that goes back to the queen?
How often should it be used?