Timber Exposed to the Weather - type of treatment?

[StandStruc]

I am working on residential project where the architect is using exposed timber that extends outside of the house. There are also some exposed timber columns and headers that are exposed to weather.

IBC 2006 - section 2304.11.5 states that naturally durable or preservative-treated wood shall be used in such situations.

I have not really had much experience with timber exposed to weather, so I apologize if my questions are naive.

1) The timbers are Doug-Fir. Should the timbers be pressure-treated? If so, will they look green like the pressure treated 2X's you get from Hope Depot?

2) Is it possible to just use a sealer in exposed exterior applications? It would seem that the IBC does not allow it.

3) The architect seemed to think that Douglas Fir is a naturally durable wood, but I don't think that it is... correct?

4) Does anyone have any pointers for treating or addressing timbers exposed to weather?

thanks!
StandStruc

 

[hokie66]

Douglas Fir is definitely not a naturally durable timber. Architects periodically come up with this idea, with predictable disastrous results in a few years time.

 

[peal724]

call AWPA

 

[Gumpmaster]

You can get a clear pressure treating, but it's probably special order. The wood should look fairly similar to un-treated wood.

Douglas fir is definately NOT naturally durable. In an exposed condition you'd be lucky to have it last 5 years. It also needs to be sealed with a UV resistant sealer. The pressure treating helps with decay but not weathering. A sealer will help keep the weathering in check.

 

[MiketheEngineer]

DF is definitely not naturally durable. Only two I know of that are readily available are Redwood and Cedar. You might get more than 5 years - but probably not much.

Then - how do you repair it?? Tear the house apart to replace a few beams??

I have seen these beams sheathed with redwood then well caulked and painted. If well maintained - should be OK.

 

[a2mfk]

I sometimes feel like I have to ask the stupid and obvious questions, so, where is this?

Big difference between Florida, where I live and would suggest to not leave any wood exposed except maybe a deck, and the SW USA...

Oh, and what do you mean exactly by "exposed"? Will these columns and headers not support some type of roof?

 

[rscassar]

I only specify seasoned hardwoods for external applications. Would not use douglas fir.

 

[StandStruc]

Thanks for all of the responses. the project is in the Midwest, so I am concerned with the weathering.

 

[hokie66]

Another thing about architects is that they like to use clear coatings or stains on wood which is exposed. They look fine for 6 months to a year, depending on where located, but the clear coating provides little actual protection. "UV resistant" is just a marketing ploy with clear coatings.

 

[csd72]

StandStruc,

The midwest includes the whole gammut from barren deserts to alpine regions. You will have to be a bit more specific than that if you want decent advice.

 

[StandStruc]

The project is in Missouri.

I have done some additional reading and have some additional questions for everyone.

1) Treatment appears to be the way to go. What are people's thoughts on pressure treating vs. brush applied treatments?

It would seem that pressure treating is better, but would it be overkill for beams that are interior for 20' and exposed for about 2' outside the structure. Also, some of the columns will be exposed for about 3' on one surface.

Brush applied would be easier and could be applied after members a cut and fit-up.

2) What treatment would you recommend for Douglas Fir?

I have been looking at borates or Copper napthenate. There is a lot of conflicting information available on what is best. Borates possibly are more prone to leaching. Copper Napthenate seems to be the recommended field applied product for bridges, but has a green color.

It seems that none of the preservatives penetrate Douglas Fir very well unless it is incised. I am pretty sure this will be unacceptable to the architect. I am hoping that the preservative will have "reasonable" penetration if the wood is not incised.

3) Can sealers be applied over the various treatment options?

I would hate it if a preservative that I specified was not compatible with the preferred sealer product.

Thanks everyone!

 

[csd72]

make sure that the treatment is not water soluble.

 

[hokie66]

You can paint over pressure treated wood. Use a 100% acrylic coating, which should last 10 years before recoating is required. Clear "sealers" won't last a year, and don't provide much protection.

 

[apsix]

Some of the Sikkens clear coat products are very good, but will require recoating more often than an acrylic paint.
I suggest contacting the distributor for advice.