Thread: External decking joists choice

You definitely need to use treated for pine joists used externally.
They just will not last.
I can't think of any way to save the situation, if what you said is correct, other than a complete rebuild, but there may be some more optimistic souls than me here, with better lateral thinking skills.
Good luck,
TM

On a similar vein, back in the early 80s, a mate of mine used untreated pine lining boards to line his shower. Looked great for about a month. when I questioned him (I was in my early 20s then) he assured me he had put several coats of clear waterproofer on.
TM

Two questions.

Is it weather exposed?
How high is it from ground if someone goes through?

If weather exposed and untreated pine (treated timber even if treated with a clear solution will have a brand on it with a H level) your options are pull it down now... or have it fall down by itself in the next few years.
Hence question 2 because if its far to fall you need to pull it down rather then wait for it to go with someone standing on it.

If its not weather exposed... you might get away with a remedial treatment. Go big green shed or similar and you're looking for copper napthenate. Preferably as a concentrate not as the watered down stuff they like to sell. (It wont be copper napthenate by name but thats the active ingredient you want.) Mix with turpentine and paint on. After about 10 coats you'll get close to a H3 retention level.

Or a good shot of creosote will fix it.

There should be a stress resistance "F" grading as well as the hazard "H" grading to all timbers being used. https://www.softwoods.com.au/timber/...tress-grading/
If only the F grading is apparent, then the lowest H grading is assumed, ie. H1, for internal use only. There are F5 treated pine with a H3 hazard grading for example.

Also F5 is only for stud wall, you'll need F7 for joist construction.
Before we can say you'll have to pull it all down and rebuild. We'll need a bit more information.

1. How far are the joists off the ground?
2. How far apart are the joists? Are they sitting on top of bearers? and are the bearers sitting on stumps? What materials are the bearers and stumps? how far apart are the stumps and bearers?
3. What are the dimensions of the joists, bearers and stumps?
4. Are the decking boards reeded and are the reeded side down?

I have rebuilt my balcony (2nd storey high) about 2 years ago. The balcony was built in the 80's (not by me) using tassie oak beams and Oregon joists. The balcony was exposed to the elements but was reasonably sheltered. The construction was solid and the timber was of the correct size and grading. There was no protection at all. I noticed the rotting about 5 to 7 years ago and I rebuilt it about 2 years ago. So even without treatment, the balcony lasted 30 years. I used oregon as beams and joists this time as well, which is of the correct stress grade but obviously not of H3 grading. But the balcony has a solid floor and is fully water proved so the joists and beams are no longer exposed to the elements.

I am afraid you have the wrong stress grade and the wrong hazard grade for the joists. If the deck is low and the dimensions of the joists are sufficient, which I very much hope they are not 4 x 2, and if they are correctly spaced. You might be able to save them by pulling up the boards (hopefully they are screwed down not nailed). put some protectadeck on top of the joists to save them. That shouldn't take too long. A lot will depend on what your reply is.

Internal, untreated F5 timber is totally unsuitable for an exposed deck frame. Painting the exposed, visible timber is only going to prolong it's life minimally. Undiluted copper napthanate would be the best option,but would require several applications to gain some penetration.
Your best option would be to replace the joists with H3 or H4 treated timber.

Hi the joists are apparently MGP10's
At the highest point of the deck 1mtr off ground running to 3 inches above ground
joists are all 400mm apart with noggins as well, there are H4 treated bearers 1800mm apart sitting on H4 posts on stirrups. each bearer post runs all the way up to rail. so 1800mm apart
the joists are 90x45, posts are 90x90, bearer is 145x45
The decking is reeded side up
this is attached to an already existing deck which was constructed on C purlings which run through the entire house.
Thanks for your advice

The down side of 'reeded side up' is that they are there to help moisture disapate when sitting on the joists. This means that there is a much more likely chance of water sitting on your mpg10 joists enhancing the problem (mpg10 is still not suitable for external use)

I hate to say it, but I believe a pull down to bearers (that all sounds suitable and sturdy) and redo the joists is the only safe option.

Both your joists and bearers are undersized unfortunately.
Given that, you're best to pull it all apart and rebuild, and choose H3 for the joists
The max span for MGP10 90 x 45 is 1400 for single span and 1700 when they are supported over more than 2 points. the max. span for for 140 x 45 bearer is approx 1300.
Your joists should have 2 spans or more in total to use the 1700 number, otherwise it is 1400 only.

agree with Gab, have the reeded side of the boards down to promote water drainage. treated pine rots too when they are constantly wet.

I've never use this web site before but it could be handy for sizing your components: Deck Calculator - Full Plans and Costs - Metric

Sorry for the bad news.

A fail on all points. Need to book this one down to experience.

To sum up all the above comments/advice, pull it down and redo correctly. There is no substitute for doing a job properly.
The other factor not touched on is that it affects your house insurance and public liability if anything happens.
If you doubt the authenticity of the advice given, talk to your local building inspector.