Large Span Deck

Hi All

Great forum.

I have built more decks than I care to mention and have always built in hardwood. I recently finished decks on 3 sides of my house, 2.4m deep 15m wide across the front and 20m long down the side accross the top 2 levels of my house. Building the decks is great, doing that much handrailing and painting all the balustrading, not so much.

I am looking for suggestions for the joists on my rear deck. Rear deck is 4.8m deep and 15m wide. Ground floor has 2.7m ceilings so 1st deck will be 2940 off the ground, 2nd deck is 3240 above that and then the roof starts about 3m above that. I am used to dealing with the heights so not that much of an issue.

My issue this time is that underneath the deck the 4.8m width is for a double carport, so don't really want any posts, at least in the 1st 6m underneath the 1st deck. For a single storey deck that would not be a problem, I would just stick in a 250/25 UB to cover that 1st 6m and also a 250/37 UB across the 4.8 so you didn't have a post at the entrance to the carport either. I would then use 150x50 f14 (matches the verandahs on the side and ads a bit more strength). If I was to span the whole 4.8 then I would have to use 250x50 which at ~$20m as opposed to ~$8m for the 150x50 when there is 34 joists x 4.8 = almost $2000 more for the joists. The Ub's would cost me an extra $700. However as there is another deck on top of this one and I don't want any posts in the middle of it then there is not much point using the UB's underneath as I would still have to use posts on the 2nd deck if I wanted to use 150x50. To use a UB across the 15m of the upperdeck would not only be expensive and would still require a post but I would have to up the UB over the carport entrance to take the pointload of the top deck, which combined with the pointload of the lower deck is getting quite interesting.

Normally for long spans on floor joists I would use a Hyne I Joist or similiar however, as I am laying hardwood decking these joists are not suitable for being out in the weather. And with a 3m roof on top and then over 3m between decks there is a large amount of the joist that will get wet. I know you can treat an LVL to H3 but am still unsure about the durability. I have built/ reno'd quite a number of houses and built up to my family house, this one is the last, I will probably be here for the next 20 years or more so Iam keen to have a joist that lasts and don't want to be replacing it in my 50's.

So after that long winded explanation you can probaly understand my dilemma, if I use the 250x50 f14's I am up for around $7k just for joists. Any suggestions would be appreciated


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