Deck underfloor questions

[DiverDan]

Hi guys and gals,

I've been watching the forum for some time and have gleaned a lot of useful infomation. This is my first post and have some question that I'm sure you can help me with.

I am building a deck. This is my second deck. I recently transformed my upper storey deck from tiles to timber as you can see in the photos. I also got rid of that hideous balustrade and went with the stainless steel wire. It looks great and really opened up the view.

My second deck is to be built as a free standing deck in the middle of the back yard. I haven't done exact measurements yet but it is approx 6m x 7m and will predominantly be a rectangle with cutoff corners at one end (bit like the shape of a boat).

The plan is to have bearers mounted directly to stirrups with the stirrups imbedded in concrete. The deck height will require one step, so perhaps 500 off the ground. Probably use 450 stirrups.

In looking at the layout I was planning to put in 5 rows of 5 stirrups at 1.5 metre intervals.

I've had a look at the span tables and although I'm good with computers I can't make work out the tables.

Q) where is says 2/90X45 does this mean there are two pieces of wood to make up each bearer?

Q) would I just nail these 2 together and put them into the stirrup?

Q) would having the 2 pieces touching create a water retention problem? Should they be blocked apart?

Q) if I use a single piece of wood e.g. 125x75 I should centre it over the stirrup and use a filler on both sides?

Q) given my 5x5 foundations is 2/90x45 suitable? Am I over the top with foundations?

Q) I'll need to join the bearers to make up the lenght so should I use a scarf joint inside the stirrup? This would only allow one bolt through each end of the bearer, would this be strong enough?

Joists will be 90x45 at 450 centres.

Thanks for all the help you have already given me and for the future.

Cheers,
Malcolm

[silentC]

If you're making up a 90x90 from two 90x45s you need to nail them together face to face with no gap between. The nails should be minimum 2.8mm and spaced no more than 180mm part staggered vertically (one near the top, one near the bottom and so on).

If the span tables say a 90x90 will span 1500 then you should be OK with that.

I think a half lap join would be better than a scarf so you get two bolts through each.

I don't think there's any requirement for a narrower bearer to be over the centre of the stirrup. Someone else may know different.

[DiverDan]

Thanks for reply. It confirms most of what I was thinking.

I think a half lap join would be better than a scarf so you get two bolts through each.

If each bearer is made up of 2 pieces of wood and then we create a half lap join then wouldn't one of the pieces of wood be unsupported by the stirrup? This would effectively defeat the purpose of having 2 pieces of wood. How about a quarter lap join (if there is such a thing)?

I don't think there's any requirement for a narrower bearer to be over the centre of the stirrup. Someone else may know different.

The main reason for asking this was that if the bearer is off centre then it would impart a small lateral load on the stirrup that may over time weaken the structure.

[silentC]

If each bearer is made up of 2 pieces of wood ...

Yes I forgot about that. To be honest, i don't know how you would best join a bearer like that. The only time I've personally encountered long runs that need more than one bearer timber in length, they have just been placed side by side on a brick pier and bolted together. Obviously a stirrup doesn't give you that option.

I'm not sure why you couldn't stagger the two bearer members so that there is only one butt join over each stirrup. There may be a reason I'm unaware of. Again this nail laminating of bearers is not something I've ever had to do.

With the nailing I forgot to mention that you either need to nail from both sides, or use long nails and clinch them.

[bpj1968]

generally when there are two bearers you put one each side of the stump. I would think again about having 450 mm stirups above the ground, especially on a freestanding deck.

Assuming F7 trested pine

I just ran your figures through "timber solutions" and your joists are good for a span of 1260mm for a single span and 1520 for a continuos span, ie.e supported on at least 3 bearers.

Your bearers are good for 1560mm only if your bearers are 1200 apart.

Even if you use the spans I have listed you are near 100% load.

What tables did you use?

[ausdesign]

I get for F7 seasoned pine:
90*45 joists at 450 - 1400 single, 1700 continuous
2/90*45 bearers with a load width of 1500 - 1550 single, 1900 continuous

[DiverDan]

Thanks for the assist guys.

Looks like I should be ok with 1500 posts and continuous bearers and joists.

I'll think I'll go with posts and use the 2 x 90x45 one each side of the post.

Cheers,