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Large Low Level DIY Deck - Am I In Over My Head?

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  1. #1
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    Default Large Low Level DIY Deck - Am I In Over My Head?

    Hi All,

    Have been in my house for a few years and am now getting around to the most important part the Deck. I have read through other threads on this forum and they have all received helpful advice so I thought I would try my luck given I haven't done this before.

    I am currently in the PLanning stage before I launch into the construction stage hopefully in a few weeks. I have had the deck designed and this is the info I have.

    Deck Size - 18.1 mtrs x 5 mtrs following contour of house
    Stumps - 100 sq Reinforced Concrete??? or Redgum???? Founded 1000mm into natural ground level (includes 200mm sole plates). Not sure which one or why??? It seems quite deep at 1000mm, I assume becuase the soil classification is Class H?? Is it possibly easier to use stirrups???
    Bearers - 2/90x45 F7 (H3) - I assume this is 2 at every stump line?? These will be spaced at 1700mm running parrallel with the house.
    Joists - 90x45 F7 (H3). These will be spaced every 450mm.
    Decking of choice will be merbau.

    I am not sure whether to use a ledger board to attach to house or leave as free standing?? Is there any pros or cons for either??

    I am also planning on covering with a pergola so will be having that put in place first...I assume this will be best.

    Hoping that I can post some photos as I progress to assist with true novices in the future.

    Sorry for all the questions but I read the key to a good deck was good planning?? Am I missing anything???

    Any advice appreciated.

    I have tried to post some pictures of where the Deck will be located. Hope these help.

    Cheers
    Attached Thumbnails Attached Thumbnails deck-002.jpg  

  2. #2
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    With a H class site I would try and use the least amount of stumps possible. (Changes in ground moisture will cause stumps to rise/sink, A 20mm change over a 3m length is much less noticeable than over 1.8m)

    I would use a 190mm joist and butt the joists into the bearer (as opposed to sitting on top) there will be extra cost in fixing hardware, more than likely offset by fewer stumps (less holes, concrete, ect. ect.) I would attach to the house less digging again ( check you are not voiding your termite protection)

    Good luck.

    I you have the depth (top of deck to ground) assuming a deck 5m from house to edge of deck.

    190 x 45 MGP 10 will span up to 4100 with a 1200 overhang you would need only one row of stumps. The joist would need to sit on the bearer in this case if you used a duragal bearer you would have very few stumps. (A good thing in H class soil)

  3. #3
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    Quote Originally Posted by Eastwing View Post
    190 x 45 MGP 10 will span up to 4100 with a 1200 overhang you would need only one row of stumps. The joist would need to sit on the bearer in this case if you used a duragal bearer you would have very few stumps. (A good thing in H class soil)
    190 will span up to 3.6 in a deck situation need to go to 220 x 45 mm (harder to source) so 240 x 45 mm would be your option

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    Thanks for the advice.

    Obviously the fewer stumps the better given the soil type. I am a bit restricted as I only have about 250 - 500mm (between ground and top of deck) as the ground slopes away from the house. It could be an option to excavate out to allow 500mm the whole way out. It is possibly only fill anyway.

    Any suggestion on the type of stumps to use??

    If I was to use the larger size (i.e. 190mm or 240mm) as suggested what would the measurements of the stump grids become??

    I have posted a design of the stump grids that were provided to me. As you can see they are currently approx 1600mm x 1600mm
    Attached Thumbnails Attached Thumbnails deck-footings-2.jpg  

  5. #5
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    Quote Originally Posted by cherub65 View Post
    190 will span up to 3.6 in a deck situation need to go to 220 x 45 mm (harder to source) so 240 x 45 mm would be your option
    Yeah I'd have another look at your span tables, as the figure I provided did not come out of thin air.


    http://www.timber.net.au/images/down...of_F7_Pine.pdf

  6. #6
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    Quote Originally Posted by Eastwing View Post
    Yeah I'd have another look at your span tables, as the figure I provided did not come out of thin air.


    http://www.timber.net.au/images/down...of_F7_Pine.pdf

    Are we talking about joists? Cause the Span table you are referring to are Joist spans.

  7. #7
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    Quote Originally Posted by Jodon27 View Post
    Thanks for the advice.

    Obviously the fewer stumps the better given the soil type. I am a bit restricted as I only have about 250 - 500mm (between ground and top of deck) as the ground slopes away from the house. It could be an option to excavate out to allow 500mm the whole way out. It is possibly only fill anyway.

    Any suggestion on the type of stumps to use??

    If I was to use the larger size (i.e. 190mm or 240mm) as suggested what would the measurements of the stump grids become??

    I have posted a design of the stump grids that were provided to me. As you can see they are currently approx. 1600mm x 1600mm
    if you used 190 x 45 MGP 10 joists (450c) you would need only 1 bearer 4m from existing building 1m in from the edge of the deck + a plate against the house, and for the bearer size. (assuming your sizes are 7500 wide, 7500 wide, 3000 wide) you could use 2x 140 x 45 MGP 10 with stumps at 1800 centres.

    Without sizes to confirm you would only need 13 100x100 concrete stumps that is 43 less stumps, 43 less holes, less concrete, less mess, less dirt to dispose of, and less cost, 43 stumps at $20 = $860, the same 43 stumps sitting in another $20 of concrete, another $860

    increased cost of timber due to larger sizes with no allowance made for waste is only $550 + a few hundred for joist hangers and dynabolts.

    Good luck if you post up some sizes i'll help you some more.

  8. #8
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    Quote Originally Posted by Willo_1 View Post
    Are we talking about joists?
    Yes

  9. #9
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    Quote Originally Posted by Eastwing View Post
    Yeah I'd have another look at your span tables, as the figure I provided did not come out of thin air.


    http://www.timber.net.au/images/down...of_F7_Pine.pdf
    You will experience bounce in the joists using that size (check there notes), this maybe acceptable to those standards listed but we and many others use AS 1684.2 & .3 span tables as the live load is more acceptable . For a greater outlay now disappointment will be avoided on completion
    Cheers Paul

  10. #10
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    Quote Originally Posted by cherub65 View Post
    You will experience bounce in the joists using that size (check there notes), this maybe acceptable to those standards listed but we and many others use AS 1684.2 & .3 span tables as the live load is more acceptable . For a greater outlay now disappointment will be avoided on completion
    Cheers Paul
    Ok so interpolating spans using the standard table for floor joists all that needs to be done is change the joists to 410 centres. I did say can span 4100mm not will be spanning 4100mm.

    45mm Against house
    3900mm Span
    90mm Bearer
    965mm Cantilever

    I would use a rim joist, and construct the framing as you would if you were building a stud wall on the ground (leaving enough room to nail the joists to the plate against the house) slide the whole lot against the house. Dynabolt to house, skew nail joists to bearer. then you could add the joist hangers to the plate.

    Since the joist length will be 4910mm and timber comes in 4.8 or 5.4 I would reduce my deck length to 4890mm. Or if you have to have 5m the off cut from the joists could be used as midspan blocking further stiffening the floor.

    Good luck

  11. #11
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    Also most treated pine is now f5, not f7

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