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I'm building a deck in the garden, have planned, purchased and started building the deck. I've been laying out some joists on the support beams (wider timbers bolted onto the posts) that I've temporarily put in position, and I'm slightly worried that the joists I have purchased are not going to be strong enough. They feel firm enough when laid out as a test, I just don't want problems a couple of months / years down the line.

On the not so worrying side:

The timbers were listed at the merchant as joists. They don't flex significantly when I stand on a single joist and jump up and down a bit, and when I've laid a couple of decks across as pictured and stand on the deck, it doesn't feel springy.

I have enough timbers to lay the joists with 220mm centres, which seems pretty close.

The deck board I have is quite good quality - 32mm thickness.

What's worrying me:

The only thing that is really worrying me is that they 'look' a bit on the thin side, and everything I've looked up online seems to reference minimum 6 inch joist depths.

Measurements:

Max spacing across main support beams: 900mm

Joist spacing center to centre: 220mm

Joist with: 50mm

Joist depth: 75mm

Picture:

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I have read that "In general terms, joists spaced 16 inches on centre can span 1.5 times in feet their depth in inches". If this is true, they should be able to span about 1300mm, I'm only attempting 900mm. I hope that's right!

Any advice appreciated.

  • My initial sense is you are within the range of reasonable, but I'm not used to layouts at this scale. We would need more information to make an accurate determination. If your area receives any snow we need the design ground snow load for your area. We also need the grade of the lumber or the maximum unit bending and shear stresses and the modulus of elasticity. – bcworkz May 15 '14 at 22:13
  • Thanks, I can say that we don't get that much snow here (south west England). I don't have timber grade information available, but I'll see if I can find out more. – gb2d May 16 '14 at 6:05
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I can't find a floor joist span table that goes smaller than a 2x6. However, the American Wood Council calculator gives an allowable span for #2 Southern Douglas Fir 2x4s (~35mm x ~90mm) at 16" (405mm) centers at 5',8" (~1750mm) with a wet condition 10lb dead load and 40lb live load.

If the dimension measurement is accurate, the joists wouldn't get past my local inspector (around here it's 2x6 minimum for deck joists), but even at 12" (300mm) centers you could probably park a small car on it with 900mm spans.

  • FYI cars are actually not that heavy on a weight per area basis. I just did the math on a Honda Civic and it came out to 33 psf. In some areas you can get more than that with just snow. – Hank May 16 '14 at 0:41
  • @HenryJackson - Yeah, my Mazda would do just fine on most decks too. I was aiming for hyperbole and missed. I should also add the disclaimer that I don't recommend or endorse the use of cars for testing or demonstrating the strength of framing. – Comintern May 16 '14 at 0:55
  • Interesting trivia: parking garages are typically designed for 40 psf loads, but office buildings are 50+ psf. So if you wanted to convert a parking garage to a "regular" building you'd have to strengthen the structure. – Hank May 16 '14 at 1:39
  • Thanks for your comments and advice. Based on the fact that it feels pretty steady, the calculators seem to suggest it should be ok and no one here has posted 'omg that wont last the week', I think i will carry on as I am. – gb2d May 16 '14 at 6:00

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