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:
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.
Best Answer
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.