F & H ratings? Do I need to reduce bearer spacing?

[Williamstown]

Sorry about all the posts, however I just remembered another question. I understand the F rates on TP; however am a little confused about the H ratings. I’ve been told different things by different timber suppliers. I always thought H (for treatment) and F (strength) ratings were for 2 different things; however I’ve now been led to believe they are somehow similar and H3 = F5 and H4 = F7.

I have my bearers at 1.625mt spacings, on which I’m laying 90x45 TP joists. I was planning on using H4 (which I’m told is F7); however I was having trouble getting hold of it so instead will purchased H3 (which is suppose to be similar to F5).

Anyway the bearer spacings were based on F7 (H4?), so if that drops to F5 (H3?) I probably need to reduce the bearer spacing to accommodate which I can do by adding another bearer. What bearer spacing should I use (I don’t have the tables)?

[brynk]

williamstown, gday

before i can give you a span you need to tell me the size of your bearers! :U (otherwise my answer is 42).

the 'h' classes have little to do with the 'f' ratings directly. 'h' is for hazard, and the h class on the timber relates to the hazard class it is suitable for.

it may be the case, however, that a natural durability lending to a higher h class will also have a higher f rating because of the properties of the timber species make it a stronger member in construction! this does not always hold true when applying a treatment to a stick of timber - the various forms of treatment that allow one to bring the stick to h-whatever may be applied for a longer period of time/stronger mix/etcet to bring it to an even higher h-class.

if you are talking treated pine, there are generally two f-grades - f5 and f7. if you assume f5 (ie, worst case) unless it is otherwise specified on a stamp on the timber itself (try ringing the manufacturer of the timber & they should be able to help you more than your supplier has)

here below is a link that will go into the hazard classes & give good information on building durable timber structures...

http://www.timber.org.au/resources/Revised%20Datafile%20P4%2072dpi.pdf

regards
brynk

[Williamstown]

thanks for the info much appreciated, that makes sense now.
Total deck area is 7mtrs wide x 3.2 deep. Merbu decking will run the depth, away from the house (i.e. 3.2m long). I thought this would look better, as there is a chance I will extend the deck width (7 to 10mtrs) when we move the driveway in the future and didn't want an ugly join. Also running away should carry water away from the house to the grass at the end of the deck.
The bearers are 90x90 and 3.2mtrs in length, will be supported at either end (maybe 200mm in from end) and also in the middle (i.e. 3 stirrups over 3.2 mtrs). The deck is approx 7mtrs wide and I'd planned for 5 bearers therefore the spacing between the bearers is just over 1.6mtrs (joists will overhang the bearer maybe 250mm).
The Joists are 90x45 and 7mtrs in length (2 seperate pieces joined) running across the bearers with a current span of 1.6mtrs as above. I'm planning on spacing the joists every 450mm - should I go closer?
Given the above, should I introduce another bearer which would reduce bearer spacing for the joists to approx 1.3m (or I could introduce 2 if required).

[brynk]

according to my f7 tables you need joist-spacing of 300 to span 1600. therefore i would conclude that the span for f5 (i dont have these tables would be the same or less) so i would go with 300 spacings for the joists to build the deck within code. :U

r's brynk

[Gaza]

i think that 90x45 treated oine is 1350 span at 450cc we never go over 1200mm span for 90x45, 140x45mm we dont let span past 1800mm.

in the past the old F7 treated pine (now not sold) used to go 1500mm according to span table.

[Connollys]

Im a carpenter and I never go over 1300mm spacings between bearers and 450 centers on joists, When we sell a deck, we suggest that if you put our joist at 300 centers you will find you will have minimal offcuts with the decking as the timber comes in 300mm increments, Hope this helps :2tsup:

Cheers