What joist size to use for low-level deck?

[Zog]

Hi everyone. I have assembled the framing of the modular decking for my low level deck (see photos). What is in the photo is essentially a walkway, the main deck will be under the pergola and slightly higher. I've gained some confidence from what I've done so far and am considering using tuffblocks and building a framework rather than using the modular decking for the main deck. My question is just about the joist size. I'm not going to use a bearer and joists system, just a framework of joists, mainly because the Pergola is already there (before I bought the house) so I need to preserve the ceiling height of the pergola as much as possible.

I'm unsure about joist size and span of tuffblocks. The Tuffblock website has an article where they're using 2"x6" (45x150) joists, I've seen joists of 45x90 used in Australian based articles and the joists of the Good Times modular panels from Bunnings are 45x70. I'll probably use 140x19 merbau decking.

The pavers will be staying, they're not perfectly level, a small part of the deck will be on soil (the far sides in the second photo).

The total size of the main deck will be about 6.4m x 7.4m or 47.36 m2. It's a lot bigger than any articles I've seen about building a  low level deck. Any advice will be much appreciated.

[EricL]

Hi @Zog,

It all depends on the type of edge design you'll be doing. Will you be having the fascia at the same level as the decking panel to create a small thin finishing edge? Would you prefer having an overhang? I suggest having a quick look at the Top 10 most popular low-level deck projects. You'll see some excellent examples of low-level deck edge profiles. I also recommend looking at this guide How to build a deck by @MitchellMc. You'll see a combination edge finish in that guide. 

For example, if you were having an overhang past the fascia that would technically be the thickness of the fascia plus your desired overhang. The same thing would apply lengthwise. In regards to the fascia, it is usually screwed on to the side of the decking frame. To be more specific, a minimum of two screws at the end and two more supporting screws along the length of the fascia every 600mm. For aesthetics to try line the screws up with the screw lines on the top of the deck.

If you need further assistance, please let us know.

Eric

[Zog]

Thanks Eric,

That's all sorted now.

I'm close to ordering the decking boards now and I see a choice between 140 merbau set length 5.7 mtrs and 140 merbau random lengths at almost $2 per mtr cheaper. Can you tell me whether the random lengths tend toward longer? I'm interested in saving some money but if they're, for example, mostly 20-30 cm long, that would obviously be unsuitable.

[JacobZ]

Hi @Zog,

The 140 x 19mm SpecRite Merbau Pre-Oiled Decking - Random Lengths come in a big pack with lengths varying from 1.5m up to 5.7m in increments of 300mm. There is not a set amount of each length between packs, so it really is luck of the draw as there is no way to guarantee that the exact lengths you are after will be available in the current pack and it is usually not possible due to storage limitations in stores to pick and choose from multiple packs. You can speak to the team about trying to get specific lengths, but there are no guarantees.

The 140 x 19mm 5.7m SpecRite Merbau Joined Set Length Pre-Oiled Decking guarantees you get 5.7m lengths, but they are finger-jointed. You can see in the photo below there is a line across the board where two lengths have been joined together.

This may not bother you, but I know a lot of people prefer not having these visible joins on their deck.

Let me know if you have any further questions.

Jacob

[Zog]

Hi @JacobZ 

So the 5.7 MTR pieces are actually two shorter pieces joined to make the 5.7 ?

[JacobZ]

Hi @Zog,

Yes, it is multiple shorter pieces joined together to make one longer piece.

Jacob

[Zog]

The total length is 7.2 MTRS so I need to have joins anyway.

Seems a bit odd that random lengths or joined set lengths are the only choices. (Merbau trees must be really small 🤣).

Maybe the random lengths is a good choice.

Do you have any idea what most opt for?

[JacobZ]

Hi @Zog,

It's more about how they are milled, packaged and shipped. 

The 5.7m set lengths take advantage of smaller pieces that could not be used otherwise. By joining these small pieces together, you can use up the smaller lengths that otherwise would go to waste. 

5.7m is also just under 20 feet meaning they will fit nicely in a shipping container, which is either 20 or 40 feet long, without much room to spare.
 

If you're going to have butt joins regardless, there is not a lot of point using the 5.7m finger-jointed stuff. Aesthetically, most people would prefer a butt-join here and there over many finger joints in every piece of timber.

From experience, more people use random lengths of decking.

Let me know if you have any further questions.

Jacob

[Zog]

Hi @JacobZ 

Going with the random lengths - The decking board butt joints have to be on a joist, joist centres 450mm apart so there will probably be a lot of small pieces less than 450mm left over. So I'll have to order more than the exact required length which is 303 mtrs (rounded). Is there any rule of thumb used of how much extra to buy ?

[MitchellMc]

Hi @Zog,

The more effort you put into using the closest length in the timber you have delivered, the less waste you will have. Typically, you should order at least 10% more than your exact lineal meterage required, so 333 metres.

Mitchell

[Zog]

Hi guys,

the joists have been delivered but some of them are curved. One is curved unevenly about 4/5 of the way along - see the photo where the bend is circled. It's not easy to see in the picture but it's a significant bend. Another has an even curve but it's curved in both planes - you can see it on the right in the lower photo.

Some others are also curved but evenly on one plane only.

When I was building the wooden walkway, one joist was curved so I put it on bricks for a few hours and it straightened. Now it's several and the're curved in a more complex way.

What is your advice?