Our deck was built in two parts. First the floor and sub-structure and later the roof. We did this because we needed the floor space quickly and couldn’t really comprehend how we were going to cover it initially. Our building skills definitely expanded during this project. We were also quite naive on how important it is to keep timber out of the subtropical weather and thought we might get away without a permanent roof.
I picked up a little book at the Lismore University bookshop called “Acceptable Standards of Domestic Construction by the Acceptable Standards of Construction Committee (NSW) Incorporated” and goes into the details about what building codes relate to what kinds of building work and their specifications. I have found it to be a valuable reference.
The Australian Standard for foundations is spelled out in BCA Housing Provisions Part 3.2.4 AS2870.
I used my Acceptable Standards hand book to establish that I am on Class A Stable soil. It is sandy here but the sand isn’t loose.
The standards state that footings on my soil type should be minimum 400 mm concentric width and 400 mm depth. I increased this on our site to 450 mm concentric width. For the footings that were going to be potentially supporting the roof I made 1000 mm deep the others I made 600 mm deep.
The ground wasn’t excavated flat so there was a bit of shuffling of the footings to dig in the easiest spots. We didn’t think this would be a problem and it was able to be worked around years later when it came time to line the beams up in the roof!
Working out the volume of concrete required. Roof Pole Supporting foundations:
.45*.45*1 (length * width * height) = .2025*5 (volume * number of roof foundations) = 1.0125 (cubic meter volume)
Stump Supporting Foundations .45*.45*.6 =.1215 * 10 =1.215
total foundation volume 2.2275 = ~2.3 cubic meters
I did a bit of internet research and found that when the ingredients were delivered they were going to be dry and loose, and would compact when wet. This dry to wet factor could be as high as 1:1.6 Meaning we actually needed 3.8 cubic meters of material.
Concrete ratio (by volume) for footings. This ratio is for weak concrete, but that is ok for our solid block footings. It would not be suitable for a high wear area or floating slab.
Cement : Sand : Gravel 1 : 3 : 6 1+3+6 = 10
Cement bags 3.8 * 1/10 (total volume of materials by the fraction that is cement) = .38 cu m / .0166 (volume of cement divided by the cubic meters per 25kg bag) 22.89 bags, use 23 bags.
Sand 3.8 * 3/10 1.14 cubic meters of sand
Gravel 3.8 * 6/10 2.28 cubic meters of gravel
As we were making concrete for the composting toilet at the same time, I ordered a cubic meter of sand, but this turned out to not be enough for both the toilet and the deck projects, that were occurring side by side so I decided to order way too much with another 4 cubic meters.
I ordered a 6 cubic meter truckload of 20 mm”blue metal” gravel, which is the capacity of a 9 ton truck. It has many other uses such as a high quality road base, paths and drains.
Concrete is made up of cement, sand, gravel, water & air. The cement is the most expensive component and it is made up of limestone that has been ground up to a very fine powder and then baked at very high temperature. When the cement is mixed with water again it causes a chemical reaction called hydration and begins to harden. From when you mix the cement with water the clock begins to tick to get all the ingredients mixed and the resulting mix where you want it.
In our humid subtropical environment, cement that is not used quickly will absorb moisture from the ambient air and go off “in the bag”. It is very important to only buy as much as you need, and if you do have too much, to use it all anyway and make things like pavers, steps or small slabs under washing machines.
We are quite biased against poisoning the soil and water around our house so elected to use steel stirrups that keep the poles out of the ground and act as a physical barrier for termites. Termites will not go out into the light. Their bodies are translucent and their gut flora that breaks down the lignin and cellulose in the wood can not survive the awesome power of the sun.
They will build tunnels out of mud though and make their way to a feed. So it is important to check the crawlspace under the deck annually (at least) for any sign of attack.
Timber for the Posts
The posts were sourced from another share on the community. They were tallowwood trees.
Tallowwood is a hard durable timber rated as Class 1 but the sapwood is susceptible to powder borer so it isn’t recommended for internal use, that’s no problem under the deck though. We found some of the sapwood in the wet parts of the deck also developed a dry rot, this stopped at the solid wood though.
The posts were all put in at least 30 cm too tall, then we ran string lines and cut them to the right height with a chainsaw.
Then notches were cut out for the bearers. We used 12 mm galvanised threaded rod to tie the posts to the footings.
We sourced the green rough sawn timber from the Kyogle Saw mill, it is the closest supplier as well as offering the best price. I found them to be very easy to deal with and continued to use them for our other projects.
They couldn’t tell me the particular species of the timber however we identified a softer yellow timber, which we suspect is either Yellow Stringybark or Tallowwood and a harder red timber which we suspect is Sydney Blue Gum.
The bearers were 4×3 (100×75) hardwood, for the short spans this seems to be fine. It is more common to use 6×2’s for floor bearing and I would use those instead next time for additional weight bearing capacity.
Ideally 12 mm threaded rod is used to tie the bearers to the posts. In our situation there was no power and boring big 12 mm holes through all that hard wood was quickly draining the cordless drill we were using to drive the bit.
So we decided to save that extra step of putting the rod through the bearers to later and used coach bolts which we could drill short holes and fasten in by hand.
The joists are 4×2 (100 x 50). We used low-cost length limit is 4.5 m so we ordered the span of the deck in two pieces. We took a bit of a punt, often when timber comes from the mill it is a bit longer than you ordered so you can cut the ends property. Sometimes it is a lot longer.
Due to the unusual spacings of the foundations one side needed 3765 mm lengths and the other 3200 mm. We ordered 25 x 3600 mm pieces. Thankfully it worked out well and we had so much extra that we could place them side by side over the centre bearer for maximum strength.
Once everything was up we oiled it with linseed oil and turpentine.
We shopped around for decking. There are quite a few options depending on the fit and finish. We decided to go for a 250 m pack of kiln dried dressed spotted gum seconds from the Nimbin Building Supplies. We used every piece!
Putting the deck together is something of a giant jigsaw. This job is best done in a team, with a couple of people lining up the boards and someone attaching them to the joists. The person doing the pre-drilling and nailing is on their knees for hours at a time so it is best to have knee pads.
There are a few tricks to remember. Decking is not end matched, so you have to meet the ends over a joist to prevent spongy spots. Try not join pieces on the same joist side by side. The eye doesn’t see the joins if they are randomly dispersed around the floor but if there is a continuous line of joins it is quite obvious and looks bad.
I try to use galvanised everything, it lasts longer and isn’t that much more expensive.
We used a non-toxic milk based stain called Cooee timber treat on the deck surface. There are several different brands available in this area, even one made locally in Byron Bay.