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Hi Bunnings community,
I am in the planning stage of a 50sqm Merbau low-level deck located next to our pool. 30sqm of the deck will be under the awning, protected from weather, and attached to the property with a 90x45 stringer/ledger board at 60cm intervals with M10x120 masonry anchors. The finished level of the deck will be 40-50cm from the ground, which is garden soil. Deck boards will be 140x19 Merbau at random lengths.
So, our challenges, (and also our questions for the community);
- The deck under the awning will carry 4 pieces of glass pool fence 1800x1200 in size, each 70kg including spigots. Even though the total of 280-300kg extra weight will be spread approx. 7.5m, I think this is still a considerable weight. Based on the span tables, (Thanks to @EricL who shared them on the https://www.hyne.com.au/article/common-product-span-tables), I worked out most of our bearer sizes as double 140x45 with 1400-1460mm spans.
- In fact, according to the ref table from Hyne on that link, even a single 140x45 could support a span up to 1400mm if I'm not mistaken; however, considering the notes I mentioned on my drawing as per bearer, I preferred to over-engineer my bearers due to various considerations such as the live and dead loads expected on the deck, as well as the moisture exposure from the ground.
- Using 2 x 140x45 H3 Bearers will leave only 20cm clearance between the underside of the bearer and the ground. Considering cross-flow ventilation will be insufficient as the deck is very close to the ground and also surrounded by the pool and deck steps on the sides, (plus I use wide deck boards); I plan to use galvanised post anchors fixed into the wet concrete rather than using 100x100 H4 timber posts.
- Footings 20cm in diameter and 50cm deep for Pryda 300x90mm M10 Full Stirrup Post Anchor at every 1400mm should give enough foundation I believe. Once the bearers are in their positions, attaching the Stirrup anchors to the bearers, then pouring the dry concrete mix from the bag into the hole, and adding water is my way of thinking if I am not missing anything.
- For the joists, I considered 90x45 at every 45cm as usual.
- To mitigate the subfloor ventilation issue to a degree, I will consider 8-9 mm gaps between the deck boards instead of 7mm, which is the minimum requirement as mentioned on the SpecRite brochure.
- We think of a double board picture frame around the deck. This will be a challenge for the curved edges next to the pool, so, I will decide about it when the deck comes to that stage. If you have any suggestions on this I would be happy to hear.
- In terms of the breaker board in the middle, which would be necessary if the deck was a composite deck as the overall length is 11m; I am not sure if this is the general practice for timber decks too. I believe using a breaker board can be a best practice in either case but I am not sure if this is technically required. If not required, I am happy to ignore it.
- Lastly, what is the common practice for attaching the joists to the bearers? I used Triple Grip, Unitie, and Multigrip in the past on different occasions, but for decking, I am planning to use Multigrip (Pryda Multigrip 100 x 35mm) for joist-to-bearer connections as the same product enables left or right installations. Any comment on this would be nice. (ref: https://www.bunnings.com.au/pryda-multigrip-100-x-35mm_p1072509)
- Using joist tape, https://www.bunnings.com.au/cagroup-50mm-x-20m-bearer-protector-g-tape_p0224655; and protection for the bearers will also follow at the later stage as usual.
- In regards to bearer protection, I am not sure what bitumen rubber protection I should use. I have multiple options and interestingly, all these options are from the same company, Bondall. Has anyone used these options and been able to make a performance comparison? The first two items seem timber-oriented, and the last two are more general use, I think.
Monocel In-Ground Timber Protector: https://www.bondall.com/water_proofing/monocel-in-ground-timber-protector/
In-ground timber protector: https://www.bondall.com/water_proofing/bondcrete-in-ground-timber-protector/
Betta Bitumen Rubber: https://www.bondall.com/water_proofing/betta-bitumen-membrane/
Bondall Bitumen Membrane: https://www.bondall.com/water_proofing/bondall-bitumen/
Certainly, there should be some performance differences between these products. These products, including many other brands, are available in Bunnings.
Can I please get your opinion/suggestions about the technical aspects of the deck, and whether anything can be designed differently? Any heads-up based on your experience would be good to know.
I’ve done many DIY projects in the past, including a full kitchen, laundry, laminate flooring, multiple sheds, etc; so I am very familiar with many projects but had no chance to do a deck so far. I consider the planning half of the job, so I prefer to spend time on this before jumping into the action.
Thank you!
Hello @Baris
Thanks for sharing your questions about your decking project. I suggest having a quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc.
Due to the size and complexity of your decking project, I suggest visiting your local council and having a look at the rules and regulations regarding building a low-level deck near your swimming pool. I suspect that there will be size limitations to how big a low-level deck can be and if it will require a permit.
I recommend engaging the services of an engineer or draftsperson to check your decking plan. They will be familiar with the rules and regulations of the local council regarding deck building. Once signed off by the draftsperson, your plan becomes official and can be submitted to council applications.
Allow me to try and answer some of your questions:
- Instead of placing the weight of the glass panels on your deck, I propose building independent square concrete footings for the glass panels. In this manner no stress is placed on the decking frame and the glass panels are technically on solid footing that will not waver or deteriorate. Plus, it makes it less complicated.
- In regards to the moisture on the ground, I strongly suggest placing a drain system such as agi pipes in the soil so that water will not sit idly under your deck. Putting in weedmat and drainage gravel will also help direct the water out.
- Using galvanised post anchors fixed into the wet concrete is a good idea as concrete and steel have a longer service life than timber.
- I suggest having your engineer/draftsperson check your suggested footing plans.
- Using breaker boards is a design technique that allows you to have the decking panels at a uniform length making it easier to lay out. It also relaxes your line of sight as the boards stop after a certain length. Due to the size of your deck, it would not look out of place.
- Using traditional galvanised nails or coated screws driven in at 45-degree angles is the traditional attachment method for joists, and when combined with the multi-grip should provide a sound anchor.
Since you are in the early stages of your decking plan, it's possible that there may be some amendments that will occur to your decking plan. My best advice is to keep the assembly as simple as possible.
Let me call on our experienced members @Dave-1, @Nailbag, @Itai and @Adam_W for their recommendations.
If you need further assistance, please let us know.
Eric
Good Morning @Baris
That will be a nice looking deck when you have finished!
Things that may help you.
I have a few Merbau decks/privacy screens around the yard/ The timber decking does expand and shrink with age and weather conditions. I used a 4" nail as my spacer when I built the deck next to my pool and it is still going strong tho last time I maintained it some of teh spacers between the boards were almost non existant. (could still get a hacksaw blade down them to clean out built up dirt/leaf matter)
I prefer free standing decks, I dont like the idea of attaching them to the brickwork with a stringer/ledger board. I think over time that can cause issues and in reality a deck should be able to be solid enough on its own. I also used the stirrups for my posts. However I mixed my concrete first and then poured it into the whole. The mix and pour I am not so confidant to use for a footing.
I would probarly go the way @EricL has suggested about creating solid support bases for the glass screen. Timber does move with time and also being next to a pool I think natural expansion and contraction would cause issues if the posts are fixed to the timber.
Drainage, it looks from your photo that your low point is at the back near the steps. People will be people and water will splash out of the pool at the very least, Id factor in some drainage to take care of that and any rain/water flow issues.
That air gap between the soil and the deck near your pool side sounds pretty close, I prefer the distances you were quoting for the rest of the area. Moisture and timber togethere will age it fast.
I never put down a barrier between my boards and joists (both hardwood) 
I kind of new it existed but didnt know a lot (2010) Iwish I had but have seen no issues as yet with not having it. I like the idea of the picture frame look, it really shows up the area nicely. The breaker board is a good idea as well. Mainly I figure if there is an issue down the track you have a de-liminater to stop having to pull up huge sections to replace.
I thought I had another project that showed the mainenance I did this year. Will see if I can get it done.
I found it a lot of fun to build, I still havnt done any lighting or other electrical work to it tho should have done when I built the deck.
Dave
@EricL and @Dave-1 thank you for your replies.
I agree with you about preventing the deck from water damage. This is crucial.
Removing the surface soil for 5-10 cm to create more air space under the deck was already a part of the plan. This will also provide better water drainage at the front and right side of the pool. Though, I agree, these options are not enough.
@EricL , I liked the idea of using agi pipes for groundwater. Weedmat and drainage gravel are also standard as usual. Though, I didn’t find the weetmat greatly effective other than preventing the gravel from sinking into the soil.
@Dave-1, Splashed water from the pool is a good point and will always happen.
I have an idea based on this: https://www.youtube.com/watch?v=aFjK6ANYLW0
This is easy to apply between the joists or bearers, but I can’t use this approach in my case, as it will suffocate the subframe entirely. Instead, I’m thinking of using it on the ground as an alternative to the weedmat.
Because the pond liner is not as permeable as the weedmat, it should keep the ground dry and isolate the source of moisture. Drainage should also be quicker compared to draining the water from the soil. What are your thoughts on using https://www.bunnings.com.au/aquapro-4-x-3m-x-0-5mm-pvc-pond-liner_p0332777 in wet areas? @Dave-1 you're correct, the lowest point of the ground is at the back near the steps.
In regards to the Council approval, this is already in progress, but because this will be a DIY project, I still have to figure out the details myself except for engineering calculations, etc. On the other hand, the great portion of this project will be an exempt Development - Requires No Approval, according to the council. (Except pool fence regulations)
Carrying the glass pool fence on separate posts is a good idea. It helps to distribute the load to the ground directly and reduces the stress on the deck.
@Dave-1I saw your deck project, very nice one! I also have the lighting in scope but there is a long way until that stage 
Afternoon @Baris
If I had the deck above me then yeah I like the idea of a pool liner. With your deck below id actually let the ground do its thing and absorb whatever manages to fall in the area.
Please take lots of photos It would be a fun project to see unfold.
Dave
