A would suggest an initial gap of three millimetres between Merbau decking boards. Once you screw in the first board you can tap three millimetre thick nails next to it to create the spacing before laying the next board. When the next board is fixed into position you should remove your nails and continue the process across the deck. The boards will shrink over time and the gap which was three millimetres will increase to around five millimetres.
Between the middle of one joist and the next a spacing of 450 - 600mm would be suitable.
A gap of ten millimetres between your last joist and wall of your house would be sufficient. It just needs to be large enough to accommodate swelling of the timber and water drainage.
Please let me know if you need further information or have questions.
Thanks you so much. This will be really helpful.
Congratulations @Dilanka, it looks fantastic. I'm sure you will enjoy entertaining on that space with friends and family for many years to come. I have featured the deck on our Project Gallery so that other members can get inspired by what you have achieved. Thanks so much for coming back and showing us the finished deck.
Looking forward to seeing what you tackle next.