I've been building decks since I was 16 and helped my Father build a deck onto our cottage. Over the years I have built over a dozen decks for myself and for friends and family. While living in the UK, I fell into a part time job designing and selling decks. Over 4 years I designed and sold perhaps 200 decks. While designing decks is easy for me and building them is something I already had experience with, nevertheless, the professionals I worked with still taught me a lot more about decks than I already knew.It's a little more than a platform of 12' boards screwed down. It's just one part of a 2 level 650 square foot deck and is sized specifically to fit a gazebo that's 139" square.
A deck that big would take me and 2 others a half day to rip apart.
Diagonal isn't hard, I've done it several times. You run the boards a little long then cut off with a saw.
I do the same...picture framing and sometimes a board in the middle for longer spans (at GF cottage for a 24 foot span) so there is not butt joints. I have 3 adjoining decks totaling 650 sq ft. all with picture framing and no end cuts.
As stated earlier, I have more than a simple 12x12 platform. My point was that on that one section which I just added it only took me 2 hours to lay the deck boards. A larger deck of up to 600 sq ft would take me a day to lay the deck boards....I'm talking about just laying the deck boards, not building the framing.
I'm assuming you don't know very many people with DIY skills or you wouldn't be so skeptical of what can be done. I shingled my house 2 weeks ago...1300 square feet of roof...bungalow, not steep. It took me and a friend 3 hours to rip off the old roof, pull nails, and lay down water barrier. The next day I started laying shingles alone at 8:00 AM, my brother and his wife came to help at 11:00. We worked until 5:00 and it was done.
DIY is all about saving money. I have no problem with anyone who wants to do that but I take exception to the belief that DIYers can ALL do as well as the professionals in ANY example of DIYing, whether it be decks, haircuts, roofing or whatever.
If I were to do my own oil change on a car I would say I was a novice DIYer in that regard. But I can read and follow instructions and have no doubt I could do an oil and filter change satisfactorily. When it comes to decks, I would say I would fall near the professional end of the DIY through Professional spectrum and it IS a spectrum. Looking at a deck built by a DIY novice deck builder there is no doubt at all that you would be able to see it was done by a DIY novice. Looking at a deck built by a professional it would also be quite obvious that it was built either by a very experienced and therefore near professional, or by an actual professional. I don't think anyone can question the veracity of that. If you looked at 100 decks you would have no problem picking out which were done by the average DIYer vs. a professional. It is ONLY those done by the 'top end' of DIYers that you would be unable to say were definitely built by a DIYer. That would represent what percentage of DIY deck builders do you think? 10%? 50%? I would suggest to you it is certainly not a majority of DIYers. The majority will build a deck that you can tell was built by a DIYer.
So you are wrong in thinking I 'don't know very many people with DIY skills'. I know as many as anyone else probably does but I also know that MOST aren't as good as they THINK they are and that very few can do as good a job as a professional can. Simple common sense should tell us that the word 'professional' means what it means.
Do whatever you want as a DIYer to save money. Nothing wrong with that but do not think that everything you do as a DIYer will be as good as if it were done by a professional and that MORE IMPORTANTLY when you choose to DIY something, that it does not come at some other cost such as a poorer job or lost time. At my age in life, every day lost actually matters to me. Or look at it this way, I have more money than time now. When I was 30, that was absolutely the other way around and I wouldn't even have thought about hiring someone to build a deck, why would I, I could 'do it myself' for less cost. The cost for me to do it now though Prairie Guy is far too high a cost for me to be willing to pay. Not a cost in $, a cost in time.
Time is one thing you cannot bank. Money is easy to replace, time is irreplaceable.