Need help with my decking plans

mushtafa · 18 Apr 2016 at 23:04

The badly made, not to scale image above is my plan for my decking.

The joists are 3m long, 18 posts buried 25cm, leaving 15cm above ground.

I will run the decking board length ways. I will use 4.8m boards, cut to 4.27m.

My questions;

1. What size joists do I need. I have priced up 47mm x 150mm x 3m and 47mm x 100mm x 3m. Are the 100mm adequate? 150mm unnecessary? There is about £55 in price difference.

2. Do I need this many posts?

I want to build this to last well, but don't want to waste my money, and on a tight budget. This will be built directly outside the back door, and cover the entire width of the garden.

Any help will be appreciated

eniacs · 19 Apr 2016 at 00:09

I can add a little here. I've completed a similar sized deck. Approximately 3.3m x 5m. I used 200mm end joists and 175mm main joists. However mine do span 3.3m without a central post.

For your deck, I would do 150mm at least for the joists as the span for half of them will be the full 3m. There are tables available to tell you how long a joist can be between supports on a normal floor. Even though my deck has 175mm joists, there is still some give in the centre of the deck. I am now completing another deck, this time higher and 2.7m wide supported on beams. See the last photos, this deck has less give and was much easier to construct!

Finished item:

Photo this year after a clean and re oil:

This years project:

roadie · 19 Apr 2016 at 15:36

Just a heads up that potentially both of those require planning permission if you have not already got it.

kefkef · 19 Apr 2016 at 16:02

How flat is the ground you are putting it on? If it's nice and flat then you could just post the end joists for security and where you have the internal posts, do away with them and dig holes and put in a small concrete square/block, lay the joists on top of them with some damp proof course between the wood and concrete.

koolpc · 19 Apr 2016 at 16:07

roadie said:
Just a heads up that potentially both of those require planning permission if you have not already got it.

lol, maybe the one by the house

https://www.planningportal.co.uk/info/200130/common_projects/11/decking

mushtafa · 19 Apr 2016 at 20:36

kefkef said:
How flat is the ground you are putting it on? If it's nice and flat then you could just post the end joists for security and where you have the internal posts, do away with them and dig holes and put in a small concrete square/block, lay the joists on top of them with some damp proof course between the wood and concrete.

If I understand correctly, you're suggesting making a concrete base to sit the decking on? This would allow me to use smaller joists, but I'm guessing it'll cost a fair amount to construct the base.

mushtafa · 19 Apr 2016 at 20:37

I would never have though about planning permission for a bit of decking! Luckily I don't fall into the criteria.

mushtafa · 19 Apr 2016 at 20:40

eniacs; thanks for the input. I'm surprised to hear that you get flex in yours. Perhaps I should stick with the original plan of 150mm. I'm thinking worse case scenario, having 10 adults stood on it at a time - that's a lot of weight!

eniacs · 19 Apr 2016 at 22:36

Roadie, yes you are right, I got permission on my house to change the conservatory for a huge extension 8mx8m with a mass of decking outside. In the end I went for smaller decking. The other part technically requires planning being slightly over 300mm high but its only just over so if there was a problem i could always lower it lol. No planning people have ever visited anywhere I've lived anyway...

Mushtafa - I did have flex, you could tell the deck was suspended above the floor, ie not solid. But its not flex that I would worry about, the joists are within their design spec. I would prefer not to have flex so would next time go the extra for a method that didnt. Personal preference to be honest, you may not be as OCD as me!

kefkef · 20 Apr 2016 at 10:04

mushtafa said:
If I understand correctly, you're suggesting making a concrete base to sit the decking on? This would allow me to use smaller joists, but I'm guessing it'll cost a fair amount to construct the base.

No. Just where you propose to have posts sunk into the ground you dig a small hole, such as 300x300mm wide and about 600mm deep. Fill with concrete (or postcrete if you don't mind the expense) and make a small box to sit above the hole and give you the ability to bring the concrete out of the ground slightly. Put a small piece of DPC on top of the concrete and lay the joists across that.

roadie · 20 Apr 2016 at 14:13

eniacs said:
Roadie, yes you are right, I got permission on my house to change the conservatory for a huge extension 8mx8m with a mass of decking outside. In the end I went for smaller decking. The other part technically requires planning being slightly over 300mm high but its only just over so if there was a problem i could always lower it lol. No planning people have ever visited anywhere I've lived anyway...

These type of things can crop up when attempting to sell a house so it's something to bear in mind.

Hedge · 20 Apr 2016 at 22:25

kefkef said:
No. Just where you propose to have posts sunk into the ground you dig a small hole, such as 300x300mm wide and about 600mm deep. Fill with concrete (or postcrete if you don't mind the expense) and make a small box to sit above the hole and give you the ability to bring the concrete out of the ground slightly. Put a small piece of DPC on top of the concrete and lay the joists across that.

If you have spare bits of rock/brick/rubble etc you can throw some of that in to save using so much postcrete too

lloydsj · 22 Apr 2016 at 09:04

eniacs, nice decking, I like the smooth finish style. Few questions for you, or anyone else with similar decking...

1) What decking board did you use? (or is it just normal treated timber?) as the 2 major timber merchants near me in north Manchester only seem to sell decking board that is ribbed/ridges on both sides.
2) Do you find it slippy? Even in rain/winter? interested in your personal experience, as you have smooth finish decking.
3) How do you look after it? Any do's/don'ts? as your decking seems to look better in the original pic, before the 're-oil' but that could just be the pictures i suppose.

Cheers

eniacs · 22 Apr 2016 at 11:07

Lloyd;

The decking I have used is Yellow Balau. It is a tropical hardwood, very hard and dense and with no knots. It is beautiful in my opinion! It is roughly double the cost of a softwood however.

Wet wood by itself is not slippery. There has to be somthing on the wood to make it slippery usually an algae grows on it to make it slippery. My balau deck has not been slippery. However it is in the sun and is able to dry out whenever a dry day comes along during the winter. I have a bit of decking around the north side of my house which is slippery every year, it is ridged softwood boards, I clean it several times a year and it soon gets slippery again, its deadly its so slippery, I think I will be ripping it up and replacing it with gravel.

The balau deck has only been down a year, the first 12 months it wore ronseal ultimate decking oil which was rubbish and has now been cleaned off (I also got a full refund). Now I am trying Liberon decking oil. It has gone much darker this year, athough I am not sure as to the cause of this, I am wondering if the wood was dry enough to oil it. I will re oil it when the sun has been hitting it for a few days in the summer. Wood can always be bleached back to its original colour and although this year I only pressure washed it, maybe next year I will sand it back to get fresh wood again. Whatever you do with it, it is wood outdoors so it will require maintenance!

Mark M · 22 Apr 2016 at 12:24

The Balau stuff is beautiful and Im going to price it up for my up coming deck project!

Just to clarify though, if the deck gets a lot of sun then it shouldnt be slippery?

eniacs · 22 Apr 2016 at 16:35

Mark you know your garden better than I. Mine does not get slippery, however I clean it and oil it and take care of it. That said, I left it from July last year until april befor i cleaned it and it didnt get slippery throughout the winter with no maintenance in this period. I would attribute this to its position since round the back of my house the constantly damp decking is as slippery as ice, despite its ridges lol. (I should add here that while making the decking, many friends looked at the smooth wood and said "it will be slippery as it doesnt have ridges" of course ridges or not a wet piece of wood with algae growth is slippery...).

eniacs · 22 Apr 2016 at 16:45

I would also add, the price is high compared to regular softwood, so prepare for a shock when you price it up. Mine is 145x21mm smooth both sides. You can get ridged if you wanted, but there really is no point. I paid about £5 per linear metre, which is about £40/m2. But the way I looked at it was that I put so much into the structure and so much hard work into the prep that the price paid for top decking would be wasted if I put dull softwood on!