Structural raised deck - spans / timber sizes ok?

[oranjeboom]

Creating a raised deck (approx 900mm above GFL at it's highest point (see corner where Muttly's standing): 

 

 

 Looking to KISS and keep costs minimal as the composite decking itself is costing quite a bit. Obviously little point in going for composite boards if the structural deck starts rotting away in a few years time so  looking at solutions that will last at least the warranty period of the composite (20-25yrs). 

 

It will be a wrap around L-shaped deck with steps leading off one corner into garden and a small set of steps down the side of the house. Will need balustrade of some type with perhaps partly glazed (keep the wind at bay from the adjacent field) and wire balustrade. No abnormal large loads such as a jacuzzi but I'd still like to achieve above imposed load 1.5kN/m2 specifications.

 

These are the plans so far:

 

 

 

Post fixed into ground with concrete. Postcrete calculator suggests 1 bag every 10cm deep...so if I do go down as far as 700mm that's 7 bags per post! And I need quite a few posts! Ker-ching @£5/bag!!! So just mixing plain concrete will be a bit cheaper!

Most 100mm x 100mm posts are good for 15yrs so I'd have to take extra measures to make them last beyond that and could try the 'postsaver'. I did also consider using ground screws that are drilled into the ground. Quick, but still pricey with 28 screws costing be around £1000. Concrete decking posts would be fine also but due to the shortage chaos caused by (take your pick) the unseasonly bad weather/COVID/brexit/climate crisis/Napoleonic wars/demise of the USSR are difficult to get hold of (and are pricey!).
 

Working from the Trada span tables, I have gone for doubled-up beams to support the 400mm centred joists. The decking boards I will probably go for are fine with 500mm (https://www.eva-last.co.uk/item/infinity-iseries/ ). Ledger board will be affixed to the slab of the house and concreted 100mm structural timber posts into the predominantly clay ground. Plan is to place a concrete block in the hole with bitumed leg of each post on top and then fullt concreted to above GFL with postsaver.

 

QUESTIONS!

 

1) First question I have is will 700mm be sufficiently deep?  That seems to be the standard to aim for.

 

2) One thing I don't understand is the 'beam size' in mm in the Trada tables. Perhaps someone can enlighten me! Using the Trada tables, I plan to do the following:

• Joist spans of approx 1.94m using 45x147mm
• Beam span at 1.8m using 45x170mm

 

 

Any major holes with these plans? I'll probably get someone to construct the main deck for me but have seen some shocking decks that 'experts' have thrown together so I want to ensure that whatever is erected is pretty much in alignment what I have planned for.

 

 

 

[markocosic]

Life expectancy:

 

- Space your ledger away from the brick wall. Sandwich something waterproof / non-rotting between the ledger and the wall.

 

- Space any "decorative boards" that cover the other end of your "beams" (a skirt around the deck) away from the beams so that the beams can dry. 

 

- End grain of post wood must stay dry. Cover the tops of posts with DPM or similar. Let the bottoms of posts free-drain into pea gravel if they're buried. Not brick.

 

- Ideally you don't bury wood. Cast concrete pillars into the ground (dig round hole, shove a concrete-in post base into the hole hanging from a temporary frame to set them all in the right position, then fill hole with concrete) Afterwards bolt the posts on top of them - again with an air space to dry the end grain. Alternatively cast your pillars (all to same level) then screw a post base down before attaching the post. Note your posts can't carry any side loads once this is done. You have to use diagonal braces. But it keeps them dry and it will effectively last forever.

 

 

 

https://online.depo-diy.lt/productItem/7839/7973

https://online.depo-diy.lt/products/7973#7864

etc

 

UK I guess the closest is:

https://www.toolstation.com/post-base/p29420

https://www.toolstation.com/post-base/p82691

https://www.toolstation.com/concrete-in-sleeper-base-anchor-bracket/p75498

etc

 

- The ends of joists finish under the centre of a deck board. The ends of joists that "join on" to a longer run don't touch each other. Again all to keep end grain of joists dry.

 

- Boards don't start/end/join on top of joists. Boards overlap the joists and drip into fresh air. You need double joists where boards join for this to work.

 

- Joists should stay drier / last longer if the tops of them are covered with something waterproof and wider than the joint (e.g. 47 mm joist with something 60 mm wide on top). Don't know how composite boards effect this. (you're protecting the joist in the gaps between boards then immediately mashing a screw through it so perhaps pointless)

 

 

FWIW if seriously considering "concreteing-in concrete posts" rather than "pouring concrete piles then bolting wooden posts on top" and local enough I found these folks very competitive:

 

https://ellistimber.co.uk/

 

[Temp]

+1 to all that.

 

Only oak posts stand much chance of lasting 25 years in the ground. We have pressure treated 100mm fence posts rotting after just 8 years.

 

If you need to cut any pressure treated wood to length, try and leave the cut ends dipped in a bucket of creosote or similar wood preserver over night.

 

 

[ProDave]

My own decking I dug the narrowest hole I could (ladies spade) about 500mm deep and used about half a bag of postcrete per post.  I don't know where you get 7 bags per post from.