Ground level decking

[Ex-repairman]

Sorry to hijack this post slightly but.........

Thermo in particular has offered much advice on ground level decking so I would be grateful for a couple of answers to specific questions if anyone would kindly oblige.

I am about to build a 3.6 x 3 m deck at the end of the garden, following advice from various web sites I had planned to put down the weed control fabric and cover with pea shingle type stuff and sit the frame on this without any further support. Unfortunately I dug out a little deep and in reality I'm going to end up with around 3 - 4" of shingle rather than the suggested 1 - 2. May help with drainage if nothing else!

Q1 is this a complete and utter no no or would it suffice?

The slope of the land means that once levelled the deck will be fully above ground level at one end but the joist at the other end will be pretty much all buried - this is across the 3.6 m length. Not ideal I appreciate but to raise it would mean it would be too high with regard to privacy etc. A colleague has suggested applying PVA to the 'underground' timber so:

Q2 appreciating Thermo's comments that the wood will rot in time, can anyone give me a guestimate of how long before I may get problems? 1, 3, 5, 10 years.......? I'm using 6" x 2" pressure treated bearers and it will be faced with a 32 mm deck board.

Q3 Is the PVA a good idea? Or is there a better method?

Many thanks for any advice that may be forthcoming, and lets hope I'm not too disheartened by the replies!!

 

[Thermo]

q1 why does it need to be buried? excavate more sil from the low end so no timber is touching it

q2 really does depend on a lot of factors. despite what some will tell you on here fence posts in the ground can last well over 10 years. but if its not free draining they can go quicker than that.

q3 when you go back in to work tomorrow, get your colleague to stand on a chair and get everyone to laugh at him very loudly. PVA is not a waterproofer and it will do absolutley nothing. Its also water soluable, so what does he think it will achieve???????????

 

[Ex-repairman]

Thermo thanks for the response. Maybe I wasn't clear with my first question. When I say the joist will be buried I meant that soil will be in contact with it's front face due to the slope of the ground. The inside will be soil free as it will be dug out and levelled, I just can't think of an easy way of landscaping the ground to the bottom of the joist on the 'outside' edge. The problem mainly is that this end will be coming away from a fence the bottom level of which will be at the top of the deck level and my neighbours garden will end up falling into mine!!

Perhaps more importantly the question was asking can I get away with putting the bearers directly onto the shingle without any other fowm of support?

I look forward to your response, many thanks.

P.S. My colleague will be suitably humiliated at around 7 am tomorrow morning continuing into the weekend!!!

 

[WabbitPoo]

Did I understand correctly - are you intending to put the frame just on top of the gravel, that is, with no support? Not a good idea, as it'll just sink into the gravel.

If you intend on using the edge of the frame to hold back soil then you'd be better to put some black dpc plastic between the two.

 

[Ex-repairman]

Thanks for the reply Wabbitpoo.

Sadly the original intention, having read some of the chain store leaflets, was indeed to sit it directly on the gravel. However, having read some of your previous posts I have come to the conclusion that it definitely needs some sort of pillars to sit on. Unfortunately that raises another couple of questions!:

I'm not keen on cementing in posts as they will rot given time so have decided on concrete pillars. What sort of mix would you recommend? Can they protrude above the ground? Unfortunately, me being me, I got carried away with the digging out for the gravel and the level is now about 2 - 3" below the level I want the joists! So, allowing for a gradual slope for water run off, the pillars will need to sit between 3 - 5" above ground level? Alternatively, should I finish the pillar at ground level and then use bricks/blocks to make up the height? How would I then ensure that the bricks don't come away? Questions, questions..............oh so many questions!!

I'm assuming that the weight of the deck will prevent it gradually moving off of the pillars?

Finally, how deep and wide do I need to put the pillars, and would you agree that a spacing of around 1.2 metres all over the deck will suffice? The joists will be at about 400 mm centres.

I've noted your thoughts on the damp proof membrane and will do as you suggest.

Sorry to throw so much back at you but many thanks for your help.

Nick

 

[WabbitPoo]

IMHO, the easiest option for you would be to use concrete blocks (not the lightweight breeze blocks) set into the ground, with your posts resting on top of them. That way you don't need to build up to your deck level despite having dug out "too much". Just set the blocks in (on a bigger pad of concrete if the ground is soft at all) and then put the posts down to them, leaving the posts too long (ie protruding above the frame for now). Once all are done and the frame is screwed to the posts, cut all the posts flush with the top of the frame.

You will still need to ensure the deck can't slide sideways, which is why concreted-in posts are normally the way to go. Find a way somewhere in the structure for that to be addressed.

 

[Ex-repairman]

Many thanks for all of the expert advice, all very much appreciated. On balance it seems that putting posts in is the way to go so off to the merchants...........

Many thanks once again and sorry for the barrage of questions. I think I'm yet another one who has fallen foul of reading 'how tos' from too many sources!!!

Cheers

Nick

 

[Ex-repairman]

Hi Guys

Following all of the brilliant advice, and having been thawrted by the weather and family commitments, the project is finally underway. Unfortunately I have more questions!!

I spent today constructing the main frame to allow me to establish where the support posts need to be. Having finished for the day (rain stopping play!) I sat down with a beer and read the instructions on the pot of 'Thompsons End Grain Protector' I'd bought to treat the cut ends of the decking. Suddenly got the awful feeling that I should have used it on the cut ends of the joists. Would this be recommended? Not too late as I still have the opportunity to dismantle and treat, although it says two coats with 24 hour drying time is required. The joists themselves are pressure treated with Osmose Naturewood (as per Richard Burbidge) rather then Tanalised, or is it a fancy name for the same thing? Again, any advice as to whether I need to treat these cut ends (exposed and non-exposed) would be welcomed.

Secondly, I bought some lengths of 100 mm square posts to cut down for the graound level deck I'm constructing. Got them from a local 'reputable' timber merchant and they are Tanalised. They were stored outside and upon cutting them it was apparent that they had a varying amount of moisture content, to the extent that the sawdust produced ranged from orange on one length to straw coloured on another. Have I been stitched up like the proverbial kipper, or is this fairly 'normal' and acceptable? Two questions arise from this; should I allow them to dry out prior to sinking in the ground? and, should the cut ends of these be treated with the end grain preserver too?

I know that the pro's on here do a good job, but given the application of two coats it seems a reasonably time consuming addition to a reasonably time consuming job.

Grateful as always for any advice that may be offered!

Many thanks

Nick

 

[WabbitPoo]

Treat all cut ends.

Pressure-treating doesn't really penetrate very far into the wood, especially so with 4x4 posts, so you'd expect to get drier wood at the centre.