Decking Drainage issue

cwcw · 7 Dec 2014

Last summer I installed some decking. Admittedly the slope I used wasn't steep enough towards the lower end but it was fine last winter. This winter, some of the boards are not draining at all at the lower end so there are puddles.

What I don't understand is:

- why ok last year but not this?

- why only some boards when all on same slope?

- why have boards expanded out at the edges and made themselves concave? This makes drainage worse as it cancels out the slope in the other direction, going against the shallow grooves, which annoyingly I did much better than the main direction slope.

I am not sure how to correct it now, other than strip off all the boards and readjust the feet that sit on the old patio. There are about 40 feet and then 15 x 5m boards so not a minor job! Anyone have any alternative ideas? How urgently do I need to address the drainage as I've read it affects its lifespan?

big-all · 7 Dec 2014

pictures will help
also construction method will help
it could be lots off things including sagging /settlement/debris/timber movement etc

cwcw · 7 Dec 2014

Construction is 4x2 pressure treated joists, 400mm centres, plenty of noggins for extra strength and then, as I mentioned, lots of feet that lift the frame from the old patio and provide the slope and extra rigidity. I had an issue with a couple of joists being warped so corrected them using plastic packers as shimmies to compensate.

Photos wouldn't really show much to be honest but will be difficult to get at this time of year, dark at 4.

big-all · 8 Dec 2014

ok what are the 4x2" supported from then are those timbers supported by posts concreted in or supported on slabs/ concrete
is the support in a grid pattern or random
and what fixings or brackets have you used

cwcw · 8 Dec 2014

The corner posts are concreted in. The joists are fastened using 3 long screws into the frame. Down each joists there are 3 or 4 feet that support them further on to the old concrete slab patio. The feet are made of 4x2 the same as the joists, 2 lumps screwed together, one shorter to slot under the joists and the longer one screws into the joists. They alternate sides along the joists. Noggins are randomly added in between joists, again 4x2 and 2 long screws. The patio beneath had been down since the 70s so should be a firm base. The whole frame structure is very strong and rigid, no bouncing.

I can understand the boards swelling more at the slower draining side, and that this makes it worse because it removes more of the slope. I don't get why the boards are swelling more at both edges though, which creates the cup shape that makes the drainage even worse. And I can't think of an easy solution other than lowering a lot of the feet and all that goes with that.

AronSearle · 8 Dec 2014

cwcw said:
- why ok last year but not this?

Newer timber may have a cleaner surface = less surface tension for water

- why only some boards when all on same slope?

Because!

- why have boards expanded out at the edges and made themselves concave?

Higher moisture contents, not to unusual and fixings are supposed to restrain some of this movement.

I am not sure how to correct it now, other than strip off all the boards and readjust the feet that sit on the old patio. There are about 40 feet and then 15 x 5m boards so not a minor job! Anyone have any alternative ideas? How urgently do I need to address the drainage as I've read it affects its lifespan?

It will promote algea growth = slippery.

It will increase moisture contents = reduced lifespan.

FYI ribbed surfaces are not supposed to go on top, but on the bottom, not sure if that is what you meant by 'shallow grooves'.

cwcw · 8 Dec 2014

The deck boards are reversible, one side the usual deep grooves and the other side a shallower groove. I chose the shallower for ease of maintenance, e.g. leaves and debris.

AronSearle · 8 Dec 2014

No, people think they are revearsable (including timber merchants).

Those shallow grooves are not supposed to be on the top surface, they were designed to stop moisture being trapped between the joist and the deck board. Boards may also be machined with the shallow grooves on the face that will cup (as yours have).

cwcw · 8 Dec 2014

Oh... they were sold as reversible. The thicker grooves would still give ventilation under the boards meeting the joists. Some have cupped but less than a third I would say. Wouldn't the ones that haven't cupped now have cupped if installed the other way round?

If this is the cause of my problems, do I have cause for redress from the supplier?

AronSearle · 8 Dec 2014

cwcw said:
Oh... they were sold as reversible.

If this is the cause of my problems, do I have cause for redress from the supplier?

No, just live and learn, the whole UK timber industry thinks they are supposed to be used on the top, the design originated from another country and was misunderstood here.

Wouldn't the ones that haven't cupped now have cupped if installed the other way round?

Typically, and it can vary, they machine the ribbs on the surface that will cup, and the groved surface on the side that will crown (as crowing will shed water better).

At least that used to be the practice, now they probably shove them through the machine any which way, especially with cheaper decking.

cwcw · 8 Dec 2014

Ok thanks. They were cheap as I shopped around for the best price but still chunky ones, not thin. I would guess they were machined randomly so cupping would occur on some whichever side I chose as the surface.

Since I prefer the groove side and I can't reverse them anyway now (some cuts round rail posts and no doubt messy screw holes) what should next steps be? Cupping is only starting and is only at the slow draining end so I'm thinking it's not too late if I improve the drainage and oil it all. However it is December.....

AronSearle · 8 Dec 2014

Probably unfeasable, but you could try screwing them flat (it's easer to stop them cupping with screws than it is to correct it).

You could clean up the decking boards and apply a water repelent decking sealent, this will require yearly maintainence.

Or just accept what is and treat them as somewhat disposable.

cwcw · 8 Dec 2014

Not sure if this is what you mean but I used screws rather than nails and they're fairly close to the edges. If I tighten more they start to go straight through the woods.

Option 2 sounds good but I still feel I need to correct the slope somehow. It was my first real project and a lot of planning and work went in and the results seemed good, so there's a sentimental side to fixing it properly as well. That said, taking most of it to bits to trim feet down all to add another 1cm - 2cm gradient is a daunting prospect....

cwcw · 9 Dec 2014

I checked the end grains on a few boards and by chance, the shallower grooved side I chose for the top means they are bark side up. I thought this was the correct way and stopped cupping?

r896neo · 10 Dec 2014

there is no quick fix here. its a pain to lift them as you know but the only real solution.

1:50 is a minimum for a softwood deck in my opinion.

90% of people dont even bother to run a string line over to check the joists are all flat and then they screw boards down creating little troughs over a low joist etc.

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Decking Drainage issue

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