View Full Version : Hi all-first post!! Cupping problems.
27th Jan 2012, 08:40 PM
My deck was built by a "professional" builder about 2 years ago however problems started at first winter. Its ironbark 13.5 x 19 mm & mostly under cover. The cupping problems are with the part exposed to weather. Laid on 40mm treated pine bearers that are bolted to a concrete slab. Gaps between planks range from 4mm to 8mm & for various reasons I would not let the builder back on site to fix it (has been back once & made a botch of that). Its been oiled twice in that time but the oil only seems to last 3 or 4 months on the exposed part (which of course makes the overall look a bit odd). The cupping seems to be caused initially by rainwater entering via the butt-joints (which dont appear to have been undercut) and the resulting lateral swelling closes the gap between the adjacent plank to the point where they are touching. This results in "trapped" water where they touch & progressivley extends the swelling along the plank. Upward pressure is put on the adjoining planks where they touch & continue to swell & cupping is the result. The cupping effect causes more water to be retained and pushes the problem further along the plank to the extend that virtually no run-off is possible & pools of water remain until I sweep the water away.In frustration with problem last winter I ran the electric saw along the touching points. This buthered some planks (can be replaced) but undercut the sides of the problem planks & resulted in a 3 mm gap. It seemed to have settled down & doesnt seem to have caused any major problems as they have dried out . All that seems to have happened is that the cupping has reversed which has resulted in the top edges turning toward eachother but still leaing a 3-4mm gap.I will have to replace the screws though as the cupping broke the heads off (8 guage X 40mm deck screws for treated pine).Your comments & suggestions would be appreciated and also any suggestion as to the replacement screws (I dont want bugle heads & would prefer not have to go to a bigger head. I think an upgrade to 10 guage x 50mm would be worthwhile. I have also seen a different screw that has a smaller tread just under the head which I assume is to take the pressure off the screw head if upward movement happened again).I have explored these issues with many, including the Timber Industry Association (who recommended a treatment of a pitch paint underneath). Overall there's 70 square meters of deck so I need to get the fix right first time!
Copied from the G'day forum.
28th Jan 2012, 11:14 AM
sounds like a combination of all the things you mentoned. oil not protecting, screws too small etc, and combined with ironbark, which i think is a strong 'determined' wood that needs to be tamed by strong deep screws. i assume your boards are 135mm (not 13.5mm) so they do need screws bigger and deeper than 8g 40mm. seems there could be waterr retention underneath but its curious that the cupping has reversed. probably best to dry them out and seal the boards on all sides with a penetrating oil such as Cutek CD50. and refix with 10g 50mm or longer if possible in stainless steel.
28th Jan 2012, 12:33 PM
Agree with renovator, but also some of the problem lies withing the board dimensions being 135x19 is not dimensionally stable. For it to be stable (ie not cup) it would need to be around 40mm think. Yeah not really probable in decking but for that reason I only use 140x32 when building decks with wide boards. Cutek seems to be the best (that I have found) in controlling cupping but even at 32mm thick you sometimes still get a very minimal amount of cupping. In addition I also use an 8-10mm gap with wide boards which is also good for self cleaning. I personally would never use a 19mm thick wide decking board which I know doesn't help but maybe for any future reference.
If you are replacing the screws and dont want future problems have a look at spax screws - cost a bit more but well worth it for quality and you want have problems down the track like you are having now.
28th Jan 2012, 01:54 PM
Many thanks for the feedback & suggestions. I will check my notes but I think Cutek was one of the applications recommended by Timber Industry Association also. I checked the Spax website & they are impressive however I was told at several builders supply outlets that stainless steel screws are a softer steel than ordinary steel & they recommended the non-stainless steel due to the problem of the heads being pulled off. The upside with SS screws would be their ability to resist rust corrosion but how long would a coated steel screw last anyway (ie those recommended for use in treated pine have a coating to prevent reaction with the chemicals which also stops rusting they say). I notice the spax screws also feature the small lock-down thread just below the head to transfer any torque caused by cupping away from the head. I dont recall who makes the steel screw with a similar dsign (meantioned in my initial post) but it would be usefull to find out whether they have a rust inhibitor also. May be reasonable compromise. As`the holes are already drilled & countersinking done, the action of the spax screws may be too sever. Thanks again for your responses. Any other comments would be greatly appreciated as this has to be fixed & has been bugging me for alsomost 2 years.
28th Jan 2012, 04:28 PM
I only use ss screws for decking and in particular with the spax screws I have never had one shear yet. If you pilot and clearance hole it won't shear.
The problem with using coated screw (ie gal etc) is that down the track when it needs to be sanded and refinished then you will sand the coating off the screw exposing it to corrosion. If you want the deck to last I think you would be crazy to use anything but SS.
28th Jan 2012, 06:05 PM
As steveoh741 and r3nov8or have said a combination of problems, all really stemming from use of those boards with wrong dimensions (there are some milling tricks to reduce this, but too late for that and access to a miller who knows how is virtually impossible - and the cost would be high anyway).
So the advice offered SS screws, don't try and save money on them and some good coatings is all OK.
But here's what I'd do: lift them, split them and re-lay them! I know it sounds a lot of work, but IMO you are never going to get a good result from the other options and they involve removing and replacing all the screws anyway. Lifting and replacing can be done in sections - start where the weather exposure is worst and 8-10 boards at a time. You would need to set up to rip them safely and accurately and you'd need to arris the edges or use a sander to take off the sharpness from the rip.
That's what I'd do, but I have the knowledge, the experience and the gear so it's easy for me to say! There a lot of work, but quite within the capacity of a DIYer - if you have the time, the patience and your lifestyle (no kids, adults etc to be injured when decking is removed for example) allows it, and you can get access to the gear or buy it, and feel confident enough (maybe find some experienced mates - beer and barbie always after the work effort . . .) then maybe that's the best option.
BTW - do they have two screws at each joist or three (with centre one)? If only two then adding a third screw might help - not just that though.
28th Jan 2012, 06:11 PM
Out of curiosity , how far in from the edge of the boards are the screw holes ? and as Bloss asks is there a third screw ?
28th Jan 2012, 07:24 PM
is the concrete under the deck allow the water to drain away or is it just sitting ontop of the slab, was the bottoms and ends coated before install? 130x19 deck works otherwise big guys like boral would not make it, but you need to fix it right, old rule of thumb is 2&1/2 x thickness for screw length. so you need like a 50mm screw
28th Jan 2012, 08:44 PM
This is really sad that a professional builder would only use 8g-40mm screws, amongst other faults....an obvious case of holding a licence means absolutely nothing in receiving quality workmanship.
As asked above can water get away from under the deck? this is important....the decking is rather close to the ground as it is & if its getting constantly wet then drying(even moisture coming from the ground could be enough), this would accelerate the cupping problem, aswell as not being held down enough in the first place.
Whatever you do apart from using 10g-50mm SS screws I would also lay out some kind of damp course on to of the joists (under the decking), plastic "snakeskin" or similar.
29th Jan 2012, 06:59 PM
The deck is on 40mm treated pine joists & the bearers are bolted to the concrete slab. Any rain water runs off the slab & is taken away via aggie-pipes etc. There are channels cut in the slab as there was a slight pooling in the slab initally - this seems to work effectively & allows the water to run off. The concrete stays wet of course following rain but dries out. I dont recall seeing any damp-course, either between the slab & joists or between the joists & the ironbark decking. I imagine damcourse could be added when I am changing the screws (& coating the underside with Cutek) but wouldnt water settle on top of the dampcourse anyway? Could you provide more info Woodchip on how this should be set up please eg is it a strip that runs along the top of the joist or only under each deck plank where it meets the bearer. Does it have a commercial name?
I am also interested in what Hammerhead has asked about "coating the bottoms & ends"`- not sure that the builder coated anything but could you fill me in on what you mean please, ie bottoms & ends of the boards or the treated pine joists. What should they be coated with & is it too late to do it now?
The screw holes are 25mm in from the edge of the boards and there is no third screw a Jiggy & Bloss have asked. However as the cupping is the edges turning up I am unsure how a third hole in the middle would stop cupping.
Something else I didnt mention but stevoh741 raised- the screws are countersunk 2 mm below the top of the board - so no chance of taking the surface off if I sand the boards. I have wondered if the water that sits in the countersink holes is componding the problem?
Thanks again to you all for your interest & feedback.
30th Jan 2012, 10:17 AM
Yes, its a strip running along on top of the joist, immediately below the decking board.
You have a few options with the "dampcourse", which essentially is what it is (brickies dampcourse), you can get what is known as "snakeskin" its a black plastic roll in various widths from 110mm, you could cut the 110mm( or even if its 100mm?) in half, it also comes in aluminium, or there is a product specifically made to lay on top of joists, sorry I dont know what any of the commercial names are.
Water would sit on top of the "dampcourse " to a certain degree, but the idea is to prevent (as much as possible) moisture wicking up from the ground, via the joists.
Personally I would not bother coating the treated pine, my theory would be its impossible to keep the water out of it, it wont readily rot (H3 (H4 would be better, but I doubt it is) in this case should be fine for at least 15yrs at a guess), but having said that ....with a convincing argument I could change my mind.
30th Jan 2012, 10:51 AM
the reason the third screw was mentioned was your mention of cupping in reverse. the third screw would help hold down the centre of the board.
IMO joist protection is just that - protection for the joists, not the boards from wicking. but if you are taking up the boards theres no harm in laying joist protection. Use malthoid IMO.
i still reckon coating the whole boards with something like Cutek (also reinforced by the Shipshape's mention the Timber Assoc recommended it) is the way to go. Ripping the boards down would also help but may not be the look you are after.
shipshape, could you also use some paragraphing to make your posts easier to read. Ta :)
30th Jan 2012, 08:26 PM
Woodchip - he's way too late for dampcourse . . . :-
On wide boards adding a centre screw can help prevent cupping - but 25mm in is way to far and will be part of the problem - 10-12mm in is what I would expect.
I can't se easy easy fix - se my earlier post for what I'd do (aside from applying heat to the nether regions of the bloke who built it!)
31st Jan 2012, 07:57 PM
25mm is too much and probably the main reason.
31st Jan 2012, 09:01 PM
I'm thinking its possible that the ironbark isn't reeded on one side? if not, maybe possible to flip every board and use three screws - centre and 10-12mm for the edges?
1st Feb 2012, 07:37 PM
1st Feb 2012, 11:02 PM
x2 with the malthoid
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