# Forum Home Renovation Decking  cupped decking boards, and moisture under deck

## crash76

G'day, i have built two deck, one being a boardwalk type of deck from the front door, which is about 1.4m wide, by 6m long.  It runs from the front door, to the footpath.  There is a gentle slope in the ground running away from the house, with teh front door facing south (so not much sun). 
Second is a large deck, of about 10m x 5m out the back. 
I have used 139 x 19 spotted gum.  Problem is now that due to the moisture which gets trapped under the deck, almost all teh boards are cupped, and its killing me.   
On the boardwalk, i have also got an area near the front door that the boards get so swollen that they have pushed each other upwards (i didnt use long enough nails out the front). 
Is there anything i can do to help with the moisture under the deck?  as they are both quite low to the ground, i obviously dont get enough air flow, and didnt think too much about drainage when i put the deck down.  Can i sweep a big load of sand between the boards, so it sits under the deck to help soak up some moisture or something? 
Also, if i need to pull the boards up (hope not) but as they are cupped, should i sand the underside (as it will be mouldy), and lay then underside facing up to help push them back to flat? 
Sorry this is such a long post (especially for a first one!) 
Crash.

----------


## jkirky

Well... Could be a number of things... 
 1 what types of fasteners did you use... Nails? Screws?
2 what spacing did you use, ie 130 odd mm decking with only two nails holding it down is gunn a be an issue, regardless of moisture levels...
3. How high is the decking from the ground..
4 what spacing did you use between the decking.. 
Worst case, you may need to pull up the boards and re-fix with a more appropriate measure... But best to work out why they are cupping in the direst place.. Also, can you post a pic?

----------


## Phil B

It's almost certainly poor ventilation causing condensation and unfortunately Australian hardwoods don't seem to cope well in those conditions in some parts of Australia.  We have had problems both here in Adelaide and also in Perth and the only way to avoid it is by using decent gaps between the decking boards (5mm minimum) and leaving the sides open to allow some airflow.  On low decks this is not always possible and alternative decking boards might need to be considered.  No matter what you use, however, there is no guarantee it won't happen again but you could prolong the life of the boards by oiling the underside prior to installation.  Whether you can save your current boards or not I don't know. 
Sorry I con't be more helpful, but the questions jkirky asked are relevant and a picture or two would help.

----------


## jkirky

I'm not quite convinced that moisture is the source of the problem (although moisture may very well be an issue, is it the primary reason that your boards have lifted and cupped... 
You mentioned that boards have swollen and are lifting the adjacent boards- whilst it's true moisture will swell the timber, the problem here would more likely be insufficient gap between the boards, along with incorrect fasteners (the high level moisture is just exacerbating the problem- definitely need to ensure your drainage is adequate, but if the gap and fastners aren't up to the job your in trouble anyway) 
Wide boards like what your using really want at least 10g screws with at least 30mm embeddment. Furthermore, if you have the screws too close to the middle, the edges will cup- whereas if you have the screws to close to the edges, the middle will cup. To counter this some recommend three fasteners/ screws along the length. Also, a 4-5mm gap is needed with the wider boards. 3mm is ideal for 90mm decking. 
Wide boards are prone to cupping when not installed appropriately, so what I'm trying to say is whilst moisture may very well be an issue, before you think about repairing you really want to determine if moisture is the primary source of the problem or merely an exacerbator. 
Hope this helps!

----------


## stevoh741

even with batton screws you're not going to stop cupping with a wide board only 19mm thick. Even at 32mm thick the 140mm boards will cup and once they start then the hold water and promote further cupping. You can minimise by using the correct fixings and applying a suitable water repellent but you need to keep on top of the upkeep. If you want a wide board that wont cup there is a proflie 120x40 that supposedly is stable enough. Called deckwood (check it on the outdoor structures website:http: //www.outdoorstructures.com.au/decking_com.php ). some good decking light reading there too....

----------


## Phil B

This still happens with 90mm boards as well, and if it's not caused by insufficient gaps then what happens is the bottom side of the board remains moist thanks to condensation while the top dries due to weather.  As the top dries it shrinks more than than bottom causing cupping and this tends to happen a lot more as you move into winter months and the air temperatures drop.  In every case we've had of this happening within our company (and we build over 75 decks a year) it has been a 90mm Australian hardwood as we don't use a lot of wide boards - the market dictates otherwise.  It has never happened with merbau in either 90mm or 140mm wide boards.  Swan River Red is particulary poor in these situations - bad enough that we no longer use it anywhere, let alone low level and restricted ventilation decks. 
Stevoh741 suggested water repellant and that is definitely a good option, although no help for the original poster.  Also, like oiling the underside of the deck prior to installation, it tends to be nothing more than a stalling tactic as it will only ever get done once because you have no access to the underside of the deck after installation.  Keeping the top well oiled is critical to minimise the potential for damage.

----------


## crash76

Thanks for the replies guys, i have been away for a little while with work, so havnt been back to check on this post. 
I think i am going to have to pull up all the boards, and then relay using screws instead of nails, and have a slightly bigger gap.  I used the titadeck nails, which i will now change to screws, just means i have to do some countersinking as well.  I think i will have to take all the boards off and let them dry out first though as well, see if i can get them to flatten out a bit. 
I'll try and get some photos up too. 
One of the other problems, was that we used the Cabots aquadeck on it when it was recently laid, and that made it look aweful, so i didnt get to reoil it for nearly 12 months, which i have no doubt contributed to the problem. 
thanks again.

----------

