levelling joists for decking

[Jupiter01]

Hi All,

I am installing 4x2 joists on a concrete base. The decking will be screwed on to this.

I want to install the joists at the correct level so that water flows off in the right direction. The joists needs to be raised an inch from one side. I have plastic packers and could pile these on top of each other but it doesn't feel right. I also any to avoid any bounce in the joists.

can you please advise. Thanks

 

[foxhole]

You would adjust the height where they connect to the vertical posts.More posts less room to sag.

 

[Jupiter01]

aha. That's really helpful I think...
Up until now, I assumed that I can only level using objects that are packed underneath the joist (packers, timber, etc). My current attempt can be seen in this picture:

I think you are suggesting that I screw wooden legs to the joists and adjust the height with this method. Presumably these legs would be screwed to my joist but what would hold them to the ground? A picture would be very helpful if you have one please.

Can I construct these with the current 4x2 timber and presumably they will be required on both side of the joist as I am laying the joists flat i.e. the 4 inch side will be where the decking boards will be screwed.

Thanks for your help.

 

[foxhole]

The 4inch dimension should be the height or the structure will be very weak, you could use post plates to secure to the ground though a large deck will have sufficient weight to not make that essential. Space at 400mm centres to avoid sag of decking board.

 

[Jupiter01]

I must admit I hadn't thought about the difference between laying the joists flat versus vertically. Your point on strength seems very obvious now that you mention it...

The 4inch takes me quite close to the DPC which isn't recommended I understand? I will be around 5cm below DPC by the time the decking is on.

I am inclined to go for 3x2s instead of the current 4x2s and have them vertical at the 3 inch height. What do you suggest?

 

[Jupiter01]

does the above make sense @foxhole as I need to order in the 3x2's if the proposed plan works. Thanks for your help.

 

[foxhole]

Dpc level is ok as you need to leave a gap between house and decking to avoid damp problems.

https://www.tdca.org.uk/

 

[Jupiter01]

so am I okay to go with 3x2's as they will provide me with more flexibility in staying away from the DPC level?

 

[Jupiter01]

do the 3x2's seem okay guys? I think they also come in C16.

 

[Jupiter01]

I'm sorry to pester but would 3x2's suffice for the decking joists? The length of each will be 2.4 metres. So not a very long run in my opinion. I can put noggins in if that will further reduce any possibility of a bounce in the decking.

 

[foxhole]

At 2.4 would likely need support in center.

 

[Jupiter01]

Thanks for getting back to me @foxhole.
I can put another leg in the centre along with noggins across the joists. I will probably need to put a leg in the centre any way to get the correct fall for drainage. I will go with the 3x2 joists on this basis.

 

[Seb101]

I'm not a huge fan of using wooden 'legs' as they become a single point of failure for the deck as they are more prone to rot faster due to exposure to ground water.

The last deck I built I set engineering bricks into Postcrete beds (with the correct run-off angle) to act as the supports, then laid the deck joist on these (see pics). Probably over-kill but it should last a while!

As you already have a concrete base it might not be possible/practical.

To protect against damp from the ground penetrating the deck I always place a piece of DPC membrane between the leg and the deck joist, it's not fool-proof but it helps.

 

[Jupiter01]

That looks very nice @Seb101. A brick would take me to high up. That's partly the reason why I am using 3x2's and not 4x2's for the joists. I could double up on the legs if you think that would help?
In my situation where do you suggest I place the membrane?

 

[Seb101]

My deck uses 3x2 C16 but that is why I have put so many supports. You don't want to span more than ~70cm unsupported with 3x2 joists otherwise they may start to flex under load. My spacings were 60cm centers on the supports (what the joists sit on) and 50cm centers on the joists (what the boards sit on) however I was using thicker 32mm decking boards, if using thinner you may want to reduce that joist spacing.

DPC between the 'leg' and the main joist. Using wooden legs you will likely have to screw through it however for your situation which will create a path for water, it might not be worth it.