Laying deck joists directly onto concrete?

[sambotc]

Hi,

Basically i have a slab of concrete in my garden which i would like to lay decking over.

The slab is 150mm below DPC so i can't really raise it much, and being 8" deep i dont want to remove it.

Anyway, can i lay the deck joist's directly onto the concrete, or do i need to put DPC under them to sto water from rotting them out to fast? Also can i then fix the joists to the concrete or should i just rely on the weight of the wood to hold it in place?

Also as the joists will be 4" plus 28mm deck boards i'm going to come above DPC level. I plan to leave a gap from the house to avoid damp issues and avoid fixing a bearer to the house. What kind of gap should i leave? I was going to leave about 50mm do you think it is enough?

Cheers in advance, Sam

 

[OLD]

Use treated timber for joists with strips of dpc under fix a couple of joists to concrete with galv. brackets to stop deck moving. space boards to allow water/snow through say3-5mm use a nail etc.

 

[sambotc]

Cheers for the quick response, any ideas about what size gap i should be leaving away from house where decking level is above DPC?

Cheers again, Sam

 

[Thermo]

you dont need to worry about the dpc with decking. it is free draining and does not cause a problem as a patio would do. You can lay onto the concrete. do not put dpm under it as it will just hold the water. when we lay over concrete we use short legs of 4 x 2 to give support to the joists to stop them moving. the weight of the structure will hold it in place.

 

[sambotc]

The usual clash of opinion's in the construction industry!

That makes sense and is more practical from my point of view.

Just to clarify sorry, when you say short leg's of 4x2" how do you mean?

Do you mean to hang the deck off short leg's so the main joists are off the floor so that just the leg's are in contact with the ground?? In which case what sort of ground clearance should i be looking at?

Thanks for the reply, top stuff

Cheers, Sam

 

[Thermo]

whatever ground clearance suits really.

the height is normally goverened by door thresholds etc. once we have a datum level we normally put wall plates around the area tobe decked and then infill with joists at 400mm centres. You can then use the "legs" to support the joists to stop any bounce. normally at around 1m intervals is sufficent. Once you think youve got them all then walk over each joist to see if there is any bounce.

Make sure you fall the deck away from the house. you can shim the wall plates away from the wall if you are concerened about damp but its not really a problem.

For a better finish put the fabric over the frame not under it. Stops food rings etc falling through. weeds cant get up between the gap the floor and the deck as they do when people put it on the floor, and it means you dont see the framework either.

small tip with the supporting legs. Lift the joist just slightly before you fix the legs to them. that way they have the pressure of the joist pushing them down and there really is no bounce then.

 

[WabbitPoo]

And 50mm is way too big a gap between the deck and the wall. A child would get a hand stuck in that. Go for no more than 5mm.

 

[WabbitPoo]

Oh, and do a SEARCH as this has been answered in more depth many, many times.

 

[Thermo]

with pictures!

 

[sambotc]

And 50mm is way too big a gap between the deck and the wall. A child would get a hand stuck in that. Go for no more than 5mm.

I was referring to the gap i should leave between the house and deck to prevent damp problems, not each deck board!

Also i did search for this specific topic but it came up with no results, so posted this?


Cheers for the info thermo,

Sam

 

[Thermo]

wabbit poo is referring to the gap between the wall and the wall plate. you need only a few mm if you really want to put one in

 

[sambotc]

Sorry my bad

I don't think i'll be attaching to the wall if i can help it!

Cheers, Sam

 

[Thermo]

it will give a much firmer deck if you do. we use rawlbolts to attach them to walls.