# Forum Home Renovation Sub Flooring  Securing joist to concrete slab

## grob

I am putting a wooden deck on top of a concrete slab (mainly because we prefer the timber look to tiles). I am using 70mm by 35mm treated pine joists (about 2100mm in length) with a hardwood top. I intended using a construction glue and loxins to secure the joists to the concrete. 
In discussions with suppliers, some have suggested that I need 3 loxins per joist, others no loxins as the glue will suffice and some have suggested 2 loxins on every 3rd joist.
The suggestions have varied on what they were trying to sell me! 
Would greatly appreciate experiences or views.   
Grob

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## kenmil

Is this just a deck, or is there a vertical structure above ? That would be the determinant as to how many bolts per 7' length I would think. If just a deck, two bolts plus adhesive would be more than adequate. I prefer to use screwbolts myself, rather than dynabolts, as they can be removed, or then there are the chemical adhesive ones which bond themselves into the concrete.

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## grob

Thank you for your replies.
Yes it is just a deck with nothing above. I am an owner builder. When we designed the house we thought we would have tiles. It is a large area (38 sq mtrs) and we now think timber will look better. It is cover with the main house roof.
Would you bolt each joist or only every 2nd or 3rd one?
And (excuse my ignorance) what is the difference between a screw bolt and a dynabolt? 
Grob

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## Dazza007

:Biggrin:   Hi Grob, 
The difference being:
 (a) a screw bolt aka "coachscrew" has a screw threaded shank with a hex bolt head which is tightened by a spanner or socket.  Use for securing wood or metal objects to wood.  eg; a press drill to a work bench.  They come in a variety of sizes from 1 inch to about 6 or 8 inches long. 
(b) a "dynabolt" is a trade marked name for a bolt assembly wich expands at the base when/while it is tightened.  Usually the assembly is inserted into a pre-drilled holed in concrete or brick/mortar.  It is tightened by a spanner or socket.  In appearence, the bolt is inserted "head first" into the hole with the thread end protruding enough for the article to fastened.  The nut is then threaded onto the shank and tightened. 
The 'Dynabolt' is often used to secure large objects to cement floors or walls. :Cool:

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## Eastie

Re Dynabolts and Masonary screw-bolts: 
Ramset make "ankabolts" that cut their own thread through brick/concrete.  I would use these in preference to dyna bolts for this job any day.  Dyna bolts are bastards to install properly and are even worse to remove if you stuff one up or it decides not to 'grab and expand' - not so with masonary bolts.  The other advantage is you don't have to grind the top off as you sometimes have to with normal dyna-bolts (threaded nut insert dyna-bolts are different but I've only ever seen them available by the thousand - 200% better than the normal external nut ones).   
Masonary srews are available through any good hardware outlet and bunnies also stock them  :Tongue:    ramset ankascrews 
If you are unsure of the level of re-bar in the slab or if you don't want to penetrate the entire slab then these may be an option - a bit more alignment required than the others though:  ramset loxins  
If unsure go to your local bolt outlet and ask them.  Other than this follow stoppers advice.

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## kenmil

> _Originally posted by Dazza007_  *  Hi Grob, 
> The difference being:
>  (a) a screw bolt aka "coachscrew" has a screw threaded shank with a hex bolt head which is tightened by a spanner or socket.  Use for securing wood or metal objects to wood.  eg; a press drill to a work bench.  They come in a variety of sizes from 1 inch to about 6 or 8 inches long.   
> The 'Dynabolt' is often used to secure large objects to cement floors or walls.*

  I was not referring to coach screws, as they are only suitable for joining timber to timber or metal to timber as you mention. The screwbolts I refer to are similar in appearance, but they screw into masonry,brick or concrete with a socket or spanner, can be removed easily, and are not dependent on you getting the depth of the hole exactly right. They are just as strong as dynabolts, and easier to use. Eastie refers to them as Ankabolts, and he may be right, but I have always known them as screwbolts.

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## grob

Thankyou for your comments. I purchased AnkaScrews direct from Ramset for $120. They gave me a 40% discount and they were certainly cheaper than Bunnies at $190!
Now to put it all together.
My slab has quite a few lumps and valleys in it. Is there a smart way of laying level joists without having to chip off the high points?

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## journeyman Mick

Beg/borrow/rent a dumpy level, find the highest patch of concrete, then use this level to set up the two outermost runs of joists, wedging them up to get them to the right level. Then you can use a string line between them to level all the rest. I would probably use gal. washers on the bolts as packers rather than relying on wedges (even plastic ones) to keep it all level for the long run.  
Have fun, be safe, 
Mick

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## Pulpo

I imagine if your building your own home you would be familiar with the dumpy level. 
If you do not have one another cheap alternative is the laser level on a tripod for around $50 or something of similar design. 
If you dont get the levels right from the start you have problems forever. 
As for glue and mechanical fasteners I would use both. 
2.1m I would use 3 bolts in each joist maybe an over kill but cheap insurance. 
Dealing with the concrete levels I really have no idea I know you can buy products that pour onto the slab to level it. If you just attach the wood to the slab then planing the top surface of the wood is the way I would go than trying to shape each piece of timber to the contours of the concrete slab; level from above not below. 
Just a few thoughts 
Good luck 
Pulpo

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## journeyman Mick

> _Originally posted by Pulpo_  
> Dealing with the concrete levels I really have no idea I know you can buy products that pour onto the slab to level it. If you just attach the wood to the slab then planing the top surface of the wood is the way I would go than trying to shape each piece of timber to the contours of the concrete slab; level from above not below. 
>  [/B]

  Ther are cement based floor levelling compounds available from Ardit and Levelcrete to name two, however all the ones I've worked with are for interior use only. Try Hilti (agents for Ardit) or Ramset and see if they have a floor leveller suitable for exterior use. However galvanised washers on your bolts and between your slab and joist will give you adjustment in 1.5mm increments which I reckon is close enough for an exterior deck. Plus using a levelling compound would mean making little forms to exactly the right height at every point you want to level as the stuff is pretty runny (like thickish cream) so unless you form it up its going to run down to all the low points and you probably don't want to top the whole slab! 
Have fun, 
Mick

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## John G

Don't forget to cut drainage notches in the bottom of the joists (every foot or so) so that water that seeps through the deck can escape.
If your concrete is unlevel, be careful of areas that pool water.

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## Sigidi

Hi Grob, 
this may come a bit late, you mave have already done your deck and I'm a new member, but I hope I'm not going to be stepping on toes... 
Why hasn't anyone mentioned anything about minimum clearance for the floor sub-structure? BCA (Building Code of Aus) states a minmium clearance is needed to allow adequate ventilation to prevent timber deterioration. True drainage notches are a good idea and ponding occuring are good points to note, although the floor sub-structure needs to be picked up off the concrete to allow adequate ventilation. 
Grob you mentioned you are using treated pine for the sub-structure and hardwood for the decking. One thing to take into consideration is CCA or LOSP treats the timber against the bad stuff, but you will also need acces to allow for termite inspection.  
In Sydney, ventilation above sealed ground needs to be a minimum 3,000 mm square per metre and minimum height above ground 150mm or 400mm if termite inspection is required. 
I hope this helps and I have'nt come off like a know all, but I'm studying and having to cover all this stuff just recently. 
Sigidi

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## Sledge Forward

Yeap
 I'm undertaking the same task my question is do u vent the floor. IE to core drill 100mm dia - 150mm dia holes at say 1.2m ctrs or not.Some say to seal it with forticon/builder plastic. We are appling timber floor boards in the normal sence and in the boat deck finish externally.

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## Pulpo

Sigidi's post
[this may come a bit late, you mave have already done your deck and I'm a new member, but I hope I'm not going to be stepping on toes] 
[In Sydney, ventilation above sealed ground needs to be a minimum 3,000 mm square per metre and minimum height above ground 150mm or 400mm if termite inspection is required] 
[I hope this helps and I have'nt come off like a know all, but I'm studying and having to cover all this stuff just recently]  
[/QUOTE]  
Firstly the more comments that are posted to a listing the better, it gives greater information to all concern, even if I was not the one to post the original listing and going by the number who read the posts to those who respond Im not alone. 
So if you feel you have a vaild point to make, we all may become wiser from it, hopefully. 
Its good to see someone has the standards, and knows what is required. 
In regards to water pooling would it be a major issue as the flooring is covered by the house roof? 
Some cut outs on the bearers or washers under the bearers should solve this problem, at a guess. 
Finally you mentioned you are currently studying do you mind disclosing what you are currently studying only because it seems relevant, and we may have an expert to ask next time. 
Cheers 
Pulpo

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## Sigidi

HI Pulpo, 
thanks for the encouragement concerning posting mails. 
In response to your question regarding if water pooling being a major issue to flooring under the house roof. Maybe hinting at the roof negating any issues arising from ponding. Well, I'm just going on info that water/moisture causes all damage to residential dwellings caused by soil movement, also all floors in a residential dwelling covered by a roof are still required to allow the minimum clearance for sub-floor ventilation. I know sometimes the requirements seem to be somewhat over-the top, I'm not 100% sure why, as you mentioned 'under a roof' all sub-floor systems need to allow the clearance I mentioned earlier to comply with the Building Code of Australia. I don't write the Code and certainly can't imagine where or how they arrive at some of their stated regulations, but I would think it would be up to a forum such as this try and offer as much support to the regulations as possible 
One thing a bit off track, which may offer a perspective of where I'm coming from in all this - at times when doing odd projects in wood (the latest being a staircase I volunteered for and received fantastic advise via the routing forum...) I use reclaimed timber quite a lot and am often faced with cupped boards. To 'flatten' the cupping I simply lay the timber cup down on the concrete floor of my workshop and leave it overnight. I think this flattens the board due to the amount of moisture held by the floor allowing local expansion on the cupped surface, thus reversing the cup. (also incidently this slab is under a roof) 
So the prospect of permanently attaching a sub-floor system to a slab causes a little concern. 
I am currently studying a diploma in architectural design - drafting. When I thought of starting this course, I didn't fully appreciate just what things I would need to find out, but it's been fun so far. 
Thanks Allan.

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