Material for Floor joist

Villan11

Hi All,

I am in the process of a part DIY project to convert my garage into an office. One part I’ll do myself is the subfloor. The garage has a concrete floor which I need to fix joists to and then sheet with 18mm T&G OSB. I was thinking of using these fence posts as the joists as I would like to use 75mm square timbers. I’ll fix them with 150mm concrete screws.

https://www.murdockbuildersmerchants.ie/timber-square-post-75mm-x-75mm

I was just wondering if there’s any reason why I shouldn’t use this as floor joists??

Also, I was gonna put down a sheet of DPC between the concrete and timber just for precaution, any thoughts??

Thanks in advance!

Cian

10-10-20

Hi! These appear to be treated posts. Some people may have an issue with using treated posts. Also, if they are described as "suitable for DIY..." then that can imply that the posts aren't regularised and hence you may encounter some variation in the sizing. Maybe that's less of an issue these days than in the past, but if you will be using them for a floor then you'll need to screen the sizes. So they will probably be fine, just be cautious.

listermint

If these are to be suspended then no they aren't suitable. They won't have required strength in width.

mickdw

If you are going out to buy timber for the job, that wouldn't be a good choice.

If you have them knocking around, you could use them but will need to be supported along their length.

e87228722

not a good idea, i think 50x50mm is what you will need.However if the floor has damp course under a floating floor with insulation may be a good option in my view if there is no dampness.

Villan11

Thanks all. Will go for something else.

The floor won’t be suspended, these timbers will sit straight on the existing garage concrete floor, possibly with a layer of DPC between them.

Am going to buy timbers so will get whatever’s needed. Only requirement is that I need it to be 75mm to bring floor up enough.

Any suggestions of what to go for??

listermint

https://www.boards.ie/discussion/comment/121151207#Comment_121151207

They're fine for that then. Yes on dpc also put in foil too. And insulation between the timbers. Xtraterm or something.

Villan11

So the posts are ok as they are sitting directly on concrete and not spanning any unsupported space???

Yes, am going to insulate between the timbers. Was going to use this, the 50mm thick one.

https://www.murdockbuildersmerchants.ie/kingspan-tf70tp10-high-performance-floor-insulation-2400mm-x-1200mm

Am also planning on using spray foam to seal / insulate all edges.

Edit: Sorry, should also have included that if I’m using these timbers it would only be if they are uniform in size.

Am very open to any suggestions on alternatives tho

mickdw

If you need the 75mm height, go with 75 x 44

That's 3 x 2 in old money.

The extra width in the square posts you propose is wasted really. I'd imagine 75 x 75 would be alot more expensive too.

Lumen

I don't understand why you want to use timber. Just use continuous insulation that's appropriate for the application.

Is the concrete level?

Villan11

Mainly because I have some electrical junction boxes, pipes and cables running on the floor that I also want to protect.

Villan11

Thanks,

will just get PAO 3 x 2 and make sure all lengths are uniform. Was only using the 3 x 3 for extra stability, the price difference was only relatively small.