What kind of nails for tongue and groove boards in formwork?

[BORNER]

We shall build a casting mold for the first time using raw boards and place these upright, nailing them to the horizontal nail battens we set up. But what kind of nails should be used? I assume it should be some form of brad nail to have as little metal surface as possible against the concrete?

And how do you secure the raw boards on the opposite side? By then, you already have a completed mold side and also rebar in the way, so it might be difficult to nail "from inside"?

Would it be easier if one acquires a brad nailer perhaps?

 

[roli]

You can use any type of nail (screws also work) but the smaller the head, the fewer marks in the finished concrete. Also, be mindful not to drive the nails too deep into the form board, as this will also show in the concrete. You will need to manufacture the opposite form side separately and place it in position after the reinforcement is completed. This side should be the one that will be least visible after casting as it can be difficult to make this one as "neat."

 

[BORNER]

You can use any type of nails (screws also work), but the smaller the head, the less noticeable marks in the finished concrete. Also, be careful not to drive the nails too deep into the form board, as this will also show in the concrete. The opposite form side should be made separately and placed in position after the reinforcement is done. This side should preferably be the least visible one after casting, as it might be difficult to make it look as "nice."

Thanks, @roli! However, we have a bit of a challenging situation as the mold will stand on uneven rock, making it a bit difficult to manufacture the opposite form side separately and then place it. So we were thinking of trying to find a method where we nail up the rough timber for the opposite side plank by plank, to adapt each plank to the rock's unevenness and thus create as tight a mold as possible at the bottom.

Would it be possible to nail "from behind" through the nail rule and sufficiently into the rough timber? We will have form tubes pressing both sides outward, with a threaded rod through it taking the pressure from within, so maybe it's enough just to ensure the rough timber on the opposite side is "held in place"?

 

[roli]

Sure, it can be an option. The important thing is that the board cannot move. Whether the nail or whatever you fasten the board with is attached from the front or back is of less importance. If you attach it from the back, a screw might be easier to place than a nail that can easily move the form board if nailed from the "wrong side."

One option is to create a form side that adapts to the rock, which you then lift away to make the other. Then reinforcement, finally lifting back the first form side to its original place. Exactly how to do it on site depends largely on how what you are making looks, for example, thickness, height, and length influence, so what is right and wrong is hard to say. The only thing that matters is that it should remain in place and withstand the casting pressure during the casting stage and provide an approved final finish when it is deformed.

 

[Snickarkirre]

Nail with 60mm plain nails, that way it will be easy to dismantle the form later. If the doubling side is going to be visible, you can manufacture it on the side and then lift it into place, just sealing underneath the form with joint foam if there isn't a significant height difference. This way, you can make it on the side and skip the rule at the bottom, place the form in position, mark after the rock, remove the form, cut after the marking, and then nail the rule at the bottom. Otherwise, you can angle-screw the board from outside as roli wrote, which is much easier than nailing. The nail/screw is only there to hold the board in place because when you pour in the concrete, it won't move a millimeter due to the form pressure. So you can really use anything, but plain nails are preferable in form construction over galvanized ones or screws because it's easier to dismantle.

 

[BORNER]

Thank you very much @roli and @Snickarkirre for the good advice!