What timber for concrete frame work?

[Yak]

I am doing a little patio with some concrete pavers. Pavers will be bigger than what is available off the shelf. 

I'll be building the frame work for it. Similar to the attached photo. 

Now in terms getting it perfectly square if the timber has a bow to it I can straighten that with the frame and screws. But if it has a crook then there is potential that my pavers wont have a flat surface

  

and have a concave or convex shape as when putting the concrete down I will use the frame work to screed. 

Besides visually checking for crook timber, is there a reliable material or alternative suggestion? 

[Noyade]

Hello @Yak 

I've spent the day playing with cupped, bowed and twisted timber - never again.

Personally, I've always used steel for concrete form-work. Possibly overkill - but I also like sump oil painted on the inside to prevent the concrete sticking.

[EricL]

Hello @Yak 

Thanks for sharing your question about concrete form work. If you are after a good finish for the sides of your pavers, I suggest using Ecoply 2400 x 1200 x 17mm F14 Formrite Formply. It's designed specifically for use as concrete formwork and it is overlaid with a hard durable resin-impregnated paper, to provide a smooth concrete finish, and is reusable and durable. 

Please note that the surface is not waterproof, and a release agent must still be used on its surface. I recommend using this as the inner form of your frame in order to get the flawless finish that you're after.

If you need further assistance, please let me know.

Eric