All our flat top trellis is treated and made from 38mm x 16mm timber battens. To add further strength to the trellis we also double the battens on each end, this clamps the ends together and when slotted into a concrete post it fills the gap helping to prevent the trellis from flapping between the concrete post giving a much firmer fit. 
We recommend staining all timber products with an oil or spirit base treatment after 8 – 12 months of erecting your fence this continues to help protect all timbers and keeps your fence looking at its best.