Hi Dave,

Nothing in the building restrictions that I can see regarding grade of alloy. The following is the only applicable part I can see that effects beam materials.

"The main beam and rear beam shall each be of straight aluminium alloy of constant section along its length. Holes may be drilled in the beams for fastenings only. Holes remaining after the removal of redundant fittings are permitted.
6.2
The main and rear beam shall be; a rectangular hollow section 50 mm plus or minus 1 mm by 39 mm plus or minus 1 mm by minimum 3 mm wall. Beam edges may be rounded to a maximum of 2 mm radius."

From my experience this size only leaves Mossies with a choice of a standard section from Capral, available at most alloy fabricators/sellers around the country. Though it does come in a choice? of internal corners rounded or not, rounded internal corners is preferrable, in particular if you are not bolting beams right through.

Have you found a alternative in this size?