Bere Alston School decking

 

I am an engineer with the RN.  We are helping a local school refurbish a wildlife area which was vandalised.  The area includes a pond which has now been relined.  The school would like two decking areas placed over the pond corners.  The decks are each 3.5m x 1.5m.  The problem is the 3.5m length runs over the pond and would be unsupported apart from each end.  My interpretation of the decking regulations stipulate that the max length of 6×2 joists is just over 3m.  Can you advise what size of joist would be required to meet the regulations?  I have considered using steels to span this length if required.  Only one length will be over the pond so the opposite side can be supported along the length.  The joists will run at 400mm centres at 1.5 m length.  I am reluctant to increase the joist height to 8 inches as this would make the required step up to high and increase the overall cost considerably.  Would appreciate some help with this as we have now come to a stop with this question.  We are a group of electrical/mechanical engineers with the RN based in Plymouth helping with projects in the local community.  The school is Bere Alston primary Devon.