Outhouse shed roofs

In this situation I would personally lean towards the 18mm OSB option. Although the existing concrete roof provides a solid base, the additional cost of upgrading from 11mm to 18mm OSB is relatively small in the overall context of the project. The thicker board provides greater rigidity, reduces the potential for movement, and gives the GRP system a more robust substrate to bond to. GRP manufacturers and installers can have differing specifications, but as contractors we always follow the relevant system manufacturer's requirements and current building standards rather than trying to save a few pounds on materials. The reality is that once the roof is finished and artificial grass or decking is installed above it, any future failure becomes far more expensive and disruptive to rectify. For the relatively modest additional cost involved, 18mm OSB offers extra peace of mind and is generally the route I would take if it were my own property. Sometimes spending a little more during installation helps you sleep better knowing you've gone the extra mile rather than wondering later whether you should have done. Before proceeding, I would also recommend checking the condition of the existing concrete roof and ensuring any trapped moisture issues are addressed before the new system is installed. If you're considering upgrading the roof further, it may also be worth looking at vapour control layers and insulation options at the same time, as getting the roof build-up right can make a significant difference to long-term performance and condensation control. I'd be glad to offer further advice if you need any additional pointers on the roof build-up, vapour control, insulation details, or the best way to approach the project. Best of luck with it! regards Colin Prembuild

You maybe better with liquid rubber straight onto the concrete, putting any type of boarding down straight onto the concrete will cause the boarding to rot relatively quickly. Good luck Alex

These structures are renowned for cracking over time and generally have large aggregate stones making a ply deck a necessity, I gather if your putting grass on (an previously had decking)its used as some sort of seating area , if so I'd avoid rubber due to the traffic it'll take. You'd get away with the 11mm ply (fixed properly) if you use a 2 layer (4mm each) system, but as another answer said , for the extra couple of pounds a board you may as well future proof it . And if you plan on having any seats possibly opt for 3 layers so the feet don't dig in , hope this helps