tarp

I need to tarp the 5.245m x 3.05m flat roof of my detached garage but I'm not sure if I should buy a 6m x 5m tarp, https://www.diy.com/departments/groundmaster-250gsm-brown-green-tarpaulin-5m-x-6m-/5051641516196_BQ.prd, giving two 0.3775m and two 0.975m overhangs, or buy a 8m x 4m one...

My flat garage roof has been leaking and lack of funds means I'm having to tarp it. I have a load of old bricks so I was thinking of loose-laying a couple of courses down the centre then putting the tarp over thus forming a tent. The idea is that the tarp will last longer as any rainwater...