Advice on torching on underlay on a flat roof

I am about to replace the flat roof of my workshop. I am using OSB3 boards and using torch-on felt underlay and capping. I am torching on 2mm underlay and whilst it is clear that the capping is torched on with an overlap on each sheet, does the underlay go on with an overlap too or are the sheets butted together? Also I presume that the underlay is laid along the fall in the roof and the capping is laid across the fall. Can anyone clarify?

does the underlay go on with an overlap too

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yes overlap the 2mm underlay as well.

The underlay should be lapped. But DONT lay the felt either other way in a criss cross pattern, this creates more joints where water can penetrate.

Ideally, laying the felt across the roof start with the 2mm, cutting the roll in half (so its 500mm wide) then the next run the full 1000mm.

When you lay the next layer on top of this, start with a full 1000mm, so that the join of the felt underneath, lies directly in the middle of the layer above.

I think ive just done a great job of making that sound far more complicated than it is. Good luck!

http://www.liquidrubber.ca/video/NewVideo.html canadian answer now in UK.

Many thanks for the advice. I will take it on board when I start the job.