Flat Roof - should Fasica top be same leel as top of OSB ? or should OSB "extend" and fascia go underneath

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Flat roof install, DIYer. I haven't yet put the new OSB on, and when googling, I found some images where the OSB seems to extend out, and the fascia board is under the OSB, and others where the top of the fascia board (uPVC 18mm thick - from eurocell) is level with the upper surface of the OSB. Is there a recommended way ? Or is either "ok" ? (FYI - will ultimately be a rubber EDPM roof going on with appropriate drip battens, and endge trims etc
 
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It Depends on how you deal with the roof overhang for water to discharge into the gutter.

if the roof OSB overhangs on the gutter side, then it should run over the fascia on all other sides.

it kind of depends on the trims you have
 
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You need a 50mm batten to act as a drip into the gutter. This is fixed to the fascia and the deck board goes over both to avoid any joints which can lead to slightly different levels, collect water or cause a weak point for the covering.
 
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Like this, but I tend to have a 50mm drip to take the water more into the gutter, which helps longer term

1653597775600.png


*not my drawing, just to illustrate the principle
 
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Like this, but I tend to have a 50mm drip to take the water more into the gutter, which helps longer term

View attachment 270717

*not my drawing, just to illustrate the principle
Is that a hybrid roof?

Incidentally, the batten needs to supersede the fascia i.e. fix a (deeper) drip batten then butt the fascia to the underside of the drip batten - just so you know. And that overhang is a wee bit lean, especially on windy days.
 
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And for the (gently) sloping side [which is there to cause a slop for run-off], is it norma to try and cut the Fascia so the bottom of the fasica is "true" (parrallel with the ground) whilst top of fasica "follows" the slope ? [uPVC fascia]
 
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And for the (gently) sloping side [which is there to cause a slop for run-off], is it norma to try and cut the Fascia so the bottom of the fasica is "true" (parrallel with the ground) whilst top of fasica "follows" the slope ? [uPVC fascia]
Always fit the fascia level and trim the top to suit, if need be. Falls can be accommodated within the drip flashings and cocking edges (felt flatties) leaving the fascias level.
 

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