Any suggestions for soffit detail please?

7 Jun 2015
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United Kingdom
My sister has the following existing fascia and soffit detail, and we're looking to cap over /under:-

The loft is vented with a continuous run of 45mm wide slots along the existing wooded fascia, so I think that we have to keep ventilation. Also notice that the existing wooden soffit board is rebated into the fascia creating an 18mm down step.

The geometry seems to support this idea:-

250mm wide vented soffit board from wall to front of fascia. To be supported by a J channel at the wall, which is in turn screwed to a new treated wooden 19 x 38 batten, which is itself screwed to the underside of the existing soffit. So the batten would be at the intersection of the existing soffit and brickwork. 18mm and 19mm are kinda equal and this puts the new soffit board virtually horizontal. I'd be tightly trapping the soffit board between the new fascia capping and the wall as 250mm = 250mm. Is this a problem as regards longitudinal expansion?

Are there any simpler solutions? Most houses around her have the hollow soffit board fitted transversely, and just either ignore ventilation or put a few 70mm circular vents every couple of metres. (Aren't these meant to be every ~400mm as per regulations?)

I'd thought of the vented hollow board mounted transversely, but I've not seen that done in about 20 houses I've looked at. People probably thought I was going to burglar them. Would it be weird?
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i normally cut bottom 18mm of old fascia so new soffit comes to face of fascia then once new fascia put on the lip covers the new soffit joint if over capping
also when putting vented soffit over vented soffit the slots never line up can you not just remove all and start a fresh and use vented soffit or over vent
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@^woody^ ,

I love it thanks, but just a quickie... Would you glue the blue vent piece to the soffit board? This is what my sister has now, with a plastic vent and wooden soffit board. And she's experiencing soffit droop. As the wood's only clipped into the vent channel, over time it's rotated down into a vee shape. Does modern stuff do this too, hence the glue idea? Do you not like the soffit with the moulded in vents?

PS. How much to hire that guy in the corner? I've never seen a happy builder.
The vent strip is normally a tight fit and further wedged by the soffit board. But you could add some silicone or external no nails type adhesive between the new and existing soffit. (PS the gap is not that big in reality).

Alternatively the soffit vent strip is wedged behind the fascia lip and held/wedged by the existing fascia/ soffit.

My happy chappy is extremely busy at the moment. Sorry.
And , would you not bother with the 19 x 38 batten between the J channel and existing soffit? You'd just have a slightly sloping new soffit?
I would screw a baton to the old wooden soffit to even thing up and then screw the J trim to the baton, slide vented soffit into J trim and screw to underside of fascia. New fascia clamps new soffit and hides fixings.

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