Need help to visualise roof construction

[wozza]

 

Hi All,

 

Would someone be able to visualise or understand my roof construction on my extension - I cant get my head around how the architect has planned for it to be constructed - I get the majority of it but I cant work out how she has planned the insulation etc, this is what she has written on the plan.

 

Proposed roof tiles to match existing to suit roof pitch on 38x25sw treated battens on Tyvek Supra breathable membrane on rafters and ceiling joists as specified on plans on sw wall plate anchored to block inner skin with 30x5 gms straps min 1.0m long and at 2.0m max centres. The first 3 No. rafters and ceiling joists strapped back to wall with bat 30 x 5 x 1200mm gms restraint straps at max. 1800mm centres and supported with 50 x 75mm sw noggins packed tight to wall. All roof timbers to be tanalised. Roof voids to have 150mm quilt insulation laid between joists and 150mm laid above at right angles. Insulation to roof slopes to be min 100mm Celotex between and 40mm below rafters with min 25mm void to underside of roof membrane.

 

Thanks, Wozza.

[Miek]

I would refer to the "as specified on plans" part, a picture tells a thousand words and all that. 

 

 

[Russell griffiths]

It sounds fairly straight forward 

do you have two types of roof, one standard with a cold loft area, the other with a vaulted section. 

 

You really need to show us a section drawing. 

 

In the standard cold loft area you will have 150mm of quilt between the joists and then 150mm laid at right angles across the top of the joists in the loft area. 

 

In your vaulted section you have 100mm of celotex in between the rafters ensureing you allow 25mm gap between the celotex and the membrane, then 40mm of celotex under the rafters. 

 

Does that make more sense or just as confusing. @wozza

 

thinking about it, I bet you have a raised ceiling joist, that’s why you have the two different insulation types. 

Edited February 2, 2019 by Russell griffiths

[wozza]

Russell,

 

I think you may have hit the nail on the head, we are having a wrap around ground floor extension with a vaulted roof and also an extension above the garage, so there are two different types of roof - I had them confused as one roof, doh.

 

I have attached a plan.

 

A couple of  questions, the plans show Celotex on the sloping roof, 100mm between and 40mm below the rafters:

 

1 - How much difference would it make if I upgraded the 100mm to 125mm (would it be worth the cost etc)?

2 - How do people fit downlights in a roof with PIR insulation?

 

Thanks, Wozza

 

 

BR-001 REV A.pdf

Edited February 2, 2019 by wozza

[Russell griffiths]

How deep are your rafters, as it said you must leave 25mm above celotex 

as for upgrading to 125 that’s something only you can decide, you need to weigh up cost versus benefit 

 

roof windows, triple glazed at the very least, fit the best you can afford as this will be a big heat loss area. 

As for fitting it’s no different then any other roof, rafters doubled up on each side,

fit window

fit celotex in between rafters

fit celotex under rafters

batten and plasterboard 

this will leave a slightly deeper looking reveal around the window due to the 40mm celotex and batten,

 

[wozza]

Rafters are drawn as 150mm

 

We have planned for triple glazed Velux

 

Is my diagram below how it should be done? or would it be better to fill the area directly behind the plasterboard?

 

Or is there a better way?

[Russell griffiths]

IMHO

that looks like a really stupid design 

so yo install bloody expensive insulation and then cut a hole in it for a light

DUR. might as well leave the window open 

pull the 100mm celotex down and touch the 40 mm against it, then find a shallow light or increase the batten depth, how is she proposing to provide your airtight layer if your going to cut a sodding great hole in it. 

Sorry that’s rubbish. 

You can have the best insulation in the world, but if it’s leaky and gappy you might as well leave it out. 

[Ferdinand]

19 minutes ago, Russell griffiths said:

so yo install bloody expensive insulation and then cut a hole in it for a light

 

Agree with @Russell griffiths on this point.

 

Surely that will be a large number of "bloody great holes" !

 

(Personally I think that recessed spotlights are the spawn of the devil; I have about 70 installed by the previous owner, and the lecky will charge you One Point for wiring each one.)

 

In my view Occam says use either multispot fittings or the more modern LED panels. Plus perhaps spend the recessed spotlght money on a feature light fitting for the hall, an artwork, or whisky.

 

Ferdinand

 

Edited February 2, 2019 by Ferdinand
Further enlightenment

[wozza]

The diagram was drawn by me - I was trying to see how it would all work  ?

 

So would this be better?

 

 

 

 

 

[Russell griffiths]

You need to add a vapour control layer between the 40mm celotex and the batten

this can be achieved many ways

a vapour control membrane taped on with all joints taped and taped to walls

or tape all joins in 40mm celotex with the aluminium tape and seal to walls. 

 

That second roof makeup is probably a million times better than the first with just a little bit of juggling. 

[wozza]

Thanks Russell.

 

Off to look for Led Panels now.

 

 

 

Edited February 2, 2019 by wozza