Hi All

Sorry if this is a silly question but I'm trying to work out the detailing for a warm roof design (all insulation above tafters) where the rafters overhang 500mm outside the outer wall.

My design has both 4 and 25 degree roofs which are to be clad in a standing seam roof and both roofs are essentially the ceilings in the single storey element and room in roof element.

I appreciate the benefits of a true warm roof and came across a system called VIEOROOF when looking at the Tata (Corus) Colorcoat Urban steel standing seam roof.

This system has circa 240mm of Rocslab mounted above the rafters using special fixings that hold the insulation in place and provide a mounting for the metal sheet roof.

The construction would be as per attached image (1) with standard 220mm rafters resting on the roof plate of a 150mm full fill cavity wall construction. The rafters extend 500mm beyond the outer wall to create the overhang.

My questions are

1. How to make this detail airtight? I presume I would have to plug the gap between the rafters as the exit the inner leaf? And then rely on the plasterboard/skim below the rafters?

2. If the insulation is above the rafters, surely heat wil rise into to the void between the rafters and espace out through the sides? If I plugged the ends with enough insuation would this work?

3. Would I be better off having a hybrid warm roof with insulation between the rafters and then making up the balance above? Would this help with either of the issues above?

I’m going to attach a couple of images to help explain the above better, but please say if its not clear
Thanks

Hi Crofty,

I had a similar set of issues with my roof (going in as we speak).

Here's a brief summary of my construction:

The pitch of my roof is 30deg
The walls are 100mm block, 200mm glass wool, 100mm block.

I am using 200mm deep rafters sitting on the inner wall plate but not extending across the cavity.
Over the top of the rafters is an 18mm OSB (grade 3, exterior) sheet layer. This is primarily for structural stability. This extends over the top of the cavity to link to a bevelled wall plate on the top of the outside blocks. It then flies past to provide the overhang. This achieves several things: It ties the tops over the block wall together for stability, it creates the eaves overhang and it makes air sealing of both the inner and outer joint easier.
I then have a layer of breather membrane (Tyvek or similar)
Then a layer of 50mm of Steico Universal wood wool over the top. This also acts as a water-proof layer.
Where I have Veluxes or other roof penetrations I am putting a strip of Tyvek from the ridge down to the window/chimney etc. This is simply because it makes sealing easier as most of the products assume you have Tyvek as top layer.
Then counter battens, battens and tiles.

I'm insulating between the rafters with glass wool (not sure which one yet, see later). 2x2 noggins between rafters will help hold the insulation in place. The bottom of the noggins will be flush with the rafter bottoms and installed as the insulation goes up.

A VCL will then be stapled and taped to the underside of the rafters. At this point I am intending to fix plasterboard (not foil backed) to the underside of the rafters (with the noggins catching the edges of the boards).

You could put a service void in at this point, in which case the 2x2 noggins could probably be omitted.

At the eaves and verge, there isn't any reason to carry the over rafter insulation much over the inner wall leaf, and certainly not over the overhang. So I am stopping the over rafter insulation above the inner wall. to make up, short deep counter battens are fitted. This stiffens the OSB overhang so that it is strong enough, the top edge of the stub counter battens also helps support the over rafter insulation and the counter battens that fit on top against "sliding" down the roof (i.e. shear of the connectors)

Things to watch out for:

At the rafter feet you almost certainly will need noggins as timber is far from straight and un-twisted. The noggins will also help stabilise the rafters. This means that the insulation will be interrupted at this point. I compromised by leaving a small gap at the top of the noggin, with some wool stuffed in. This reduces the thermal bridging.

Fitting veluxes with over rafter insulation can be a little bit more tricky but I think we have got it cracked, I'll let you know.

Timbers are nowhere close to straight, you must not assume (as I did) that if the ends are at 600mm centers the middle bits will be at 600mm centers!

With that in mind, I would urge you to consider very carefully the 4degree slope on the flat roof (assuming it's not a typo). With the variation in building tolerances, especially timber, holding a slope of 4 degrees will be difficult, resulting in low spots and ponding

EDIT: I forgot to mention, the room-in-roof roll has been discontinued. Knapf have not yet updated any of their literature or even their sales people, but if you try to order it you can't.

Do you have goggle sketchup? If not download it for free from google, it's really great for early stage planning. If you do, whisper me your email and I'll send a model of the roof construction over for you to fly through.

Good luck

Hi VH
What is the purpose of the 25x25 battens inlaid into the EPS (top layer I presume to fix the ply decking, but the bottom layer?)
How do you cut the EPS for the 25x25 inlaid battens, then how do you fix the battens into the EPS?
Peter

OK I thought that would be the reason, there remains the question as to how do you fix the battens once the slot has been cut, or is it just a friction hold which would seem a bit tenuous to hold ply decking etc.
Peter

Hi VH Thanks for that, its clear now.
On the separate note! I do not build houses, but my sons partner is a newly qualified architect and works in Budapest and her English is better than my Hungarian.
Peter

Posted By: spoonandfork So with your first layer of EPS all your battens line up directly with the rafters below? And your picture above shows the first layer ready for the next layer (which will have perpendicular battens)?

That's the way we did it.

Yes, you're correct Spoon, I misread your question, here's the detail we used.

Another warm-roof question which I don't think is answered anywhere above (sorry if I missed it):

The builders have put a warm flat roof over our loft room, with 90mm PIR above the rafters. We are adding 50mm PIR between rafters (making it a 'hybrid' warm roof). What do we do at the last rafter each side?

(1) bring the insulation down the face of the rafter to join up with the wall insulation?
(2) leave the rafter bare to keep it 'warm' (at the expense of some heat loss through the thermal bridge)

To complicate things, neighbours on one side have a loft room (with a weathered joint between the two flat rooves). On that side, it's hard to say what conditions the last rafter will be exposed to.

On the other side there is a dormer cheek with tiles, membrane, and OSB so the last rafter will be relatively cold and well ventilated.

Posted By: Viking HouseYes, you're correct Spoon, I misread your question, here's the detail we used.

http:///forum114/extensions/InlineImages/image.php?AttachmentID=2017" alt="Ext Ins TF Roof.jpg" >

are you tempted to add an airtight layer above the EPS? tape the joints between the sheets / another barrier?

Based on the above definitions, anything with a cavity between the insulation layer and the outer skin would not be classed as a warm roof?

We are planning a new warm roof (I think/hope) for our renovation in New Zealand as follows:
1:20 fall
metal tray roofing (Dimondek 400 tray, width 200mm, 45mm high rib, sheets clip together -2 trays/sheet)
roof batten
counter batten (structurally fixed to rafters)
100mm celotex with taped joints (2x50mm, offset joints, NZ distributor only imports 50mm sheets)
15mm ply sheathing (bracing plus support for celotex)
exposed rafters @ 1200 spacing

We are planing to build "clip on" eaves, so the wall and roof insulation (both celotex) will be able to be sealed together at the roof/wall junction with no penetrations through the insul boards (except for fixings)

Does this qualify as a warm roof?

Super impressed with your details Seamus. With the flat roof detail below do you slope the whole structure/ceiling to form any fall required? And what do you do if you need to connect solar thermal or PV panels on top of this construction? Any recommendations on membrane material?

Posted By: Viking HouseThis is our flat roof solution! The roof is airtightened from the outside using silicone between the rafter and the Ply.

Posted By: Mike GeorgeA warm roof is simply a roof with ALL of the insulation outboard of the STRUCTURAL element. [In your case, the rafters]. There can be many variations to Timber's example above, including the inclusion or omission of vapour control layers/barriers and breather membranes. The choosing of these dependent upon the other materials physical properties. Not least their vapour permeability.

So what do you call a roof with some insulation outboard of the rafters and sarking and some inside the rafters sarking?

Posted By: borpinSo what do you call a roof with some insulation outboard of the rafters and sarking and some inside the rafters sarking?

This is what some call a hybrid warm roof.

Although Mike's definition is the text book one, an area for potential misunderstanding is with the way the roof is ventilated. A roofer will often refer to any pitched roof where the ventilation goes below the sarking membrane/board as a cold roof & any pitched roof where the ventilation goes above the sarking membrane/board as a warm roof.

So I've taken to calling my chosen build-up "a hybrid warm roof with ventilation to batten cavity".

Tiles;
Tile battens;
Counter battens (ventilation to eaves & ridge);
Breather membrane (taped);
22mm T&G woodfibre sarking board;
400mm mineral wool between I beam rafters;
18mm T&G OSB3;
Vapour control layer (taped);
Plasterboard.

Posted By: borpinpage 5 of the PDF shows 18mm sarking but would this work with OSB?

This looks like the standard Scottish way of laying slates where the norm is to leave 3mm gaps between square edge sarking boards. Large sheet sarking materials are normally specifically excluded from the BBA certificates of breathable membranes which allow this approach.

Even with cut timber you may have issues with a building control officer in England/Wales. So if you need the OSB for structural strength then either place it below the rafters (where it will be kept warm) or put breather membrane & counter battens on top to form a ventilated drainage space between slates & breather membrane/OSB.

David

What about squirty foaming them together?

Apparently there is a low expansion squirty foam. I think you need some expansion as a glue, IMO, would not fill the gaps enough. Glue tends to work when two surfaces come very tight together and I am not sure if insulation fits this description. This is only a guess, I will be gaining practical knowledge this spring (hopefully).