Advice for warm roof design for new build.

[dabba]

Hoping I can get some advice regarding a warm roof on a new build detached bungalow, two gables one hip, no dormers just velux type openings,  planning consent has been granted.

Just getting building regs drawings sorted and I'm thinking a warm roof design may be best? sadly designer has not had much experience with warm installation so very limted information.

Would very much appreciate some help and guidance please.            

[ProDave]

Is it room in roof?  the velux windows suggest it will be.

 

I thoroughly recommend what we did, and it is not difficult.

 

Make it a cut roof supported on a big ridge beam. (the ridge beam will probably need intermediate support from internal walls)  Then all the rafters span from the ridge beam to the wall plate, putting no outward load on the walls and giving unhindered use of all the roof space with no need for collar ties etc.

 

You simply design the height of the ridge beam and rafters to allow for fitting insulation over the top of the rafters so by the time it is then battened and counter battened and tiled, your finished roof height matches what the planning drawings say.

 

We used 100mm wood fibre above the rafters, and then 200mm Frametherm in between the rafters making it a hybrid roof.  the air tight layer is immediately inside the rafters before plasterboarding (with or without a service void)

 

Very easy, very simple, and performs very well, and very easy to get the air tightness detail done well.

 

I will edit in a minute to add some photos.........

 

 

 

 

[dabba]

Many thanks Dave, I should have added I've been advised to use attic trusses as I need to keep the roof height to a minimum due to planning constraints.

[dabba]

PS

Yes two small rooms and shower room in the roof.

If the two gable ends were opposite it would be easy to use steel for the ridge sadly the hip elevation is opposite one gable and the other gable is at right angles to the other.

[ProDave]

1 hour ago, dabba said:

Many thanks Dave, I should have added I've been advised to use attic trusses as I need to keep the roof height to a minimum due to planning constraints.

That is my point.  It matters not whether you put the insulation on the outside, warm roof, or put the insulation on the inside, cold roof.  The thickness of the total roof buildup will be the same.  So as long as you design it correct, both give you exactly the same finished roof height and internal head height.  A fact some people seem unable to grasp.

 

Roof trusses are the architects cop out.  They stop working when you get to the hips so that part would need to be cut, and they have restrictions on the internal space.  But are equally suitable for a warm roof, you just build them a little lower so that when the external insulation is on the finished roof height is correct.  You don't lose head room inside because no need to put insulation inside like a cold roof.

 

Steel could do the ridge beam and the 2 steel legs to support the hip end of the ridge beam.

[Iceverge]

Drawings please if you have them and I'll try to do a sketch.  Include any height restrictions too. 

 

 

A cut roof is probably better for a warm loft but not always. 

 

You can have a WARM LOFT with

 

1. A COLD ROOF. ( Insulation between and below rafter. )

 

2. A HYBIRD ROOF. (insulation between and below rafters. )

 

3 A WARM ROOF ( insulation only below the rafters)

 

 

 

 

 

 

 

 

 

 

 

Edited April 1, 2023 by Iceverge

[ProDave]

17 minutes ago, Iceverge said:

1. A COLD ROOF. ( Insulation between and below rafter. )

 

2. A HYBIRD ROOF. (insulation between and ABOVE rafters. )

 

3 A WARM ROOF ( insulation only ABOVE the rafters)

Have you got a little confused there?  My alterations in bold.

[Dave Jones]

Pretty misleading

 

a warm roof is sealed on the cold side to make it vapour tight. You also have to seal the warm side to stop it filling with water and rotting. Not many do this correctly.

 

cold roof which is what your after , insulation between and below the trusses with the top open to air , a gap and then the roof tiles .

[ProDave]

27 minutes ago, Dave Jones said:

a warm roof is sealed on the cold side to make it vapour tight. You also have to seal the warm side to stop it filling with water and rotting. Not many do this correctly.

A breathable membrane is normally used to stop actual water getting through but allow water vapour out.

 

And you should do a proper condensation analysis.

[Iceverge]

1 hour ago, ProDave said:

Have you got a little confused there?  My alterations in bold.

 

Yes thanks. 

 

 

 

[Thorfun]

I would suggest the OP do as @ProDave has suggested with a cut roof. having been through the process of filling our attic trusses with 195mm glass wool and adding 100mm PIR underneath I can tell you it is a pain on attic trusses! in our vaulted ceiling entrance hall we had a cut roof and it was a doddle not having to cut around trusses. 

 

but might I also add the suggestion of using very deep trusses as filling will blown cellulose instead of mineral wool and rigid insulation combo.

 

if I had my time again I would do a cut roof with blown cellulose for sure!

[JohnMo]

We basically did insulation between rafters, our pozi rafter are 256mm deep and we counter battened below 100mm and then spray foamed.

 

Pozi joist come precut to size, and are a doddle to construct.  Big ridge beam, wall plates and just assemble.

 

Pozi joist company designs the whole thing, structural engineer approved and includes within the structural engineering certificate (for Scottish building warrant).

 

From this

 

To this 250m2 of roof ready to slate about 3 to 4 weeks, team of 3 guys. 

[dabba]

I think to acheive the latest regs I will need insulation over the rafters and also between.

Finding roofing joiners for a cut roof is proving difficult, space is limited on the first floor and i had thought that perhaps attic trusses would provide max space possible.

I'm flexible as to the roof construction so any guidance welcome.

 

[Dave Jones]

You will get more useable room with attic trusses, less labour. Just spec them deep enough 

[Dave Jones]

15 hours ago, JohnMo said:

We basically did insulation between rafters, our pozi rafter are 256mm deep and we counter battened below 100mm and then spray foamed.

 

Pozi joist come precut to size, and are a doddle to construct.  Big ridge beam, wall plates and just assemble.

 

Pozi joist company designs the whole thing, structural engineer approved and includes within the structural engineering certificate (for Scottish building warrant).

 

From this

 

To this 250m2 of roof ready to slate about 3 to 4 weeks, team of 3 guys. 

 

lovely design.

 

With hindsight, id warm deck the whole roof, not faff about in-between rafters, leave them exposed. Deck straight over the top 200/300mm 

 

Would look nice and chunky and could be done in a couple days.

 

 

 

[JohnMo]

6 minutes ago, Dave Jones said:

 

lovely design.

 

With hindsight, id warm deck the whole roof, not faff about in-between rafters, leave them exposed. Deck straight over the top 200/300mm 

 

Would look nice and chunky and could be done in a couple days.

 

 

 

You are correct, I had proposed this to the architect but got quite a lot of push back. It would have quicker and way less work for me.

[Dave Jones]

19 minutes ago, JohnMo said:

You are correct, I had proposed this to the architect but got quite a lot of push back. It would have quicker and way less work for me.

its hard to sometimes to stick to your guns when the so called professionals push back. I learnt that lesson on our first build. Never again, now i listen to them and make the call. Need a build under the belt to feel confident enough.

[Nickfromwales]

Posi's and a bit of additional timber to get you up / around 350-400mm depth, and blown-in cellulose wins the day for me. Just one days work for one guy, chilled out, and staggeringly good performance for both insulation AND sound-deadening.

For the OP, I'd seriously consider getting spray foam in, but only if there's a robust interstitial condensation mitigation plan in place.

 

15 hours ago, JohnMo said:

and then spray foamed.

Did you cap the open cell layer with a layer of closed cell, or did you go for an internal membrane instead? In talks with one client atm to go down this route ( vs membrane for AT plus obligatory's ) as the roof space is nigh-on impossible to get a man into, let alone cut and fit insulation AND ten cut and tape an AT / vapour membrane. A very bust attic trussed roof indeed, and just looking at the planning for insulating it "manually" was making my brain bleed. Worse than that was the assurance that a builder would just put a 'decent labourer or two' on it, and the results would be dire at best.

[Dave Jones]

whats the u value and m2 price of the cellulose ? 

[Nickfromwales]

1 minute ago, Dave Jones said:

its hard to sometimes to stick to your guns when the so called professionals push back. I learnt that lesson on our first build. Never again, now i listen to them and make the call. Need a build under the belt to feel confident enough.

Yup. I used to look up to these professionals, after 5 years of being a PH M&E, and listening to the majority, I look sideways and down. It would be a breath of fresh air to hear / see these types of prfessions stating what they do / don't do, and what for them to just excel at what they're best at doing, eg designing great houses. And just stop there!!

[Nickfromwales]

Just now, Dave Jones said:

whats the u value and m2 price of the cellulose ? 

almost always POA after a survey. U-value similar to PIR sheet format, but if the spray company try to cut corners and do too much in one go, it ends up like swiss cheese and will have cavernous pockets of air everywhere. Just seen one example of this, what a crock of crap they turned out  Took days to rectify.

[Thorfun]

2 minutes ago, Nickfromwales said:

Worse than that was the assurance that a builder would just put a 'decent labourer or two' on it, and the results would be dire at best.

doing mine myself I thought many times that I'd hate to see the sort of job a builder and their labourers would do (not all of them though I'm sure!). the methodical and detailed work we did cutting PIR, membrane and taping I can guarantee that a builder on a price job wouldn't spend the time to do.

 

I'm very glad I did ours ourselves but I wouldn't do it that way again as it was a horrible and slow job.

[Dave Jones]

1 minute ago, Nickfromwales said:

almost always POA after a survey. U-value similar to PIR sheet format, but if the spray company try to cut corners and do too much in one go, it ends up like swiss cheese and will have cavernous pockets of air everywhere. Just seen one example of this, what a crock of crap they turned out  Took days to rectify.

 

my concern would be if it gets damp or clumps on wall ties etc reducing its U value. Much of an issue ?

[ProDave]

6 minutes ago, Nickfromwales said:

For the OP, I'd seriously consider getting spray foam in, but only if there's a robust interstitial condensation mitigation plan in place.

 

For me, not until surveyors have got over their fear of it.

 

I have just been organising the home report for our old house, and one of the very first questions the surveyor asked was "is there any spray foam insulation in the house"  the implication being it would be a problem if there was.

[Nickfromwales]

2 minutes ago, Thorfun said:

(not all of them though I'm sure!)

Defo. I should have said that. The builders on one of my 2x PH M&E projects in Leicester are absolutely mustard, an absolute breath of fresh air.

The other project not so. After seeing F-up after F-up, after F-up, I simply went to the clients and asked them to sack him. Which they did. We managed to stop the poor work, covered up property damages etc left in his wake at circa £35k.

 

If anyone wants to know which builder to actively avoid in the Lutterworth / Leicester area, PM me. Absolutely some of the worst work / work ethics I've seen in a few decades. Of course, his website looks absolutely stunning, shame they don't know which side of the saw has the cutting edge. It's not very often that I am stuck for words, as were the building control officers, drainage & groundworks engineers and others.