Plastering loft conversion

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Matt_North

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Hi All,

I am revamping my lost conversion and I am after some advice! I am new to the forum and do my own DIY.

Bought the property 3 year ago and the loft conversion was done by previous owners in 1993. There is stair access and a dormer and the structure seems very sound.

The plaster has some cracks where the plasterboards meet and this is pretty much throughout the loft space.there are up to 10mm gaps in the corners of the room and plaster has simply given way meaning it looks shocking and I’m going to reboard the lot.

Ive started stripping the plaster board off (back to joists and rafters) and here’s where I’m not sure what’s going on:

Plasterboard is 9mm thick
Looks like a pre coat of grey plaster on top of the plasterboard between 6-10mm thick!
Then some pink skim about 2mm thick

Why is there such a thick pre coat and why is this needed? I thought you could plaster with pink skim straight on top of plasterboard? There’s no chance of plastering on top of this as the existing skim is way too thick, unreliable and I’d rather have piece of mind knowing done a decent job with fresh boarding and skim.

To top it off the previous owners did not white wash the plaster so the paints is brittle and comes off in big shards.

Out of curiosity I’d like to know what the thick grey plaster is and why it was needed? And if I’m replacing the boards, is there a particular board I should buy?

For added info, there is a velux window either side of the room, there are vents in the roof and there is enough space between the rafters and the Battons for airflow the circulate.

Any advice would be mich appreciated!
Cheers

Matt
 
first up... building regs

youve removed over 25% of a "thermal element" good (y) it needed doing


means youve gotta bring the insulating properties of it (the u value) up to current regs for a renovation (not a new build)

youre gonna need to insulate between the rafters with a polyeurethane insulation board such as cellotex or kingspan, as much as you can fit in but still allowing a 25mm air gap between it and the undertile felt

then got over the top of that with 25mm cellotex (or equivalent) covering the rafters..

then either tape the joins up with silver foil tape, or cover the whole lot with polythene or board it with 12mm foil back board... using 60mm drywall screws into the rafters..
same all over...

personally id go the foil tape route... cheaper than foil backed board and easier and better than polythene
 
Last edited:
Chris W said:
first up... building regs

youve removed over 25% of a "thermal element" good (y) it needed doing


means youve gotta bring the insulating properties of it (the u value) up to current regs for a renovation (not a new build)

youre gonna need to insulate between the rafters with a polyeurethane insulation board such as cellotex or kingspan, as much as you can fit in but still allowing a 25mm air gap between it and the undertile felt

then got over the top of that with 25mm cellotex (or equivalent) covering the rafters..

then either tape the joins up with silver foil tape, or cover the whole lot with polythene or board it with 12mm foil back board... using 60mm drywall screws into the rafters..
same all over...

personally id go the foil tape route... cheaper than foil backed board and easier and better than polythene
Click to expand...
Thanks for the response Chris

There are battons which have been nailed to the rafters (bringing the ceiling forward) which has created sufficient room between the undertile and the plasterboard... can I just put thick cellotex between the rafters then go over with some 12mm foil backed board? Why do you say cellotex between the rafters and on top of the rafters too?

Does it need to be foil backed board of the cellotex if foil backed?

Thanks again!
 
its called a thermal bridge. it stops heat travelling up the rafters. part of the regs. then the foil tape seals the gaps up, this is a vapour barrier. what that does is stop moisture from the INSIDE travelling through porous plasterboard and ending up in the space where the timber rafters are. What youve created is known as a "warm roof", however you should really have ventilation from eaves up to ridge with vented ridges but thats another job, another day
 
weed.jpg
 
if you need a full breakdown check out "planning portal" look for "approved documents", look up document L1b and find "renovation of a thermal element"
 
Bricktop said:
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Click to expand...
seen the spec? hed have to have his roof off, cross batten it and re roof...
theyve been trying to get that stuff past building control for years... 3 layers of the stuff on a newbuild might just do it but ive never seen a building inspector recomend it
 
Chris W said:
if you need a full breakdown check out "planning portal" look for "approved documents", look up document L1b and find "renovation of a thermal element"
Click to expand...
Thanks Chris will check this out.

the loft space isn’t exactly huge but it’s ideal for an office and a small bedroom/games room. The problem is I dont want to lose any more height in the room.

can I remove the battons from the original rafters, put cellotex between the original rafters (rather than the added on battons) then follow the steps you recommended? I.e. cover again with cellotex and board the lot? I’m pretty sure there will be 25mm+ of circulation between cellotex and the undertile

assuming it’s okay to skim straight on top of plasterboard once taken steps to insulate in line with regs?
 
im trying to think of a reason theyd batten the underside and not use the extra depth for cellotex in the first place tbh...
could be because the old rafters were shot to bits with woodworm or something worse.. dry rot maybe?
without seeing it i cant give you a definitive answer..
what i can say is technically, by law, youre supposed to follow the regs.. BUT
building control cant make you pull the roof off just to gain the extra insulation value or anything similar..
lets pretend these extra battens didnt exist and the rafters were sound and a building control inspector rocked up...
a good un would let you fill between the rafters with as much as you can get in and 25 over..
a jobsworth would make you stick 50 over instead of 25...
the rule of thumb is "as long as you dont make it any worse and do what is reasonably practicable to recover the fuel costs over a 2 year period", or maybe 5 years i forget
if it was my house id say nowt to nobody, stick what you can in between and go 25 over

if you really wanna nick an extra inch pull a batten off the worst looking rafter and have a look, send a 30mm screw in and try pull it out with a claw hammer... if it feels like it aint coming out with a 3 foot crowbar youll prolly be ok

on a newbuild youre looking at 150mm cellotex at least then 25 over but they would be attic trusses 7" deep
last flat roof extension i did about 3 years back had 225 in... but thats flat as opposed to pitched..
if youve only got 4" rafters youre only gonna get 80mm in between which to be fair youd have got away with prolly 10 years ago..
theyve been upping the requirements since 2000 due to something called the kyoto agreement..
upshot was they supposed to bring emmisions levels down to pre 1986 levels by 2010..
we are the ONLY country to stick to it apparently... look at the usa, china etc...
gas prices only gonna go up...
cellotex saves you an absolute fortune in the long run it used to be dirt cheap till 2000, wish id bought shares in the stuff...


either way, it makes more sense to get as much in there as humanly possible and dont wear high heels
dont fall for the false economy that many people do..
 
p.s. yeh once its boarded just skim it, maybe you could look into the superquilt as an addition or replacement for the 25 over provided building control arent all over it but it wont do anything for the thermal bridging once its squashed againt the rafter...
again... more insulation is just better but its up to you mate, ill bet half the plasterers on here didnt know about the 25% thing so the general public isnt expected to and no ones gonna see what youre doing anyway unless you stick a big flag up saying so
how old is the original building/roof? if its 100 year old them battens are probably there for a good reason...
 
something else you could do is gen up on how much heat loss a given thickness of cellotex gives you... theres u value calculators out there on the net by cellotex and kingspan, you type in what you got and it tells you how much you need for a given u value..
you could get really architect on it and work out exactly how much it would save you a year by converting the wm2k to btu and then again to kilowatts of gas etc...
youd be surprised just how much difference just 100mm makes... single skin garage conversion done right needs a lil 600x600 single rad to keep it toasty as opposed to a massive 2400 double...
 
red
Chris W said:
p.s. yeh once its boarded just skim it, maybe you could look into the superquilt as an addition or replacement for the 25 over provided building control arent all over it but it wont do anything for the thermal bridging once its squashed againt the rafter...
again... more insulation is just better but its up to you mate, ill bet half the plasterers on here didnt know about the 25% thing so the general public isnt expected to and no ones gonna see what youre doing anyway unless you stick a big flag up saying so
how old is the original building/roof? if its 100 year old them battens are probably there for a good reason...
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cheers Chris

the house is approx 100 year old

had a look at the original rafters and they aren’t shot but the new rafters/battons that have been bolted to the side of them are are much better nick and look more sound.

I can get 100mm between the rafters leaving 50mm airflow. If go straight over the rafters with foil backed board, will this be detrimental to my house? Or will it just not be as well insulated?

the insulation there at the minute May aswel be non existent....the fibre glass is missing in places, all squashed in some places and the board is standard! I’m pretty sure I’m going to gain on thermal performance whatever steps I take

are two vents at the front of the house enough to keep the roof insulated?

appreciate the advice mate
 
you only need a 25mm air gap so if you can get 125mm in the extra 25mm will help a lot
you can board straight over but just be aware its not to regs but nothing to worry about really
dunno what you mean by 2 vents at front of house.. by ventilated i mean airflow from behind the gutter up to the ridge and chances are you wont have a vented ridge.. you might not have vented eaves ...
1588610174398.png

these are over fascia vents...



1588610260715.png


this is another way to save you having the first row of tiles up, felt support tray and over fascia vents installed..

the reason for the ventilation is to eliminate the build up of moisture within the roof structure thereby stopping your rafters from rotting...

its another job another day...


just pull all the old fibreglass out and get rid, crack on with the insulating, boarding and skimming and if you can when you get chance tackle the ventilation problem
 
Chris W said:
you only need a 25mm air gap so if you can get 125mm in the extra 25mm will help a lot
you can board straight over but just be aware its not to regs but nothing to worry about really
dunno what you mean by 2 vents at front of house.. by ventilated i mean airflow from behind the gutter up to the ridge and chances are you wont have a vented ridge.. you might not have vented eaves ...
View attachment 48760
these are over fascia vents...



View attachment 48761

this is another way to save you having the first row of tiles up, felt support tray and over fascia vents installed..

the reason for the ventilation is to eliminate the build up of moisture within the roof structure thereby stopping your rafters from rotting...

its another job another day...


just pull all the old fibreglass out and get rid, crack on with the insulating, boarding and skimming and if you can when you get chance tackle the ventilation problem
Click to expand...
Nice one cheers mate. I have two tile roof vents which sit to the left and right on one side of the roof. I’ve pretty much stripped all the plasterboard and fibreglass out and I can see the two vents let a load of airflow in between the rafter.

will size/thickness of plasterboard make any difference to thermal performance?9mm/12.5mm?
 
mate youre starting to sound like you squeak when you walk :rolleyes: The only time i use 9.5mm is to overboard an existing ceiling.
next to nothing is your answer to the thermal difference between 9,5 and 12mm, I cant see how much difference 2.5mm is going to make to the height in practical terms..
if you want to save even more money just buy a roll of cheap polythene and pin that over the rafters before using standard 9.5mm plasterboard. Itll do the same thing as using foil backed board but wont be as strong as using 12.5mm.
Each roof vent will ventilate the gap between 2 rafters. nowhere else. So no, theres isnt "plenty of ventilation"

you came on asking for advice, and you got it. Very good advice at that but it seems what you actually wanted was confirmation to do what you wanted to do in the first place. What you are going to do is on contravention of building regs, which are there for a reason. Youll end up paying out more to heat your house which will burn more fossil fuel doing so. Thats what the regs are for.

I hope when you sell your house you dont tell people youve just had your loft conversion redone and its now all cosy and up to regs cos it just wont be. Plenty of people have tried and got away with it in the past, in fact I had a bloke try it on with a property developer friend of mine recently and all it took was for me to pop out a non fire regulation downlight from the SHOWER area to find there was NO insulation in the roof void

You crack on mate, do what you want, its you thats liable, youre responsible for it in the eyes of the law.

I once heard mohammed was a tight b'stard and thats why Im not religious...
 
Anyone fancy taking a wild guess at the name of the bloke who tried to have my property developer friend over? its a very popular name....
 
Chris W said:
:rolleyes: yep thats him
Click to expand...
I missed most of this thread Chris
Was told to cross batten by suppliers of bubble / multi quilt suppliers when I questioned thermal bridging , a problem with loft height situation , any tips
 
yeh, read the thread bof... cross batten your roof if thats what supplier said but obviousy thats gonna cost you a complete re roof on top of the simple insulate and board job you could do from underneath. cross batten it from underneath and you lose the extra height you could have had by re roofing AND the extra insulation 25mm of cellotex will give you as opposed to squashing a bit of bubble wrap tin foil sandwich between 2 bits of wood. As i said Ive yet to see this done on a job that actually conforms.. even the sips houses I got booked soon as this crisis is over dont use the stuff and youd think they would know what the crack is?
 
Chris W said:
mate youre starting to sound like you squeak when you walk :rolleyes: The only time i use 9.5mm is to overboard an existing ceiling.
next to nothing is your answer to the thermal difference between 9,5 and 12mm, I cant see how much difference 2.5mm is going to make to the height in practical terms..
if you want to save even more money just buy a roll of cheap polythene and pin that over the rafters before using standard 9.5mm plasterboard. Itll do the same thing as using foil backed board but wont be as strong as using 12.5mm.
Each roof vent will ventilate the gap between 2 rafters. nowhere else. So no, theres isnt "plenty of ventilation"

you came on asking for advice, and you got it. Very good advice at that but it seems what you actually wanted was confirmation to do what you wanted to do in the first place. What you are going to do is on contravention of building regs, which are there for a reason. Youll end up paying out more to heat your house which will burn more fossil fuel doing so. Thats what the regs are for.

I hope when you sell your house you dont tell people youve just had your loft conversion redone and its now all cosy and up to regs cos it just wont be. Plenty of people have tried and got away with it in the past, in fact I had a bloke try it on with a property developer friend of mine recently and all it took was for me to pop out a non fire regulation downlight from the SHOWER area to find there was NO insulation in the roof void

You crack on mate, do what you want, its you thats liable, youre responsible for it in the eyes of the law.

I once heard mohammed was a tight b'stard and thats why Im not religious...
Click to expand...
Thanks again mate. I’m changing a few things based on your advice. There might be a bit of squeak when I walk, nothing wrong with getting the right balance between all the gear no idea and saving a few £££ and still getting a good job done. I have no intention of selling my house and the door size/type & ceiling height don’t meet regs anyway so not much I can do to get it 100% correct. Will convert properly in about 5 year.

Luckily I’m northern so can appreciate your terrible banter Thanks again
 
mate i was born in morecambe.... it might feel hard when presented with the options but ive been in this game for 28 years and thats after i did 2 years at british aerospace and another couple fabricating the staircases for the jean michelle jarre docklands concert but the bottom line is I wouldnt give you advice if i wasnt sure of the benefits its gonna give you

"converting properly in another 5 years" is just a cop out

like f**k your gonna upgrade what youve just spent money on

do what you want mate, youre gonna do that anyway, what you wanted was a tutorial and to be honest, thats what you got

I just wouldnt wanna buy the house off you when you do sell it...

never buy a house off a builder or a car off a mechanic....

never buy a house off a tight f**k*r either..
especially one he reckons hes done up and its all bang on... bollox
 
Chris W said:
mate i was born in morecambe.... it might feel hard when presented with the options but ive been in this game for 28 years and thats after i did 2 years at british aerospace and another couple fabricating the staircases for the jean michelle jarre docklands concert but the bottom line is I wouldnt give you advice if i wasnt sure of the benefits its gonna give you

"converting properly in another 5 years" is just a cop out

like f**k your gonna upgrade what youve just spent money on

do what you want mate, youre gonna do that anyway, what you wanted was a tutorial and to be honest, thats what you got

I just wouldnt wanna buy the house off you when you do sell it...

never buy a house off a builder or a car off a mechanic....

never buy a house off a tight f**k*r either..
especially one he reckons hes done up and its all bang on... bollox
Click to expand...
Mate, it’s a bit of DIY Calm down son, you have no idea about my background or my intentions for my property.
 
so back to why you came on...
and ive had a few tonight so beware..
you come on looking for advice and at every stage you trying to push your luck.. it was an easy job, easy advice just do what youve been advised and youd you be sorted for years to come
do what you want mate, all the advice you need is in the thread
questions like "why cant i use 9.5 as opposed to 12 mil"? you were told early on...
just do what you want youve had a load of free advice from a bloke with 28 years in the game its on your head
you popped my head when you asked that question and it became blatantly obvious it was about saving a few pennies in the short term

laters mo, im done
 
Chris W said:
so back to why you came on...
and ive had a few tonight so beware..
you come on looking for advice and at every stage you trying to push your luck.. it was an easy job, easy advice just do what youve been advised and youd you be sorted for years to come
do what you want mate, all the advice you need is in the thread
questions like "why cant i use 9.5 as opposed to 12 mil"? you were told early on...
just do what you want youve had a load of free advice from a bloke with 28 years in the game its on your head
you popped my head when you asked that question and it became blatantly obvious it was about saving a few pennies in the short term

laters mo, im done
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Haha! Priceless
 
Matt_North said:
Mate, it’s a bit of DIY Calm down son, you have no idea about my background or my intentions for my property.
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In Chris's defence , you be one in 20 at best mate , the amount of times in my early days I've been talked into helping someone short cut cos of whatever reason , only to drive past in the not to distant future to see their lifetimes home up for sale , some even have the cheek to ring again to compromise , cos money's tight and this is the one

Lol

No offense , just saying how it's possible to get such a reaction
 
bof said:
In Chris's defence , you be one in 20 at best mate , the amount of times in my early days I've been talked into helping someone short cut cos of whatever reason , only to drive past in the not to distant future to see their lifetimes home up for sale , some even have the cheek to ring again to compromise , cos money's tight and this is the one

Lol

No offense , just saying how it's possible to get such a reaction
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The thing is it’s never been about saving money hence why I said you know nothing about me. I enjoy a bit of home DIY nothing serious. I genuinely want to do a sound job but not chuck the kitchen sink at at. In no way am I pro but I came on here to ask genuine questions to the people who know their s**t.

If you don’t like answering questions, don’t answer them, no one forced you to pal.
 
Matt_North said:
The thing is it’s never been about saving money hence why I said you know nothing about me. I enjoy a bit of home DIY nothing serious. I genuinely want to do a sound job but not chuck the kitchen sink at at. In no way am I pro but I came on here to ask genuine questions to the people who know their s**t.

If you don’t like answering questions, don’t answer them, no one forced you to pal.
Click to expand...
Got me wrong , I meant no offense as I said , just thought I'd explain cheers
 
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