roofing and its vapour barriers e.t.c.

[LarryS.]

been searching the forum for an hour fruitlessly, hoping someone can help...

we have an extension on our kitchen which has a small sloping roof (6ft in total). i am planning to take out the ceiling currently in place and install velux windows, then plasterboard to them i.e. along the line of the roof, so losing the current small attic space that is there (currently a flat ceiling).

i've read loads of posts on workshop builds to look at the roofs on them to check if i need to install vapour barriers and insect barriers but can't find a simple picture of what the differing layers should be, though i know in the past i have seen one, does anyone remember which thread it was in, or do they know what the answer is anyway ?

exasperated paul.

 

[MikeG.]

I'll have a go at this at lunchtime........
........in the mean-time, if you could give me more details it would help. Depth of rafters, joists (if any), type of roof covering (tiles, slate etc), type of sarking (felt?) if any.........

Mike

 

[MikeG.]

Paul,

firstly, those ceiling joists you are looking to remove may well be serving a structural function...........they could be preventing the spread of the roof. Worst case scenario is that your walls get pushed over as the roof collapses. That is not something I can give advice about without visiting the site, so you should consult a structural engineer prior to removing anything. However, there are always ways around these things, so even if the joists are acting as a tie then there will be ways of removing them and leaving everything sound.

Assuming that the joists have gone, the next issue is the rooflight. Ideally, this should fit between existing rafters. If it doesn't (ie, you have to cut a rafter), then the rafters either side should be doubled up, and trimmers fitted to form the opening.

Insulation: you should really use about 125 or 150 of Kingspan, or 250 to 300 of mineral wool insulation. Presuming your rafters are only 100 or 125 deep, then you are likely to be restricted to Kingspan. The most critical thing is that there is a 50mm deep ventilated void above the insulation and below the felt. This void should continue to the eaves, where a proprietry ventilator strip should let air in and keep insects out.....and, most difficult of all, there should be a ventilator where the roof abuts the outside wall of the house above (if it is a mono-pitch lean-to type roof).

This depth of insulation is unlikely to fit between the rafters.........the answer is to fit whatever you can between the rafters, and the rest should be fastened to the underside of the rafters (no gaps anywhere....use expanding foam!). To enable the fitting of plasterboard afterwards, have horizontal battens the depth of the insulation running across the rafters at 600 C's (15mm plasterboard) or 400 C's (12.5mm p/b).

Vapour barrier: if you have a properly ventilated kitchen (ie extractor, trickle vents), then foil-backed plasterboard will be perfectly OK. If the room isn't adequately ventilated...............fix it!!

Thhis should get you started.............if I have confused you or you need to know more, just ask.

Mike

edit PS If the lean-to roof is made up of trusses, rather than seperate rafters and joists, then you won't be able to remove the ceiling. At least, you will have to take the roof off and build a new one.

 

[LarryS.]

firstly, those ceiling joists you are looking to remove may well be serving a structural function...........they could be preventing the spread of the roof. Worst case scenario is that your walls get pushed over as the roof collapses. That is not something I can give advice about without visiting the site, so you should consult a structural engineer prior to removing anything. However, there are always ways around these things, so even if the joists are acting as a tie then there will be ways of removing them and leaving everything sound.

I've checked with a builder mate of mine, the joists are there just to provide a ceiling so fingers crossed no issues there

Assuming that the joists have gone, the next issue is the rooflight. Ideally, this should fit between existing rafters. If it doesn't (ie, you have to cut a rafter), then the rafters either side should be doubled up, and trimmers fitted to form the opening.

I've measured the rafters and bought rooflights to fit between them (from memory think they were at 15 inch intervals). There will need to be one rafter cut per window (as they are approx 30 inches) so will double up as suggested

Insulation: you should really use about 125 or 150 of Kingspan, or 250 to 300 of mineral wool insulation. Presuming your rafters are only 100 or 125 deep, then you are likely to be restricted to Kingspan. The most critical thing is that there is a 50mm deep ventilated void above the insulation and below the felt. This void should continue to the eaves, where a proprietry ventilator strip should let air in and keep insects out.....and, most difficult of all, there should be a ventilator where the roof abuts the outside wall of the house above (if it is a mono-pitch lean-to type roof).

Kingspan is the preference, take your point on the 50mm void above.
Not sure on the ventilation abutting the house, will have to see what I can find for that. Currently the facia board is flat against the brick (attached to the end of the rafters) so not sure how I can put ventilation in (as there is no downward facing surface).

Vapour barrier: if you have a properly ventilated kitchen (ie extractor, trickle vents), then foil-backed plasterboard will be perfectly OK. If the room isn't adequately ventilated...............fix it!!

We will have an extractor but its the internal re-circulating type (due to cooker being nowhere near an external wall). So looks like we might need to install an extractor specifically, as I am assuming trickle vents on their own won't be enough

thanks for the advice Mike, really appreciate it, especially after the hour of my life I lost last night looking for it !

 

[MikeG.]

I've checked with a builder mate of mine, the joists are there just to provide a ceiling so fingers crossed no issues there

Hmmmmm.......no disrespect, but that leaves me feeling a little nervous.

I've measured the rafters and bought rooflights to fit between them (from memory think they were at 15 inch intervals). There will need to be one rafter cut per window (as they are approx 30 inches) so will double up as suggested

Rooflights are a planning issue.........you should check with your local authority as to whether these will require Planning permission

Currently the facia board is flat against the brick (attached to the end of the rafters) so not sure how I can put ventilation in (as there is no downward facing surface).

Difficult to say without seeing the detail, but the normal approach is an over-fascia vent, running under your felt and tiles. This will require the fascia to be lowered or trimmed by about an inch.


We will have an extractor but its the internal re-circulating type (due to cooker being nowhere near an external wall). So looks like we might need to install an extractor specifically, as I am assuming trickle vents on their own won't be enough

Correct. A kitchen extractor should extract 60 litres per second if not adjacent to the hob, and ideally, be triggered by a humidistat.

thanks for the advice Mike, really appreciate it, especially after the hour of my life I lost last night looking for it !
No problem......its taken about the same out of mine though!!!!

Cheers

Mike

 

[Deejay]

Hello Paul

'Not sure on the ventilation abutting the house, will have to see what I can find for that. Currently the facia board is flat against the brick (attached to the end of the rafters) so not sure how I can put ventilation in (as there is no downward facing surface). '


http://www.monier.co.uk/fileadmin/bu-fi ... ICKME.html

might be useful.

Cheers

Dave

 

[LarryS.]

looks like over fascia vent it is, going down local plastics / pvc supplier to source some hopefully

had another builder check the roof, he's confirmed that its not supported or tied in at all by ceiling, in fact its supported by a 4 x 10 beam which runs from wall to wall (massive thing) that will stay in place

thanks for the advice guys, been a real help. i'll post some pics when we get round to working on it (in the next week hope to take the ceiling down)