lots of questions :( PIC UPDATE 20/11

26 May 2008
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United Kingdom
firstly hi and thanks in advance if you help with my questions :)

me and a friend are going to be putting a pitch roof up where i currently have a flat roof we are both joiners albeit bench hand for me (a long time ago lol ) but my friend is an all rounder and i definately would not try doing this job if i wasnt sure we could do it, anyway bit of info to give you an idea first.

the roof that is over single story stretches 12 feet from the main house but there is only roughly 30" from bottom of bedroom windows to the flat roof i.e not much pitch.

we plan on pitching the new roof, ply boarding ontop of the rafters, then vertical laths. felt across with half width overlap, then horizontal laths and finally tiles.

(we are taking the tiled roof off in the pics to raise the brick coarse to match the same height and having a uniform degree of pitch on the whole lot, with a valley from the corner)

ok heres some questions if anybody could help me with please

1. what size rafters should we use to span 12ft with no support ? my friend thinks smaller than me but i want a rock solid job, is 7" x 2" at 500mm centres ok ???

2. again i know its not really needed but i only want to do this job once, so, can anybody recomend a type of waterproof paint/bitchumen that i can roller on the boards/horizontal laths to toatally waterproof them 'just incase any water should get through' ? this wont make the board sweat will it ?

3. what width lead do people recomend for the flashing to the main house where the roof butts up ?

4. would/could one of those 'light tunnels' be fitted ok to this type of roof, and would it be liable to leak ?

5. even tho we are boarding and double felting the pitch is going to be real low so what tiles can people recomend ?

6 would people recomend a ventilation brick (or similar) in the end wall to allow air movement under the new pitch ?

7. we will need to fit a lintel to a wide door/window (see pic, above the rotten wooden door ) which wont have any brickwork ontop (to keep it in line with the walls height wise) my friend says a concrete one ? would it make fitting the wall plate awkward ? or would we be better off bolting some 7"x2" wood together for the lintel ? wood will make fitting a new door/window below easier to fix aswell ?

8. what decent felt can people recomend ?

i will probably add a couple more questions as i remember them lol anyway heres some pics, its not a very straight forward job :(



p.s first job is to finish off the bodge job main roof (like that before we brought house)
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Looking at your first pic, it's just as well you're doing it yourself, many wouldn't touch this with a bargepole. 5 roofs for the price of one, nightmare!

Anyway, that's too many questions to answer at once, and it's late so my first thoughts are:

Use a breathable membrane such as tyvek, over the plywood and under the first run of battens. If you were just battening over the rafters then you wouldn't need the vertical battens, as the felt/membrane would sag between the rafters giving any water an escape route under the batten. But as the ply will render the membrane perfectly flat, then battens in both direction are a good move.

You shouldn't need to treat the ply (make sure it's WBP though!) as a good breathable membrane over the top will keep all moisture out. (and even if it did get in, it could evaporate through the membrane)

I think tyvek can be left exposed to the element for up to 3 months before you need to cover it, and it's near impossible to tear. Great stuff. There are many similar brands like Marley and Klober perma-forte etc etc.

You will need special tiles for a low pitch (how low exactly?) Several types available, I'm not particularly knowledgable in this area but here's some ideas to get you started.

As for rafter depth, your 7" will do it. Deeper rafter's give improved insulation depth at the expense of internal ceiling height (unless you are going for a flat ceiling)

Sunpipes are good but you could probably install a normal velux (with an upstand) for more light. I believe low pitch velux's are available.
make sure your underlay has a current BBA (some dont, even big names) and that they are waterproof some have a saturation point and then will let in.
thanks for the replies guys :D

so rather than something that can be painted straight onto the ply, your saying i am better off with a membrane off a roll ? if i have the right end of the stick :oops: how is this attached to the boards ?

also what is a current BBA ? :oops:

thanks again guys ;)

p.s i had lots of quotes to do the job, all around the 6-7k mark
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British Board of Agrement. It's like a kitemark, for building products, to let you know that it's been tested and works.

Membrane can be weighted down after being rolled out, then the vertical battens that get screwed down will hold it in place. Big overlaps (there will be a manufacturers recommended lap) and it will protect your roof without having to paint on messy roof coatings.

If the roof and covering are installed properly then no water will get past the tiles, unless some fall off or get damaged, but any sensible homeowner would get that sorted quick sharp anyway.
thanks for that deluks.

another few questions :rolleyes:

9. instead of the underlay felt is there any new products i could use instead ? i have seen black sheeting before but not sure of any names to look for ?

10. what type of bolts should i use to fix the 3"x2" to the wall that the joist will sit on ?

thanks in advance :)
The breathable membrane I mentioned will be the best product to use. and it is fairly new too (in building terms). There's a cheaper non- breathable underfelt on the screwfix website, which would also suffice. but you would need to make sure that there is adequate ventilation under the boards.

3x2 won't cut it, use a 4 at least, or better a 6.

Rawlbolts, expansion anchors, resin fixings, or my choice would be hammer in 'thrubolts'.
a 12' span with 30" of rise gives only an 11degree pitch.

this is considered as flat.

you could use a separate weathering system beneath the tiles ie. onduline or ply and mineral felt as this will deal with wind blown or capillary ingress.

7" x 2" rafters at 400mm c/c will be fine.

any interferences with the natural pitch of the roof ie. valleys, roof lights, sun tunnels, could potentially be troublesome on such a shallow pitch.
quick update (thought i would as many dont) the span was 10ft not 12 oops we lowered the front wall and have around 15 degrees pitch now yippeee

brought redland regents which go down to 12.5 degrees so should be ok hopefully

also got to big velux to go in aswell, now we have the minimum pitch required for them

lots of work but will post up some pics when finished

thanks for all the help :)

another quick question, when using grp valley on a boarded and double lathed roof where is it fixed ? any help appreciated thankyou
you fix the grp valley to a couple of lathes strung up each side of the valley boards.

the valley boards should be fixed between the rafters and flush with the rafter top.

there should be enough width on the valley boards to accommodate not only the valley gutter but also to catch the lathes.

if you are using solid sarking i.e. ply then there may be no need for valley boards.
thanks noseall, we have boarded ontop of the rafters, then we are covering the boards with vapour permeable barrier, then vertical laths,more barrier then laths again.......... i am sure it will become apparent where it all fits when we start to put on the laths, just thought i would ask first to perhaps get an idea :D
we are covering the boards with vapour permeable barrier, then vertical laths,more barrier...

Woah! :idea:

I read that, went to walk away from my computer and turned round and sat back down again after mulling it over for a second.

Wouldn't adding a second layer of vapour permeable membrane halve the overall vapour permeability of the roof? :confused:

I don't know the answer but I think you should find out before proceeding.
ummm now you have me worried lol

heres what we are doing >

the rafters are covered with shuttering ply. ontop of the ply we are covering it with the barrier (straight onto the boards)

then we are puttting vertical laths with another layer of the vpb ontop of the laths (so there will be a gap between layers)

finally the horizontal laths and tiles ontop ?????????

we were told to do this as it is a low pitch at 15 degrees to try to insure the roof would be watertight, and as a secondary protection incase any water made it under the tiles.

any input welcomed ? what are peoples thoughts ?????
What product are you using and who told you to do it this way?
One layer of membrane should be enough, for lower pitches you simply increase the overlap.

I'm not sure what the answer is, but here is my theory explained, as if I was your science teacher. (straggly beard, faux leather elbow patches on a corduroy jacket etc..) :LOL:

Imagine pouring water into a collander, it empties as it fills up but not necessarily as fast as it goes in. Now place that collander into another identical, and pour in water, thus.
The water will 'back up' in the first collander and take longer to empty. That would happen with moisture in the form of vapour trying to escape through your roof.

I guess it all depends on how much moisture (if any) will need to escape, and how permeable the membrane is.

I just don't see that you need membrane over a membrane any more than you'd build a another roof over a roof.

Better off than on, but it'll probably be alright though tbh ;)

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