Advice on finishing new shallow pitch roof

[mattu27]

Hello,

Wondering if anyone can offer some assistance on an issue that we are having installing a new pitched roof.

Current progress is as follows:

Breathable membrane fitted
Tile lath fitted
800 Redland Regent tiles waiting to go

The roof is an L shape with a 16 degree pitch on the rear half and a 13.5 degree pitch on the side half. The issue is that with such a low pitch we are concerned about the guttering. The father in law/builder has noticed that the final tile at the gutter needs raising in order to improve the aesthetics of the finished roof and has suggested putting a second lath on top of the bottom roof lath in order to raise the tile up.

I see no problem with this however the next suggestion has me concerned, he wants to cut the membrane just behind the final lath and place 12" DPC underneath it to provide an additional water barrier (The membrane we have used is UV resistant for only 3 months so will begin to degrade after this), he has then suggested putting the DPC and the membrane over the top of the last lath in order to create a drip tray that the fascia can be slid underneath.

Problem is, with this shallow of a pitch the additional lath almost makes the final section horizontal on the 16 degree roof so the 13.5 degree roof will effectively have a trough so to speak.

I understand that there should be no water ingress through the tiles in a normal scenario and this should not be an issue of concern however cutting the membrane and reducing the pitch seems a bit strange to me.

I have attached an illustration of the current and proposed scenario so that it hopefully makes more sense.


how to make screen shot


http://postimg.org/image/ba9tqz32v/ Note: the angle shown is a bit steeper than it actually is.

Just a thought, is there supposed to be a batten at the bottom of the roof or can the tiles rest on the fascia board?


Thanks, Matt [/img]

 

[oldbutnotdead]

Not a roofing expert but currently (well not today, wind got a bit fierce) reslating a roof.

Have you checked Marley's fitting instructions for the things or are you relying on the father-in-law? http://www.marleyeternit.co.uk/~/me...g Guide for Interlocking Tiles and Slates.pdf

You should be using felt support trays under the membrane- they'll help with your shallow/negative pitch (otherwise any water will just pond in the DPM behind the facia batten). Not sure about the aesthetics but with that pitch I'd be going for a working roof rather than a pretty roof.

 

[mattu27]

Thanks oldbutnotdead for the reply,

sorry they're Redland regent tiles, we were going to use mendips but they only went down to 15 degrees where as the regents go down to 12.5 degrees.

We've both been scowling the internet but we have been doing other parts of the extension whilst waiting for the tiles to arrive (which they did yesterday) so it's been on the back of our minds as a job to do

From the Mendip fitting guide on page 7 it shows pretty much what I was thinking which has eased my mind. I'd prefer not to cut the membrane but slide the DPC underneath if possible, however I see your point with the eaves trays as they would provide a lot more rigidity to the membrane beneath the tiles.

Do you think a 20mm thick fascia would support the weight of the tiles effectively?

Thanks, Matt

 

[oldbutnotdead]

Doh! Where did I get the idea they were Mendip? Sorry about that...

Moniers' website really isn't useful is it? Or rather they don't really supply generic fixing info.If you've got 3 days to spare (can't see you doing much ladder work this week) you could try this thing http://www.monier.co.uk/services/fixmaster.html and see if it comes up with anything useful.

20mm facia board- is that plastic or timber? The tiles come in at 49 kg/sq metre, your facia batten will be carrying a bit more than half the weight of one row (300mm) so each row comes in at 49/3.3 so say 15 kg per metre. So as long as the fixings are up to the job (something a bit beefier than 1" x 8 ) then I reckon 20mm batten will be fine.

With the weight of the things then DPM under the membrane (but do keep the membrane running all the way to the gutter) would probably work for a while but will eventually get brittle and fail, not to mention being noisy when the wind blows. Felt support trays cost coppers (sure I saw Jewson or someone had them at 94p at the moment), they're UV stable, they save you having to mess about with tilting fillets- whats not to like!

You aren't using a valley between the 2 pitches are you- they do mention minimum 15 degree if valleys are involved...

Have fun

 

[catlad]

Tiles generally sit on the fascia.

 

[mattu27]

Thanks again for the reply.

I typed mendip initially as that was what we were using until we realised how low the pitch was on the side of the extension.

He was supposed to be starting the work today however this seems to be the debate of the month as we both have conflicting ideas and so he is reluctant to start as we can't agree.

Monier have one 148 ish page document about fitting that isn't the greatest or least time consuming document to read through and has little information regarding tile fitting on the angle that we have. I might try the fixmaster link and see what it throws up just out of curiosity however I would like to get cracking as I've seen the storms they're having in america again and if they head this way then it will be interesting.

I was thinking 20mm plastic fascia would be sturdy enough to support the tiles, the tile battens are 2" x 1" at 450mm centres so they're strong enough to walk individually. There would be a batten near the bottom of the roof so I'm hoping this wont be an issue.

I spoke to the father in law last night and yes adamantly stated that felt supports/eaves trays aren't the way to go as they will reduce the pitch of the tile too much. I can't see this personally but apparently I'm wrong!

Hes saying that putting the eaves trays on is a waste of time and money as they will take the pitch the wrong way. The ones I've looked at are 210mm deep which as far as I'm aware would reduce the pitch but allow it to be constant.

I've attached another picture of the 16 degree roof. No there's no valley, the l shape is wrapping around the house so to speak so just two hips and a gable end.

Thanks for your help so far,

Matt

 

[mattu27]

Plus 1 for some extra visualisation

Does this seem right? Eaves tray under the membrane.

Thanks,

Matt

 

[oldbutnotdead]

Morning,

You're probably up on that dodgy scaffold right now.........

Your sketch of the eaves tray, membrane etc is exactly what Marley show for their slates (except for the double batten- you can't kick up Eternits like that) but if your FIL is a highly experienced roofer then I'd defer to his knowledge.

Again I'm no expert but the felt as it is now in your pic will flap about in the breeze and eventually perish causing any water that does hit the felt to drain down between the gutter and the facia board. You'll also get windborne Stuff blowing up under the tiles unless you fit the combs you showed in your sketch.

I'm assuming Building Control aren't involved here (they'd get a bit excited about the scaffold) so think Fixmaster will be your friend. Those battens look fine, as long as the facia board is solidly fixed 20mm will be fine as well.

And that roof pitch looks more than 15 degrees to me.....

Don't get too worried about the weather- if all the battens are down the felt will be fine.

Good luck

 

[mattu27]

Hello again, no no im at work at present however it seems a nice day out there! The building trestles have now been replaced for something more substantial!

From what I've researched that solution seems to be the best way forward. The second batten that I've shown is thinner than the first so it basically replicates the overlap that you would have if there was a tile below. Its just for aesthetics mainly.

I totally agree with what youre saying and for the sake of a few quid a Meyer extra I'd prefer to have the combs and the trays fitted. Going to pop jewsons on my way hone and pick them up so I can demonstrate to the father in law.

I thought the battens would be fine as we went with the thicker stuff as its more substantial. The roof shown is the rear elevation, if you can see on the photos the side roof is a lot shallower. If I remember right its about 4.2m span.

Thanks for all your advice so far, it has eased my mind, now just to convince the builder.....

Thanks, Matt

 

[oldbutnotdead]

Good work. yes 25 x 50 battens are reassuringly solid, especially compared to the skinny things I've taken off. Get a price from Jewson for the bits before you buy- they can be a bit naughty sometimes (chargel you list if you don't ask first).

 

[ree]

Before you load up with tile you should give a thought to flashing.

You dont appear to have cavity trays installed, and given that you have a reasonable overhang at the eaves soffit and the window cills, only a couple of brick panels are perhaps exposed.

You should grind out and fix your flashing before tiling.

Its difficult to tell with the camera angle but the last batten seems to be in the same plane as the upper battens?

Typically, you should first fix your fascia, and then set out the roof from 40mm beyond the fascia ie. the overhang into the gutter.
And, as mentioned above, the eaves tile rests on the fascia.

For what its worth, batten cuts/joins should be staggered for strength - you have matched yours up on the same rafter.

 

[catlad]

That first baton looks like it has been fixed permenantly and its not normally needed, ask your fil how is the water going to drain into the gutter after the fascia has been fitted if you do not use trays.

 

[noseall]

I look at the image (rear elevation) and can't help thinking that there seems an awful lot of brick coursing above the lintels for an extension fighting for roof pitch.

Whist a shallow pitch roof does mean that the soffit is likely to require a course or two above say a 'standard' finished brick height, I'm just curious as to whether the wall plate is as low as it could be or is there a ceiling height reason why it isn't.

My guess is that it is the camera angle and I'm being daft!

 

[mattu27]

Good morning all and thanks for the replies, I am once again worried!

Went to jewsons yesterday but they wanted 8 quid per one! And they weren't in stock! Got them from our local roofing store at 3.50 plus the vat for a 2.5m length which didn't seem too bad.

Anyway, got back and removed the bottom lath and membrane, refitted the lath and an additional smaller lath. Then cavity trays were fitted which has increased the height of the bottom tile at the eaves. The lath has effectively simulated the thickness of a tile to keep things looking right. If I remember I'll take a photo later on. Question, do the eaves trays need securing to the roof carcassing at the rear as well as the front? Can't seem to find any images showing this.

Just need to secure the membrane over the cavity trays then I can cut the mortar for the flashing. Thanks to Ree for suggesting that. If only I'd had known all the things I've now found out, this roof might look a whole lot different! Hindsight eh!

The angle of the cavity trays now that they're installed is almost horizontal And this concerns me a little but hopefully this will never be an issue as the roof shouldn't leak!

Noseall, you have a good point however the ceiling in the kitchen at the moment is lower than a standard ceiling by far and I dont think the house would of looked right with a shorter French door fitted. We probably could have gone another brick which would have increased the pitch but what's done is done unfortunately.

Last question, at the gable end the concrete board has been fitted under the battens to support the mortar when the tiles are on, how much are the tiles meant to overhang the side of the roof? The depth of the fascia board then half the gutter?

Thanks again,

Matt

 

[Alastairreid]

Oversail the undercloak aprox 40mm.