Any suggestions for the flaunching mortar mix for a valley gutter? I've seen 1:4 recommended for flaunching around chimney pots, so I assume this is not wildly out. What about the sand -- sharp sand or building sand?
Guess who's repairing his roof today...
I've got to re-do mine soon, I bedded the valley originally 3:1 building sand to cement, and it has broken out over the last 10 years. I'm wondering if lime mortar would be better?
A 4:1 would be fine.
I would use building sand for workability, but add some sharp sand too for weather resistance 2:1 0r 3:1 ratio. Some PVA will also help.
But remember, its not so much the mix, but the trowelled finish that will give it the tough surface and greater weather resistance.
dg
Thanks.
Didn't get to the flaunching today -- that will be tomorrow, or Wednesday if I need to buy more tiles.
I would like the tiles to overhang the flaunching slightly, so that even if the tile to flaunching bond breaks, the water still goes into the gulley. However, that makes it really hard to get a good finish under the tile edge. The way it was done was the flaunching was brought up in front of the cut tile edges and made level with the tile top. This may be because, having removed the flaunching, I can see the cut tile edges are not in a very straight line, which the flaunching was making up for.
Still thinking this one through...
Pointing to a tiled valley can course more trouble. Before I comment I would like to know, which type of tile is on the roof, concrete single lap interlocking or plain tiles, and is the valley lined with Lead, GRP or concrete valley trough. By the way 4 to 1 is useless for this type of work.
2 to 1 stiff mix and PVA if you are leaving the old mortar in. BS 5553 states " Open cut lead valley's with plain tiles should not be bedded or pointed"
Concrete interlocking tiles (similar to Marley Ludlow Plus), GRP valley trough. The trough has no upstand at the sides (other than a pair of small ridges) so I suspect it would overflow easily without forming a mortar gulley. It has a strip down each side to bond to the mortar.
I've stripped out the old mortar, which had developed a number of cracks, and come unstuck from some of the tiles.
I called up Lafarge's helpline, and the chap there was very helpful. He said to use 3:1, and sharp sand for best result. I asked about PVA (which I do normally add when I want a strong mortar) and he wasn't happy about that in mortar exposed to such moisture. He said if I really wanted a bonding agent, use SBR(?). I commented that I had waterproof PVA, and he was happy with that (he might have said that is SBR -- didn't fully catch the comment). He also said to coat the sand areas on the plastic gully with a slurry of cement/waterproof-PVA/water in a 2:1:1 ratio to make the mortar bond well, and apply the mortar whilst the slurry is still tacky.
Anyway, I did one side of the gully. You know that nice feeling you get when you're doing something and it's going perfectly? Well, I just wasn't getting that. Mortaring in tiles further up was causing the ones lower down to move and come unstuck. I had visions of this last part of the job still leaving me with a leaking roof, screwing up all the re-felting, replacement of battons and tiles I had done.
In the morning when it had set, the mortar did look better, and the tiles did all look stuck, but I wasn't going to try tugging on one to see. I decided to call a roofer to mortar the other side and redo the side I had done. Roofer came to have a look and said my side was fine and didn't need redoing. He's going to do the other side for me though.
Roof was just tested with a torrential downpour. I was up in the loft looking for leaks, but not a drip to be seen, and that's with one side of the valley still unmortared. I guess that's a good sign at least.
I took lots of pictures whilst I stripped the roof around the valley off, and redid it. When I get a chance, I'll put them up somewhere. They also turned out to be invaluable when I was retiling afterwards, as a reference to how it had been tiled before I started.
styrene butadiene rubber
PVA in a set cement is resistant to moisture, even submerged, as long as it is used in the usual amounts or less. SBR is styrene-butadiene rubber, a synthetic rubber, first developed in the 1930s. Due to the occupation of Malaysia by the Japanese, and the consequent shortage of rubber (no tyres, rubber dinghies ,Mae Wests etc.),this spurred the allies into producing it for the war effort. As dried film, waterproof PVA (actually EVA - Ethylene Vinyl Acetate) is little better than PVA. In set cement matrices, performance is similar. Adhesives suitable for swimming pools contain redispersible PVA. On that subject, I received an email asking why redispersible PVA powder was being used on the creases of cricket pitches. A bit of research revealed it was to stabilise the surface for more consistent behaviour during a match. Yet another use for an amazing material!
In hot weather it is a good idea to damp mortar and concrete down every so often. If the mixture dries out, the hydraulic cure will not take place properly.
John Schmitt
Thanks John, I did do this. After the neighbours saw me walking down the valley gully sprinkling what probably looked like grass seed (but was actually dry sand to remake the mortar bonding strips on the GRP gully), and then saw me watering it all afternoon, they probably think I've either given it some pagen ritual blessing, or they're now waiting for the roof garden to spring into life ;-)
BTW thanks for your comments and your FAQ which I browsed before starting. One area in which it seems very difficult to get much hard advice is the sand/cement ratio, type of sand, and other additives for any given application. I think I had ratios between 1:2 and 1:4 suggested. My roofer tells me they used to use 1:3 for this, but nowadays it's done with 1:4 using a 50/50 mix of building sand and sharp sand. It would be good to see a chart somewhere ranging from the strongest through to the weakest mixes, and typical usages of each.
1:2 is a bit too strong, with the risk of shrinkage. Bearing mind that most mortars are gauged bt volume, either shovelfuls or bucketfuls, rather than by weight, the mix ratio will by nature be approximate. Fortunately mortar is quite tolerant to exact ratio. As for the chart, that is a very tall order. Admixtures, water ratio and the particle size distribution and shape (rounded, subrounded, subangular, angular) all play their part. I did try writing a program to analyse this, assuming spheres (the simplest maths, but still brainpain)but that was when I only had BASICA, this being before Live Aid had even become an idea. In the end it was down to trial and (mostly) error. There was a company which supplied monotypical (same size and particle shape) sands, but they appear to have gone under. In the end the project I was working on struck gold. There was a company which used cyclones (think a ginourmous Dyson) to grade sands and one of their products was perfect for the application. The boss thought I had rigged the results at first until he himself made and tested the development product. He was as gobsmacked as myself. Market leader in six months. I'll have a look in Fry's to see if there is any guidance. Cover price £70, picked it up for 30p. :-)
John Schmitt
Roofer came a did the other side. It was very useful to watch him working and I picked up a number of tips. However, I didn't like some aspects of what he'd done (no effort to bond the mortar well to the valley sand strip or to the tiles, amongst other things). When he'd gone, I pulled out all his mortar and did it all again, using what I regarded as the best parts of his technique, and the best parts of mine. All still seems waterproof following another rain storm.
I didn't regard his visit as a waste of time though -- the chat with him and watching him work were worth the rather small fee he charged, and effectively went towards improving my technique.
Had a read of Fry's Concrete on the weekend. The short answer to strength of cement mortars is as follows. Particle size distribution, as I propounded, is more or less impossible to correlate with anything in particular. Angular aggregates tend to result in a lower workability than rounded ones, which rather stands to reason if you think about it. Compressive and flexural strength are well correllated with the elastic modulus of the aggregate. To the best of my knowledge all the building sands normally available in the UK are silica (quartz) sands, so there should be little variation. I do hope this isn't too dry going. Fry's is in excess of 500 pages of this sort of stuff.
John Schmitt
replying to Andrew Gabriel, Stevie Murray wrote: Haha, I Love that you pulled out the mortar he did and redid it as you knew it should be and could be done! You kept a great attitude about it too, seeing it as a positive learning experience, which is commendable. Well done mate..! ??
replying to Andrew Gabriel, Stevie Murray wrote: Haha, I Love that you pulled out the mortar he did and redid it as you knew it should be and could be done! You kept a great attitude about it too, seeing it as a positive learning experience, which is commendable. Well done mate..! ??
Even though that was a 2005 post, Andrew is still around. Welcome to uk.d-i
-y.
NT
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