Replacing corrugated asbestos-cement garage roof

[Simon]

My garage has corrugated asbestos-cement garage roof, which is cracked beyond repair (I've emergency-sealed cracks and new ones now appear) so I want to replace it. (I know my local tip will take it with advance notice).

The company that built it will replace it for not a great deal of money (about 250 quid I was quoted last year I think) but they will use fibre-cement sheets.

However I've seen that you can get bitumen type corrugated sheets from B&Q/wickes etc for about 7 quid a pop.

So the question is, which is the best material for a garage roof? I kinda get the feeling that with fibre-cement it'll have the same brittle characteristics and will degrade over not many years. Are the bitumen sheets any better/worse?

Thanks,

Simon

[Chris Bacon]

These are good, as is corrugated iron (which is probably cheaper, too).

This is called "Onduline" and is AFAIR made by Johnny Frog.

Well, about 40 or so...

They sag. They sag, lots. They're better if you put them on Sterling Board, but that adds to the cost.

You could DIY a pitched roof for a few hundred.

[Thomas Prufer]

They will not take to being stepped on well, and will crack if stepped on in winter. (Mines are on a double layer of bitumen felt on wood, so are supported well.) The colored ones lose their color in a few years. No way will it last 40 years or so.

Thomas Prufer

[meow2222]

fibre reinforced cement usually lasts very well, its one of the best for longevity. That doesnt guarantee every batch will, but for long life its your best bet, short of concrete house tiles or slates.

You can make your own flat fibre cement sheets if you want, it isnt hard.

NT

[Chris Bacon]

Thanks, I didn't know that.

I shouldn't think they would, either. Do they (IYE) seem to encourage moss growth?

[Chris Bacon]

Interesting. How?

[Autolycus]

As others have said, the bitumen-fibre sheets are sold as Onduline, Coroline, and as a Wickes own-brand. Worth looking at the onduline web site for recommendations on pitch, supports, and fixings. I've used it on a moderately-pitched (20-odd degrees) roof successfully (so far). Watch out for a few gotchas: it's only available in short, (2m) sheets, with a cover width of about 850mm; matching ridge cappings are quite expensive; and the sheds make their profit on the fixing screws.

Box section steel is cheaper, and can span further, but is susceptible to condensation on the underside, which then drips everywhere, unless you use a "proper" roof construction. Several ebay sellers offer it in various finishes and two thicknesses. Beware of the thin stuff (0.5mm) and a rather rum outfit in Erdington, although I've just finished a 25 square metre roof using both.

The big advantage of steel is that you can buy it in long lengths, so if your garage has a very shallow front-back pitch, you can avoid end laps, which are a bit of a pain, especially in fibre-cement sheets where you have to saw chunks off the corners of the sheets to avoid having four thicknesses of the stuff.

The SLE cladding site has some useful stuff

but they're not especially cheap.

[Simon]

Hmm so it seems the concensus is that the fibre cement is the better material, so I'll go with that approach. Is it actually "better" than the old asbestos stuff, or could my original one just have been badly installed? I say that because the last roof is probably only 10-15 years old, way less than the 40 years lifetime suggested for fibre cement.

Thanks all

Simon

[Thomas Prufer]

The surface of my Onduline is rough, a very fine cross-hatch pattern. Moss and algae grow on it, but no worse than on the adjacent fiber cement. But it doesn't take to pressure washing well, whereas the fiber coment does. (Don't pressure-wash asbestos fiber cement, btw, as it releases the fibers, so they tell me.)

Also, the fiber cement may be available with an acrylic coating, usually colored. ISTR something like one or two quid extra for color-coated. Might want to check.

Thoams Prufer

[Chris Bacon]

I doubt it's actually better. Could be, though - I'm not sure how it is made.

It's possible. The 40 years I suggested was based on experience of asbestos cement roofs that have not been mucked about with - if the roof's been bumped, or structure isn't quite rigid, I bet it could fall to bits early.

[Autolycus]

It's different. For some applications, it may be better, for others, e.g. as I suggested, long, low-pitched roofs, it may not be. It's certainly expensive and heavy.

Is it actually "better" than

If it's that new, I'd be very surprised if it is asbestos-cement. Could second-hand sheets have been used when it was built? There was a thriving trade in s/h asbestos-cement sheets long after fibre-cement became the new norm.

If they really are that new, and have failed, I'd be looking very hard at the underlying structure, to see if it is adequately stiff. If it isn't, then you've either got to strengthen it or take a chance with a less brittle material. Or stop exercising the elephant on it.

If you do use fibre-cement, think hard before painting the underneath: if you make it any less absorbent, you'll get condensation, just like you would with steel sheets.

[keith_765]

The bitumen sheets are only compressed card board impregnated with bitumen, They tend to sag and curl and they look unsightly being black in colour. The only advantage is that they are light weight. Cement fibre corrugated sheet will out last the Bitumen. The down side they are a lot heavier.Have you consider plastic coated steel sheets. They come in lengths to go in one, are light weight and easy to fix and come in various colours. Down side is that you may have to under line with an insulation sheet to stop condensation, but well worth it.

Keith

[meow2222]

asbestos sheet _is_ fibre reinforced cement, the fibre used was asbestos. New fibre cement boarsd simply uses other fibres, eg nylon, polypropylene, glass.

NT

[meow2222]

The material is simply fibre reinforced cement board. 1:3 mix with 5% alkali resistant chopped glass fibres works fine. 1% polypropylene or nylon fibre is also used for other purposes, though whether that gives as much tensile strength as the glass I dont know.

The mould base is a sheet of glass, or ply with polythene on it. 6-8mm wood is attached round the edges. The mix is made, it must be mixed properly, and its tamped into the mould. Applying a sheet of polythene on top will smooth it and keep it damp while it cures.

Flat sheet is less strong than corrugated, hence you may want to use thicker sheet than the corru. It also looks a lot better.

NT

[Chris Bacon]

I thought all glass was "alkali resistant"?

? 1% polypropylene or

a) I've used this in render, b) getting it evenly spread in the mix is nigh impossible, c) I doubt it very much.

I assume this is all theoretical. Hm?

[Aidan]

Regarding fibre cement roofing, I think the main product is Marley Eternit. Besides roofing sheets, the material is also used for tiles, slates and cladding panels.

list of stockists for roofing is odd (the list of 'your nearest towns' doesn't include London!) but the list is more extensive for the tiles & slates section. I can recall using Marley's asbestos cement slates in the late '70s, I'd think they've just changed the material used for the fibre reinforcement.

[meow2222]

forgot to mention moulded sheets use a very fine sand. A mix of fine and the usual would work.

NT

[ag.jones]

These sheets are nothing but cardboard covered in bitumen. They sag with any weight of rain or snow on them and need bracing at 600mm intervals max. I did my son's garage last year and now it all needs replacing. Total rubbish!!

[Chris French]

In message , [e-mail] writes

needs replacing. Total rubbish!!

Friends have this on an outbuilding, it has been fine for years. Sounds like you didn't follow the installation instructions properly.

[newshound]

Not asbestos cement then. Sounds more like Onduline or other equivalents. I did one 25 years ago and it is now about due for replacing. Don't see why they should sag with rain unless there's no slope! If you follow the manufacturer's spec they will support the weight of a sensible adult, but you would not find me on an asbestos cement roof without proper crawling boards.