Shed roof repair, re-tile, fibreglass or felt?

[Zooologist]

I've got a fairly large (4m by 3m) brick built garden 'shed' which has a tiled roof. The slope on the roof is very shallow, so much so that the tiles are practically horizontal.

The roof of the shed is now leaking (its been up there for maybe 15 years). The tiles look fine, so it looks like the leak is due to the very shallow slope which doesn't allow the rain to run off properly. There is a plastic sheet under the tiles, which has gone brittle, which is where the water is probably getting in.

The question is which of the following options should I go for to repair the roof:

1) Remove the tiles and battons underneath and replace the plastic sheet above the rafters with a new one and then re-tile.

2) Replace the tiled roof with a felt or fibreglass roof.

Any opinions and does anyone have experience of fibreglass roofs?

[BigWallop]

I'd personally go with your option one (1), but, using proper roofing felt as the underlay, rather than just plastic sheet. The roofing felt should be set as a good seal against the weather, or in other words, as a proper roofing cover.

The tiles can then be placed back on battens, which are also sealed with liquid felt to the underlay felt to stop the weather getting in, and also give some more protection and comply with your local roofing style. The battens should be screwed through to the roofing timbers with brass screws. Done properly like this the roof should last a whole lot longer than just 15 years.

[Dave Liquorice]

Aye, or even one the more modern breathable membranes though for a "shed" this is probably wasted.

You don't want to seal the horizontal battens to the felt or any water that does get under will puddle at each batten and not run down and out at the eaves.

[BigWallop]

But the battens shouldn't be perfectly horizontal. They should always slope slightly, either all to one end, or to both ends from the middle. This is always normal practice to stop anything from gathering at the top edge of the battens.

The slight run is never noticeable through the tiling.

[N. Thornton]

1 would be much better IF you use a decent quality membrane - not plastic sheet. EDPM pond liner material would last well. Note the majority of materials dont last well, as youve found.

The sheet under the tiles was always the real deal on that, tiles with inadequate slope wont be at all watertight in themselves.

NT

[Zooologist]

Someone else has suggested re-tiling with a lower profile tile (as well as replacing the felt underneath). Is this what low profile tiles are designed for (ie specifically for roofs with a low angle)?

[BigWallop]

Simple answer, no. Standard natural slate tiles are normally used for most roofing designs if the area model demands it. Preformed concrete or clayware tiles do come in profile formats, and also premoulded hybrid tile systems that are made of ground up natural slate and concrete or clay mixtures, so these do have different format patterns for differing roof pitches or border styles.

I don't think anyone has asked what type of tiles you have on the roof, so a low profile tile might help if you do have a preformed pattern tile system. For standard natural slate though, it doesn't really matter. All natural slate has the same sort of properties no matter what the roof angle. Natural slate will only differ in the headlap required for different roof angles. The headlap is the amount that the tiles overlap on each, and slower slopes are normally set with tiles that are around half way over the lower row. The headlap also helps in wind load strengths, and this is usually worked out on the area of the country you're in and the rain fall averages that are experience there.

The area you live in may also determine what material is actually used on your roof covering and its design, so it might have been part of the shed build permission to have a certain sort of tile on its roof. Aesthetically pleasing for the area they call it, I think.

[Zooologist]

Thanks for the advice and comments. I spent a couple of hours this afternoon lifting a few tiles on the roof in the area where the leak is to see what sort of state it was in underneath. There were a few pools of water, the battens were wet, but no obvious place where the water could be getting in.

The existing tiles are around 1" thick (and 12" square) and are corrugated with interlocking grooves on either side (left/right). The slope on the roof is only about 8.5 degrees from the horizontal, the lowest-profile tiles I have found specify 10 degrees minimum

As such I don't think retiling will definitely solve all the problems. What I'm thinking about now is fibreglassing the roof, and then replacing the tiles on top of the new roof. That way I get to keep the look of the original tiled roof, but should be completely watertight and durable.