Garage Roof Repairs - Design Issue?

Sebo027

Good day. A relative of mine has a double garage which was built in 2006. It's quite a big garage, can suitable fit two cars with plenty space front and back. 

The roof has started to leek. The joints are timber, the roof itself is made of plywood and it's covered with welsh slate. One half of the garage roof, presumably the side that's most exposed to the elements, has deteriorated quite badly. This half of the roof, when viewed from the inside, is black, daylight can be seen from the right angles and the plywood itself looks to be rotting in places and splitting. 

I have had a slater look at it and he is of the opinion that the pitch is too shallow to have the roof covered with slate. He also commented that the slates used to do not provide the cover to prevent water ingress. The other side of the roof has little to no deterioration and looks like it would last another 14 years easily. The slater has recommended replacing the entire half of the roof that's badly affected, and I don't disagree. He has suggested replacing the plywood with OSB board which he uses as standard, a suitable underfelt and then buying bigger slates to minimize chances of the problem happening again in the future. Alternatively, he's suggested removing all slates and replacing it with a felt roof that he thinks is a less risky option. The joists look to be in good condition. 

I am looking for general thoughts on the above - obviously my relative only wants to get the work done once. She's in a conservation area so may have to live with the slate anyway, as I think that may have been a condition of the planning permission. 

Anyone else have any experience with low pitched roofs / slates?  

Would sarking be a better option, in this case, than OSB board? I'm inclined to think solid timber may have more longevity if it's ever exposed to water/damp again in the future, but obviously I am no expert? 

sienew

Sebo027 said:

Good day. A relative of mine has a double garage which was built in 2006. It's quite a big garage, can suitable fit two cars with plenty space front and back. 

The roof has started to leek. The joints are timber, the roof itself is made of plywood and it's covered with welsh slate. One half of the garage roof, presumably the side that's most exposed to the elements, has deteriorated quite badly. This half of the roof, when viewed from the inside, is black, daylight can be seen from the right angles and the plywood itself looks to be rotting in places and splitting. 

I have had a slater look at it and he is of the opinion that the pitch is too shallow to have the roof covered with slate. He also commented that the slates used to do not provide the cover to prevent water ingress. The other side of the roof has little to no deterioration and looks like it would last another 14 years easily. The slater has recommended replacing the entire half of the roof that's badly affected, and I don't disagree. He has suggested replacing the plywood with OSB board which he uses as standard, a suitable underfelt and then buying bigger slates to minimize chances of the problem happening again in the future. Alternatively, he's suggested removing all slates and replacing it with a felt roof that he thinks is a less risky option. The joists look to be in good condition. 

I am looking for general thoughts on the above - obviously my relative only wants to get the work done once. She's in a conservation area so may have to live with the slate anyway, as I think that may have been a condition of the planning permission. 

Anyone else have any experience with low pitched roofs / slates?  

Would sarking be a better option, in this case, than OSB board? I'm inclined to think solid timber may have more longevity if it's ever exposed to water/damp again in the future, but obviously I am no expert? 

Have you thought about metal roofing? You can get standard box profile or more attractive tile effect like this.

https://www.roofingmegastore.co.uk/roofing-categories/pitched-roofing/roofing-sheets/tile-effect-roofing-sheets.html

I'd lay over exterior ply personally but OSB is fine.

I'm also a fan of corrugated bitumen sheeting if budget is an issue:

https://www.roofingmegastore.co.uk/onduline-black-corrugated-bitumen-sheet.html

The metal sheets should last pretty much forever if installed correctly. I have a shed 20 years old with the bitumen sheets and it looks like new with no leaks so even this budget solution is long lasting

FreeBear

This site gives some recommendations as to the minimum pitch (20°) and the amount of overlap for different slate sixes & pitch.

Are you sure it was genuine Welsh slate used ?

Real Welsh slate should last well over 100 years.

stuart45

Have you got any photos of the roof?. Any idea of the.pitch? Is.It in Scotland?

woody7777

FreeBear said:

This site gives some recommendations as to the minimum pitch (20°) and the amount of overlap for different slate sixes & pitch.

Are you sure it was genuine Welsh slate used ?

Real Welsh slate should last well over 100 years.

I can't see the site or is it me?

FreeBear

woody7777 said:

FreeBear said:

This site gives some recommendations as to the minimum pitch (20°) and the amount of overlap for different slate sixes & pitch.

Are you sure it was genuine Welsh slate used ?

Real Welsh slate should last well over 100 years.

I can't see the site or is it me?

No. It is me not adding the link....

http://www.sigaslate.co.uk/knowledgebase/design-guidance/headlap-tables-for-natural-slate-bs-5534/

woody7777

So, it seems that only one size of slate can be used at a 20 pitch. I have a garage with quite a low pitch, so I am interested in the tables. Would it leak if the pitch was wrong?

Mistral001

Are the slates natural slates?  If they are and you decide to replace one side with bigger slates, as recommended by the roofer, the old slates will have a value as the market for second hand natural slates is always quite buoyant, so there could be a cost saving there. 

Sebo027

Thanks for the feedback so far. I'll get some photos. I'll look at the drawings too and see if I can determine the actual design pitch of the roof. At best, it's 20 degrees. It's in the West of Scotland, so a particularly wet area. Looks like the idea that the slates are too small and provide insufficient cover is probably correct. 

The slate itself on the roof is natural welsh slate, and they're in fine condition. I suppose they could be sold to offset the cost somewhat. Although, best to keep some as they match the slate on the roof of the house! 

stuart45

If the pitch is too low water can get through with side creep. This is more likely with smaller slates. There is a system called easy slate that's been around for about 10 years. With this a slate roof can be taken down to a 12 pitch using 20x10 slates.