Best way to waterproof a shed roof?

I have a very good quality potting shed (handmade six years ago by my local timber merchant with decent quality wood) so it should last my lifetime, however it has started to leak. The existing felt is in good condition, but it is leaking through on the overlaps despite my clearly botched attempt to seal using flexible guttering sealant – I ran a bead along the overlap and also around all pin heads (none of which have pierced through the wood on the roof).

Should I be looking to start from scratch and sealing with bitumen adhesive first and putting new felt on or is there another way I could attempt a fix using the existing felt?

How big are the overlaps? Did you run a continuous sheet the depth of the roof, Or are there any joins rather than overlaps? Id start again. I did that with our shed and it’s been fine for six years. Continuous lengths, one third overlaps and a final piece on the ridge. I think I used wickes felt but chose some premium stuff and a pot of sticky adhesive.

https://www.wickes.co.uk/Onduline-3mm-Black-Corrugated-Bitumen-Sheet-950-x-2000mm/p/240039

onduline is what you need. Stop messing around with felt and gunge

g

^^ what he said.

I did consider corrugated sheet – a recent newcomer to it but just used it for a rabbit run cover. I will have to check with the boss though – she might not be keen on the industrial look it would give.

How big are the overlaps? Did you run a continuous sheet the depth of the roof, Or are there any joins rather than overlaps?

It has to continuous runs from side to side with an overlap in the middle.

Redid the leaky roof on my shed a couple of weeks ago. Paid a bit more for the thicker stuff (IKO Super Shed) as I always seem to tear the cheap felt putting it up. Took my time on a sunny day, no more leaks 👍🏻

I replaced a clear polycarb corrugated roof on our shed that got annihilated in a hail storm with onduline roofing. It was pretty easy to do.

I bought the sheets, the ridge strips & screws from screwfix with the clear rubber caps.

It looks OK and is easy to work with, although I will agree with the ‘industrial’ sentiment. It also stank for quite a while afterwards when in the shed – perhaps 2 months.

I think a well laid felt roof looks neater although you can buy the Onduline in different colours like green & a brick red which may tone it down a bit compared to black.

Ours has no OSB board or anything else underneath it. It is supported by battens running the length of the shed, which was how the previous polycarb corrugated sheets were held. I think it would have been better to remove the battens & replace with OSB, but the shed had been bodged about quite a lot & it would have been a lot more work.
I did notice shortly after installing that there were lots of drops on the underside of the roof. I initially thought it was leaking through the screw holes, but it was probably more likely condensation although there are large air gaps at either end to hopefully provide good ventilation.
I am in two minds whether to use some thin insulation held in place with plywood to attempt to prevent this condensation. I am not sure whether it would just encourage damp & mould between the plywood & the onduline.

EPDM!

Ondulines horrible for condensation.

I have a garage clad with it. I went with 2part epoxy foam to insulate and get rid of the condensation

Garage stays dry now…..it was so much condensation.

Ondulines horrible for condensation.

Good to know. I’ve only used it fixed on top of old crappy felt so that’s not been a problem. About to build a timber double garage and looking at onduline roof so will bear that in mind.

^^^ cheers trail_rat. Thanks for confirming!

I’ve got a ton of natural sheep wool and synthetic insulation from Gousto and Eat Fresh food boxes.
I was planning on putting that under the Onduline and holding it in place with some thin ply…

I have another garage that’s got a metal box profile roofing on it.

Also horrible for condensation

Any singleskin roof material will have this issue. On that roof I went with 100mm kingspan and ply.

Spray insulation much cheaper and quicker.

Did you apply the spray insulation yourself? Think I’ll do some Googling (need to reroof my garage and currently unsure if what to replace it with).