External waterproofing for a summer house

[Richard C]

I’ve got a 5m x 4m summerhouse that’s just one year old, it’s made from reasonably robust & pressure treated 20mm tongue & groove log boards & been painted with two coats of Cuprinol Garden Colour. It looks great & was fine throughout last summer but during the wet winter months & the heavy rain we’ve had, I can see that water has obviously been getting in between the tongue & groove log boards & making the inside damp & stained, mould has also formed on the ceiling & furniture. Not sure an internal membrane & dry lining will solve the problem as moisture could then sit in the cavity & may even make things worse.

I thought about over boarding the outside but that's going to be rather expensive so I’ve been looking around for a flexible, waterproof liquid membrane I could apply externally & then over paint. Being an ex-tiler, one product that springs to mind is “Aquaseal wateroroof tanking membrane” but has anyone ever used this externally on a timber shed before? Can it be over painted; do you think it would work? If not, does anyone know of a suitable product that would work?

 

[freddiemercurystwin]

Hi Richard how goes it? Staying safe and well in these current climes I hope! I don't contribute much these days either other than some crap in the GD or some random comments now and then, feels good.

 

[Richard C]

Hi mate, nice to hear from you; had a bit of a health scare couple of years ago but now doing OK & well into retirement. As is probably obvious, I don't contribute any longer either; it all got a bit tedious with "wise arses" for me in the end so I just couldn't be bothered. As wife & I are both old farts now, we are currently 17 days into total lock down trying to keep safe; we were supposed to to go to Spain for 2 months on April 15 but that's all gone out the window now.

 

[d000hg]

We were advised to only use expensive products i.e. not Cuprinol.

We haven't done the outside yet but have gone with Sadolin Classic

 

[Richard C]

Hi d000hg; I used Cuprinol shed colour not their cheap fence treatment. It is water based & not as expensive as Sadolin, granted, but it's hardly cheap @ £5/litre & I've used 10 litres! Personally I'm not sure Sadolin (which I've used extensively in the past) would fair any better as, once dry, & with expansion & contraction of the wood, it will still crack at the T&G joints & this is where the water is getting in; it's why I'm looking for a flexible membrane type treatment which shouldn't crack at the joint when dry.

I have 2 sheds & one small barn in my garden (one is now 16 years old), none of them let in moisture & they are all built using cheaper, overlapping boards. This allows water to drip off the edge of each board, it can't soak into the the board joints as there are none! T&G log lap looks great on a summer house & is rather more expensive but, with hindsight, I should have bought one built with overlapping boards & this is what I would advise anyone in the future!

 

[d000hg]

My understanding was sadolin does have done flex to it, we did look into more 'membrane' type products but they were terrifyingly expensive... Sadolin is about £12/L and is at the lower end of the high performance products, we investigated osmo as well but the really hi tech wonder-treatments are maybe double this price!

You'd be spending more on the treatment than the building!

You could paint it with bitumen paint e.g. Black Jack but then it will look like ***".

If it's the joints, is there any possibility you can seal them?

 

[Richard C]

Hi d000hg, thanks for that; agree Bitumen would look **** so a non-starter. Interested to hear which Osmo product you looked at & what others did you check out?

I did think about sealing the joints but, again, with what? Unfortunately, silicone doesn't really over paint & am unsure about the water based caulk/sealing products, I've used many in decorating &, despite what they say, they still seem to crack under expansion/contraction. It's also 5m x 4m which would take god knows how many tubes, let alone probably weeks of boredom applying it to each joint & it will probably look **** anyway. As you say, I could end up spending an awful lot without any guarantee of success.

I'm open to suggestions but, in the end, I may end up over-boarding the outside; it wasn't that cheap a building & over boarding this would add around a further 30% to the original cost but at least I will end up with a water tight building! Moral is I suppose that if you want a decent summer house, don't skimp & buy the most expensive you can afford - & certainly buy overlap, not T&G log lap!

 

[d000hg]

I've not heard that T&G is an issue... we recently built a 'house' this way but they were 70mm logs. OTOH our property has 2 (! from previous owner) 'sheds' similar to what you describe without issue.

Have you considered painting the thing rather than varnishing? That's what our pre-existing ones have.

I couldn't comment on sealants but I suppose I was thinking more akin to PVA or a waterproofing paintable treatment. I'm out of my depth there. Sorry to hear it's gone badly though, must be very frustrating

 

[Richard C]

My S/H is 20mm T&G which is a lot better than the really cheap 12mm stuff but 70mm is going to be a whole different story & don't logs that thick have more than 1 groove? Some S/H I looked at had 45mm log lap - but they were at least 4X the price!

I think that an oil based paint would fair better but, again, I think it will still crack at the joints when dry & when the timber dries out in summer & shrinks; maybe, yacht paint (which I've also used in the past) would be better but,again, it is VERY expensive!

As well as tiling, I also did an awful lot of plastering so I know standard PVA won't be suitable as it remains water soluble, even after setting; this is why you should never use on a tiling surface or over plaster before you paint. I'd forgotten that you can also get waterproof, vinyl acetate polymer PVA; it's at least 3x the price & I'm not sure if you can over-paint it but I'l check. Maybe I can mix the PVA with some shed paint & kill two birds with one stone!

You have sparked some thought in a bit of an old & tired brain so thanks for that.

 

[d000hg]

Yeah I think our logs have 3 grooves, definitely at least 2. They lock together pretty snugly too.

Good point on PVA, I hadn't considered that. I feel the principle holds but no idea what one might use.

You're very right on paint & shrinkage, I don't know how much these products are designed to flex. The small painted summer house which I think is similar to yours appears to have survived a few years OK but the logs definitely can change size a few mm each over a year.

Be interesting to see what you end up doing.

 

[scbk]

I painted the t&g cladding on part of my house with Bedec Barn paint last year, it is good stuff, similar price to sadolin.


Does the summerhouse have good gutters and ventilation?

 

[Richard C]

Looked at Bedec Barn paint & it's a water based, acrylic paint with a very similar spec to Cuprinol Garden Shades which I used. There are many similar water based acrylic products available & some are rather more expensive than others, difficult to know if any are worth the extra money. The paint itself is still looking good & repelling water after a year but, as I said, the problem is not with the paint finish itself, the paint finish has cracked at the T & G joints due to expansion & contraction of the wood & this where the water is getting into the T & G joints, I think its more to do with the construction as the log shape of the boards does not allow water to drip off as overlap boards would, it directs water straight into each joint.

The S/H doesn’t have conventional gutters as such but there is a raised timber gutter board on each side of the roof which effectively diverts water out the back & front of the summerhouse roof. There is a small ventilation port on each side of the roof at the back under the roof overhang.