How waterproof is a piece of 'treated' timber? The kind you get in B&Q for fences and outdoor work etc? I mean it's clearly not going to let water through in liquid form, but will it soak up water and transmit moisture?
Thinking of using some for my caravan repair.
I wouldn't expect timber fencing* to be in the least bit "waterproof".......just rot resistant.
EDIT : * or any treated structural timbers.
For caravan repair you need good quality hardwood ply, planed timber etc with waterproof adhesive. For external repairs you need UV stable adhesives such as Sikaflex and Tigerseal, and non-setting mastic for around windows and skylights.
You might also try dabbling in fibreglass, never tried it but it must be pretty foolproof as my brother has managed to make some stuff to bodge an LCD into his car dashboard. Its common to use it to repair rusty and leaking roofs on commercial vans.
Marine ply?
Marine ply is OTT, you just need WBP, but go for the higher quality hardwood plys, the cheap stuff just absorbs water like a sponge and falls to bits (I found out the hard way after replacing a horsebox ramp, painting it with £100 of rubberised paint, and then replacing it six months later when the 12mm ply stared to break apart like polystyrene)
Bear in mind that water should not be coming into contact with any of the ply anyway. Ally sheet or fibreglass should cover it.
I don't want ply. This is to replace a rotted out member. The interior ply I will fix later, maybe, if I can be bothered.
So it'll need to be a piece of 2x4 maybe. I should add that this is simply a bodge for a pretty old cheap thing. The trip strip will be going back on and sealant applied, as it was originally.
It will probably be exposed to road spray from underneath. I just want to know if dampness will seep through a piece of timber or not.
I just want to know if dampness will seep through a piece of timber or not.
Unless you treat it with something which is waterproof, the answer is yes.
get tanalised timber (warning contains arsenic)- probably the best for your use.
It's the only stuff that survived any length of time in our beach hut.
Ernie, that is really the question. I can assume from your reply that the pre-treated stuff is not in fact waterproof or water resistant?
Just googled for tanalised timber - it appears it it in fact the greenish stuff that you see in b&Q
Nope, it can get sopping wet. It shouldn't rot for a long time though.
Hmm. Maybe I'll paint something on from underneath then when done.
the treated timber won't be water proof, but the treatment should mean its more able to cope better with getting wet. A coat of paint wouldn't hurt but if its under the caravan its more important that it can dry out between episodes of getting wet than be waterproof, if dampness is trapped anywhere thats where it'll suffer. Given that the original timber has rotted it may be inherent in the design that moisture is trapped there [ EDIT or it may reveal something about the environment where the caravan is stored - on badly drained ground or surrounded by a lot of foliage ]
Does the bit of timber its replacing look like pine, or is it a more resilient hardwood?
Take a close look at timber you buy from B&Q - some of it is jointed together from lots of short lengths and isn't something you'd want to be standing on. If its under a floor then look for timber that is marked with 'C16' stenciled onto the wood- thats going to count for more than any rot treatment when you are jumping up and down on it
Oh dear, there's nothing worse than
a rotted out member
IGMC
its more important that it can dry out between episodes of getting wet than be waterproof, if dampness is trapped anywhere thats where it'll suffer.
Absolutely spot on........and something which a lot of people don't realise.
Even untreated softwood won't rot if it is allowed to dry out properly and often enough. The drying out will kill fungi/bacteria and stop it from getting a hold.
Water..........even more essential to life than oxygen.
Of course whilst your treated timber might not rot, it can still transmit moisture, so if that is a problem treat it to make it waterproof. Otherwise don't worry.
its under the caravan its more important that it can dry out between episodes of getting wet than be waterproof
Ah, interesting. It's protected by the body work so won't be continually exposed to moisture.
Does the bit of timber its replacing look like pine, or is it a more resilient hardwood?
It looks more like a chocolate cookie than anything else 
I hope these pics make sense. Here's how I think it looks at the moment. Brown = wood, black = metal, blue = insulation, grey spotty = rotted wood, red = the problem member
van problem by molgrips, on Flickr
This is what I propose to do. A somewhat larger red bit so that when I replace interior ply afterwards it'll have something to sit on/be bolted to.
van fix by molgrips, on Flickr
I haven't drawn the screws on, but the trim strip is screwed through the red bit into nothing much at all currently, which is why I have a problem - that corner of the van is unsupported. I need to add a new red bit so I can put some long screws through it into the solid piece behind. Access to be gained by removing lower bit of trim.
Molly, was it you who did the D&B drawings for your man's porch what fell off?
you need to cut all of the old rot out, leaving any in will just mean more problems (and quite quickly).
Hmm.. it spreads even when dry? I did think about doing the interior ply at the same time.. hmm.. won't be too hard if I can remove the seat. But then again.. I've got hardly any tools, no spares, hard to shop for stuff, and am working on the roadside 
Not sure if it's worth it though. Just needs to be structurally sound enough to get back to the UK. Question is, to scrap or repair? Last winter was not good to it.
working on the roadside
In Germany 
You'd better check with your neighbours.
Looking at your pic I'm wondering whether rain is running down the outside of the caravan and seeping in through the top edge of the trim (which appears to be covering a horizontal joint in the cladding) and getting soaked up by the wood. While the trim and cladding could be letting moisture in it won't really be letting it back out again quite so easily - a sort of reverse goretex.
really the upper sheet of cladding wants to be overlapping the lower sheet rather than being a flat joint, with or without the trim covering it. When you fit your new timber perhaps something flexible can be hooked under the upper sheet and over the lower sheet so moisture can't run in
if you're just looking to make something sound enough to survive one journey then why not just build a frame inside that will screw into good wood away from the rotten area and provide support?
"you need to cut all of the old rot out, leaving any in will just mean more problems (and quite quickly)"
is of course not true..
Seems like you do need to fix the source of water. However that might not be a leak. Caravans are ideal sources of really bad condensation since they have a external "vapour barrier" (metal skin) in precisely the wrong place. Chances are it's condensation from the inside of the alu skin which runs down anc can't get out.
Looking at your pic I'm wondering whether rain is running down the outside of the caravan and seeping in through the top edge of the trim
I think you're right in this case. I only first noticed this when the trim actually fell off over the winter. Having examined it, there was loads of sealant stuffed behind it which was all crumbly so it looks like it'd happened before and someone's attempted a bodge which finally failed again.
if you're just looking to make something sound enough to survive one journey then why not just build a frame inside that will screw into good wood away from the rotten area and provide support?
Had thought of that, but I'd rather not have a line of screws visible outside the van. I might hope to get at least a few hundred quid for it or a few more weekends in the autumn if we end up waiting to buy a new one.
My earlier comments assumed you were repairing the ceiling or walls.
I was going to suggest the same as maccruiskeen, the underneath does not need to be waterproof, commercially built caravans and motorhomes often have a bare ply floor. It'll get wet from road spray once every few months, but will dry out again. Some dealerships will offer to underseal your new caravan but the benefits are doubtful.
I wouldn't bother doing much work unless you strip out all the rotten/damp wood, fix the leak and then start rebuilding. Beware, the damp can just go on and on...
If the caravan has some resale value it might be worth bodging it to get it back to the UK, but otherwise, scrap/flog it when the time comes to leave?
Water..........even more essential to life than oxygen.
I wouldn't bother doing much work unless you strip out all the rotten/damp wood, fix the leak and then start rebuilding
That's kind of what I was thinking. I think I'll make it all secure so it doesn't fall to bits on the autobahn, then when I get home I have the option of repairing it properly or just not bothering.
When it was new it was a high spec, and everything inside still works perfectly - heating, leccy and gas hot water etc etc. It's possibly worth breaking for spares on ebay actually.
I think I'll make it all secure so it doesn't fall to bits on the autobahn
Wood hardener will stop the problem getting worse, allow you to screw into soft rotten timber if that is necessary for fixing any trim, very easy to apply, and will make the wood waterproof.
H2O ..........even more essential to life than O2.
(thank you mastiles_fanylion)
Hmm.. wood hardener.. would have been useful had I not already started clearing it all out.. bummer.
All the plumbing/air heating/hot water heater/toilet/shower will go for big money on ebay as self build motorhomers will snap them up. Paid iver £200 for my second hand water heater. Chassis will end up as a boat trailer, you may even manage to sell the interior cupboards.
Hmm, sounds like a better bet than fannying around with the damp then
Update:
Removed the bottom piece of trim and as predicted, most of the red piece fell out, the rest I could just pull out. It's only about 20x25mm though which makes me wonder about the quality of Mardon caravans. Perhaps that's why they went bust
Anyway the bottom piece of trim and the caravan wall are both screwed into the floor just with countersunk screws forced through the alu skin. So there are two rows of screws into the same bit of timber - one near the top and one near the bottom. How flippin shoddy is that? No wonder it broke apart! Plus the bit of ply that the bottom trim is made from is narrower than the red bit, so the two bits of alu skin don't even line up even when new.
This why I want an all metal Bailey!
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