Outdoor protection for hidden timber

[kenpem]

Hi folks. First time here (be gentle).

I'll shortly begin with my shed build, to my own design. The base of the shed is fourteen 70x70x3000mm bearer beams, half-lapped into each other in a cross-hatch pattern. They'll be held off the ground by adjustable feet (one at each joint), so should never be standing in water. But once the shed is built I'll never again get to these bearers to treat them, so I need to know they're well protected before I build on top of them.

I've looked at some oil-based finishes, as well as things like latex paint, but really don't know what I'm doing so thought I would ask the experts .

All advice & guidance much appreciated.

 

[Harbo]

Soak them in creosote

Rod

 

[Phil Pascoe]

Yes. You can still buy proper creosote in 25ltrs on line. It's worth buying the bulk, because even if you think you don't need much, you'll end up wanting to soak cut ends and so on so you'll need a gallon or two to use a half pint.

 

[kenpem]

Thanks. So by "proper" creosote, do we mean coal-tar based? It looks difficult for Joe Public to buy that. Oil-based creosote is easy to get.

And if I give it a good coating of the good stuff, that'll protect it for a decade or two? I'm over-engineering this shed on purpose so that it needs minmal maintenance. Might even extend it later and turn it into a mini guest-house, so I'm really looking to get things right first time, even if I have to pay a little over the odds in the short term.

 

[Rorschach]

Depending how environmentally friendly you want to be, used motor oil liberally applied over a few days will do a sterling job.

 

[Beau]

Thanks. So by "proper" creosote, do we mean coal-tar based? It looks difficult for Joe Public to but that. Oil-based creosote is easy to get.

And if I give it a good coating of the good stuff, that'll protect it for a decade or two? I'm over-engineering this shed on purpose so that it needs minmal maintenance. Might even extend it later and turn it into a mini guest-house, so I'm really looking to get things right first time, even if I have to pay a little over the odds in the short term.

Try your local farm supplies. Never been asked for any ID when buying it.

 

[Phil Pascoe]

Treat everything you alter in any way - the ends of treated timber as it comes are fine (even so it only costs pence and minutes to treat again) but every cut, shake, joint and screw or bolt hole should be well soaked as well. If you're not certain, do it again. You will see the preservative penetration when you cut a piece - on a square section, the section in the centre not discoloured will be nearly round - this makes you realise how much of "treated" timber actually isn't. On a 6" x 2" e.g. the section will be near elliptical. When in doubt, lag the stuff on.

 

[Jacob]

..The base of the shed is fourteen 70x70x3000mm bearer beams, half-lapped into each other in a cross-hatch pattern. ...

Too small and the half lapping makes them even weaker and less stiff.
Google a few shed designs.
Normal would be deep joists (175mm or more) going one way - sitting on two or three bearers going the other. With two deep enough bearers you could get away with 4 support points; concrete blocks frinstance.

 

[kenpem]

Too small and the half lapping makes them even weaker and less stiff.

Ah OK - even if each half-lap is fully supported from below? My design takes this into account, and all joints rest on ground-support; none are left hanging in the air. I was hoping the 7x7 interlocking design would give plenty of rigidity and stability. Note that every joint is fully glued and dowelled (two fat dowels right through each joint, also glued in place).

 

[AndyT]

I think it will help the timber last if it is in whole pieces without any joints or dowel holes where water can get trapped, leading to decay. You could put screws down through the shed floor into the bearers to stop them falling over. A few temporary bricks would prop them in place while you build it. Whatever you do, there will be moisture under there - you need to let it drain away quickly and easily and let fresh air blow through under the shed.

 

[kenpem]

Thanks lots for all the input, guys. Perhaps a little more detail is in order?

This is what I proposed, although I'm starting to think I've got it badly wrong:

• Sub-frame is 14 of these: http://www.wickes.co.uk/Wickes-Easy-Dec ... m/p/540523 in a 7x7 interlocked lattice,
• resting on 49 of these: https://www.amazon.co.uk/Shedmate-Adjus ... B003L5ENNG , one at each intersection,
• which in turn rest on paved ground

The ground itself is solid and has good drainage, but isn't quite level. Those adjustable feet will sort out the levelling issues while at the same time lifting the timber clear of the ground.

So it won't ever be in standing water, and there's plenty of drainage and air circulation. Of course here in the UK there's no way to avoid the damp, but at least it won't be sitting in a puddle. And then I'm going to build a sturdy shed on top of that lot, so apart from the outer edge of the outside bearers, they won't even really get wet.

 

[Jacob]

You'd be better just laying 7 joists parallel across 3 or 4 bearers at right angles with just a couple of skew nails at each crossover. No jointing required so stronger/stiffer than your lattice.
Then the bearers would only need say 3 to 4 supports each - on to concrete blocks perhaps?
You ought to read up a bit on conventional shed and building design - no point in trying to reinvent the wheel!

 

[AJB Temple]

I agree with Jacob. You are using timbers that are way too small if you really want it to last. Your foundation frame is key. I would not mess about with adjustable feet either - that is just accepting that you can't get the level right. Make some concrete or block pads and set the joists on them off the ground.

 

[Chip shop]

I agree with Jacob. You are using timbers that are way too small if you really want it to last. Your foundation frame is key. I would not mess about with adjustable feet either - that is just accepting that you can't get the level right. Make some concrete or block pads and set the joists on them off the ground.

Added to what the chaps above have said; Construction methods are pretty straight forward. I laid a really cheap & cheerful deck at my gaff last spring (OK, not quite the same thing, but the principle is the same). Made up of 6" x 6" off-cuts of green oak posts, some big old lumps of 9" x 3" native Douglas and some 8" x 2" spruce with a liberal coating of preservative.

Set out and dig post holes. Transferring levels can be done easily enough with a straight piece of timber, some bricks and a couple of bits of slate (oh, and a spirit level...of course). Drop a concrete block or a couple of bricks into the post hole, then drop the post in and back fill with the arising (checking for plumb). Tamp it down with the business end of a sledgehammer. Profiles(the pegs with cross timbers in the below picture) are dead handy for string lines. I set mine 1 metre back from construction with a zero traveller.



Once that's all ready to roll, put your bearers in...I would bolt these to the posts with big arsed galvanised bolts.



Then joists on joist hangers:



Then counter board:



Sorry, I've gone off topic, a bit, here.

Cheers,

Ed.

 

[n0legs]

￡320 just on bearers and levelling feet.
Have you considered concrete?

 

[kenpem]

Wow. Lots of great advice. Thanks.

Digging down for posts etc. - not really an option as the area is paved already (although not completely level).

Concrete - ummm that's way outside my experience. I'm relatively comfortable working with wood. I guess I could give it a go. I mean, how hard could it be? That would at least give me a flat, level foundation to build on. But I'd still then need bearers, and still need to make sure everything's level.

Another question - the sub-floor (there are two floor layers), which is exposed to the elements underneath - will be either plywood or OSB. I don't want to kill my budget with marine-grade stuff - should I just lash on the creosote there too?

 

[AndyT]

Your creosote will do more good on your floor than it will left in the can, so I would, yes.

You just have the minor challenge of getting it on to the completed floor then turning it over but with gloves and a helper or two that should be ok. I wouldn't want to be assembling creosoted components as it's nasty messy stuff if it's still at all wet.

 

[Jacob]

Wow. Lots of great advice. Thanks.

Digging down for posts etc. - not really an option as the area is paved already (although not completely level)....

You just need four concrete blocks. Lay two bearers across; level them up with bits of packing, then joists going the other way. They'll all be out of the weather when you've built the shed and won't need timber treatment - as long as the gap is well maintained for air to circulate underneath.

 

[kenpem]

...level them up with bits of packing...

as in wooden shims?

Excuse my ignorance!

 

[Jacob]

...level them up with bits of packing...

as in wooden shims?

Excuse my ignorance!

You could level the blocks themselves on pads of mortar, with or without bits of slate etc.
Or you could level the timber on to the blocks with bits of packing - slate etc, whatever is to hand.