garden seat, screw fixing, wood movement?

hi folks first post. i've read a lot of the posts here about wood movement and understand that this needs to be taken into account especially in respect to the width aspect. i have a question on how much spacing is recommended between seat slats on a garden bench. the plans are online at http://www.buildeazy.com/seat.html i have all the wood cut, and when i did a trial fitting i found it more aesthetically pleasing with the seat slats positioned tight together. the design recommends a 1cm gap. the plans use 2x4 inch unspecified exterior wood, i chose 6x1.5 inch mahogany (couldn't get 2x4). the climate is dublin = generally mild, not too hot, or cold, or humid. the reason i ask is because the instructions with the plans are just to nail the planks in to the perpendicular seat supports. surely this wouldn't be great for tolerating wood movement? i.e. if the wood gets wider with humidity, it will be pulling the nails out sideways. i intend to use three 100mm screws to give extra strength to fixing the planks to the seat supports, since the piece will be extremely heavy and people will invariably try and lift/move the bench by the slats. i wouldn't trust nails to hold it together well over time. since i've spent a few quid on the wood, there's no point risking damage to the wood just because it would look nice to have the boards together. but i'm wondering what's the expert opinion? on other benches, i sometimes found a 1cm gap slightly uncomfortable for sitting on for long-ish periods of time, especially on the back if it's sunny, no t-shirt etc., aesthetically (and for comfort) i would prefer no gap, but then as necessary i would hope for a smaller gap than the recommended 1cm. any ideas? not using any glue or anything, just bolts for the stress points, and screws to fix the slats. my main worry is that even putting 3 screws down across a 6 inch board could be too restrictive for the wood movement, but i have no hands-on experience with it so don't know if i'm making too big a deal out of it.
many thanks in advance. tim
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Gap is too allow the wood to swell and shrink.I woul dhave at least a little gap to allow for swelling and to let rain and dew drain off. A little air gap may also help the wood dry out faster which might be less swelling. Screws would be less likely to pull out. Use either galvanized screws or outdoor treated wood screws that seem to be powder coated with a weather resistant coating.

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1 cm is really too wide as you've found out. I generally use 4 mm - 5 mm but anything between 3 mm and 6 mm for a typical 10 cm - 15 cm wide slat should be fine. It will also be important to finish this with several coats of a marine varnish with plenty of UV blockers. You may want to get that from a boat supply store rather than standard woodworking channels. The premium boat varnishes hold up better than ordinary "outdoor" varnishes in direct sunlight.
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It's all relative- spacing and R.H., but you knew that. :') Actually, it depends on how the wood's stabilized to local conditions. E.g., put it down tight and oven-dry, and guess what?
You want the pieces spaced so they'll always have a gap.
I'd round-over all the edges to minimize pinching, and for appearance. And ... pre-drill for what're known over here as "deck-screws"- coated, combo-square-and-phillips drive, made of much tougher steel than hardened drywall
HTH, J screws.
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Tim_Mac wrote:

Too small of a gap can trap pine needles and such and collect dirt. I don't think you'd want to go with a gap smaller than about 3/16" (that's the old measuring system you guys used to use before you caved in to peer pressure - and common sense). When laying down deck boards, particularly pressure treated, they can be butted up against each other because they will shrink. If your mahogany is kiln dried it may very well expand, so a tight gap might become no gap.
I would use ceramic or epoxy coated deck screws, two per board. Three fasteners can easily lead to stress concentrations that will tend to split the wood over time.
Oh, and wear a T-shirt. The neighbors have been complaining. ;)
R
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hey guys, wow. thanks for the great replies. i'm already armed with 2 tins of Rustin's marine varnish which should do the job nicely. i got some wood preservative as well which i intend to put on before, apparently it's good for preventing wood worm and fungi etc., i'd be impressed if a worm could get through 3 coats of the varnish but i'm going to do it anyway, i want the bench to last forever so putting in an extra hour or 2 is well worth it. i will be burying the screws about 5mm below the surface with wood filler to keep them out of sight on my lovely boards. and i think i'll go for the 5mm gap, that seems to be the general consensus. 2 screws sounds like a good idea, thanks for the tip Ricod. we get so few sunny days here that i like to make the most of it when i can! hopefully i won't get any ASBOs :) http://en.wikipedia.org/wiki/Anti-social_behaviour_order
cheers tim
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<<<<<<<<<<<<<< SNIP >>>>>>>>>>>>>>>>>>>

Tim:
Suggest you think about counter boring and plugs vs wood filler. If you glue (water proof glue) tapered plugs in the counter bore and then clip (sharp chisel will do it) and sand them level, they will last forever and look far nicer than wood filler, especially if you align the grain. There are plenty of sources for plug cutters out there, just make them from off-cuts.
5mm spacing sounds good for mahogony on 6" width. It doesn't move as much as some.
Regards.
Tom
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Thin your first coat of varnish atleast 1:1 with minearal spirits or turpentine. This will help the first coat penetrate the wood better. Especially on end grain. I generally add some tung oil or boiled linseed oil into this first coat also. You can apply subsuquent coats straight but I prefer to thin them some also. Many thin coats last a lot better than a couple of thick coats in my opinion.
It is a good thing you built the bench to last forever because Spar Varnish takes forever to dry ;-).
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Tim_Mac wrote:

Wood filler?! Counterbore and plugs would look vastly superior.
You should figure out how you want to treat the bottom of the legs. End grain is tough to seal permanently when it is scraping along as it's moved around the patio. Wood doesn't know it's dead and keeps trying to suck up moisture. The chair sitting on a wet patio or lawn will wick up moisture, discolor the wood, get under the finish and start rotting the wood.
R
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