Will this work - any thoughts please...

[Zeddedhed]

I've been asked to make a table (outdoor) from 94 x 44 sapele.
The customer has asked for the top to be made as the image shows.
Anyone got any thoughts as to why I shouldn't (apart from the aesthetics).

 

[Phil Pascoe]

I'd have thought it would pull itself to bits.

 

[undergroundhunter]

If there are no gaps in the top and gets water on it it will just sit there and cause damage to the joints. I agree with the above also it will just destroy itself.

Matt

 

[wizard]

It will be ok if you keep it dry and away from sunlight

 

[Racers]

If the outside ring is glued up and the inner ones where loose with tongue and grove joints with space to expand it would be o/k, otherwise it would pull its self apart.
But it wouldn't be very strong and would tend to sag under its own weight, some sort of an under frame to support it would help.

Pete

 

[deema]

Depending on your client, I would not make it completely out of solid wood for the reasons mentioned. I would select a good stable base material, and them simply laminate 10mm thick or so strips of the chosen wood on top of it. The edge pieces I would make the full thickness and just rebate out to allow the base to sit inside and be hidden from view. This then gives a proper loom to the edge rather than having a laminated edging. The appearance as a whole would be of a solid wood table. If the client does not want to see the base material underneath, again I would just laminate on the chosen material.

I believe it would be a very quick build and offer your client a quality product that should be stable.

 

[Sgian Dubh]

I'd have thought it would pull itself to bits.

Not quite right phil.p. It will definitely fail.

Zeddedhed, what is your counter proposal, one that will work, that perhaps incorporates some of the visual appearance shown in your sketch? Slainte.

 

[Zeddedhed]

Thanks for all the responses so quickly guys.
I presume then that I'd be better off going for the normal style table top - all the boards in one direction with the exception of breadboard ends.
I'd like to know what the reason is that you all say it will self destruct - not that I doubt you - I just want to understand.

Sgian, I'm afraid I don't have enough knowledge of how wood moves to come up with anything other than a normal table top.

 

[Sgian Dubh]

Depending on your client, I would not make it completely out of solid wood for the reasons mentioned. I would select a good stable base material, and them simply laminate 10mm thick or so strips of the chosen wood on top of it. The edge pieces I would make the full thickness and just rebate out to allow the base to sit inside and be hidden from view. This then gives a proper loom to the edge rather than having a laminated edging. The appearance as a whole would be of a solid wood table. If the client does not want to see the base material underneath, again I would just laminate on the chosen material.

I believe it would be a very quick build and offer your client a quality product that should be stable.

For an outdoor table that would almost certainly fail too, because I'm assuming you're talking about fixing thin solid wood sections down onto something like plywood. If that's the case, it won't stand up to the rigours of exterior conditions. What is needed is something that allows each piece of wood to move independently from every other piece, and for water to run off easily. Slats, maybe 50 - 150 mm wide incorporating gaps between each slat do work. The slats can be contained within an outer framework, where frequently they are tenoned into the edge of the frame (or similar) or simply fixed individually to stout bearers without an encircling framework. Long and wide surfaces made of solid wood just don't work - rain, sun, snow, freezing conditions, etc, cause large cross grain movement and distortion, which leads to rapid failure. Slainte.

 

[Chrispy]

If each piece were tenoned at each end where it met the next slat and a gap left at each side to allow for movement and the drainage of rain water, I think it would work OK, sapele is durable and usually quite stable so should be a good choice of wood as well.

 

[Sgian Dubh]

I don't have enough knowledge of how wood moves to come up with anything other than a normal table top.

The solution is simple. Take your original sketch and incorporate gaps so that each piece is independent of the next. M&T the individual parts together, perhaps incorporating draw-boring of the critical structural elements. Essentially a series of diminishing frames within frames. I think Chrispy is pointing you in the same direction. If you can't work it out for yourself I can offer a design service ... for a fee, ha, ha. Slainte.

 

[Zeddedhed]

OK. I get the concept, and not too difficult to do. I'll be using an epoxy resin and dominos so that should be fine.

What is the reason for it pulling itself apart though? I've got a conventional tabletop outside made from Oak glued up with epoxy and domino's together thats been outside for three years and is still tight as a drum. It looks a bit tatty but the joints between boards (and the BB end) are still fine.

 

[siggy_7]

Wood grows at different rates in different directions. With the grain (ie fibres) there is hardly any dimensional change with moisture content. Radially and tangentially there is a strong dimensional change with moisture content (swelling and shrinking). Your design mixes lots of end members rigidly fixed to the pieces running up and down the table - the longer pieces will want to expand widthways but the end pieces will restrain it from doing so, until the weakest parts fail from the high stresses involved (which, if you can make joints strong enough, will ratchet up and exceed that of the wood itself).

 

[Sgian Dubh]

OK. I get the concept, and not too difficult to do. I'll be using an epoxy resin and dominos so that should be fine.

For a table destined for external use the ideal timber is air dried stuff at about 16- 22% MC because this will be fairly close to the MC it will experience in service. I imagine the proposed sapele has been kiln dried and is hovering somewhere near 10 - 12% MC. This is okay as long as you allow for the inevitable cross grain expansion during construction. Personally I'd avoid the dominos for the joinery because I'd want to mechanically lock the joints with something like draw-boring a tenon on the end of one member into the mortice of its partner, which I think would be very hard to replicate with a domino.

What is the reason for it pulling itself apart though? I've got a conventional tabletop outside made from Oak glued up with epoxy and domino's together thats been outside for three years and is still tight as a drum. It looks a bit tatty but the joints between boards (and the BB end) are still fine.

I suspect you've been lucky more than anything else. I would never recommend such a construction for an exterior table top, nor would I agree to make one like that, unless the client was willing to accept all responsibility. The problem with that sort of arrangement is the client usually forgets what they've agreed to and bad-mouths the maker anyway when things go wrong, which is not good for the maker's reputation or business. This kind of thing can happen even if the arrangement is in writing and signed by the client.

I think siggy_7 has given you a reasonable description of the problems likely to occur if you went ahead with the construction shown in the drawing in your first post. Slainte.

 

[Zeddedhed]

Thanks guys. I've always found this forum tremendously useful for gaining knowledge and getting advice, and this occasion has been no exception.
I understand the basics of timber movement, but my experience has not always been matched by the theories - as Sgian says, probably luck more than anything else.

I'm all too familiar with the situation described where clients forget the warnings they've been given and then complain - it's happened a few times which is the exact reason for the original question - I want to be able to give my client good advice and fair warning of the potential pitfalls in going ahead with their design.

I think I'll suggest a more traditional top - boards all in one direction but with gaps between and the ends tenoned into breadboard ends.
Would it be best then to oversize the mortices (hidden behind shoulders of tenons) to allow for expansion or just the usual tight fit and draw bore them?

 

[Sgian Dubh]

Would it be best then to oversize the mortices (hidden behind shoulders of tenons) to allow for expansion or just the usual tight fit and draw bore them?

Make the mortice about 2 to 4% longer than the width of the tenon, e.g., 100 mm wide tenon leads to a mortice ~ 102 - 104 mm long. This assumes your wood is kiln dried, therefore probably at something like 12% MC, and likely to expand widthways quite a bit after the table is made and placed outside. A single draw-bore should be sufficient, although you could use two in a tenon the size I've described, set fairly close together. Install the trunnels from the underside but stop them short of the top surface by 6 or so mm. That's one less place for surface water to get in, which would add to the causes of wood deterioration. Slainte.

 

[Zeddedhed]

Thanks Sgian. That all makes sense (although I had to google 'Trunnels')

One last question: as we're trying to avoid the anticipated movement from causing damage would it therefore be better NOT to glue the M&T joints but to just rely on the 'tunnels'?
Presumably if the timber can move enough to pull the joints apart as pointed out earlier in the thread then it would also wreck any glue joint? Am i right or barking up the wrong tree?

The top would be supported by bearers underneath and these would be screwed into the underside of the top 'slats' through slots in the bearers rather than holes - allowing again for movement.

 

[Sgian Dubh]

Ultimately the joints will have to rely upon the mechanical locking of trunnels or pegs because eventually the glue will give out because of its degradation through a combination of wood movement, water ingress, mould, UV light, etc, but gluing the joints would be normal. I like polyurethane glue for external furniture. I don't use it much for any other kind of furniture, but it's one of the only glues that performs well in humid environments.

On a side note polyurethane glues are also good for gluing up air dried wood destined for external joinery. The reason for that is that it's one of the few glue types that will work on wood that is wetter than 20% MC, which is often the case here in the UK for air dried wood. All the other glues will only work on dry wood. Dry wood, by definition, is wood that's below 20% MC. All the other common wood glues, such as PVA, aliphatic resin, urea formaldehyde types, epoxy resins, etc, fail if used on wet wood, but many of them work reasonably well if the wood becomes wetter than 20% MC some time after the joinery and glue-up, especially if you select moisture resistant versions of the glues mentioned. Slainte.