Thread: When to rebate table top

Hi Lappa,
Either method is acceptable as to accuracy that depends on you either requires the same attention to be accurate the only difference to me is that one you have to clean out the corners of the rebate after joining.
Regards Rod.

If you intend to laminate some old floor boards together and then capture them within a mortised frame then you may need to think about expansion/contraction. The floorboards will expand across their width with atmospheric changes. The frame in which they are sitting will alternately be liable for splitting when their expansion is at their greatest, or showing unsightly gaps when at their least.

Cheers
Arron

Originally Posted by Arron

If you intend to laminate some old floor boards together and then capture them within a mortised frame then you may need to think about expansion/contraction. The floorboards will expand across their width with atmospheric changes. The frame in which they are sitting will alternately be liable for splitting when their expansion is at their greatest, or showing unsightly gaps when at their least.

Cheers
Arron

Thanks for the reply. I followed a design used on some up market outdoor tables I have seen that used recycled timbers eg. Teak.

These floor boards I'm using were on an outdoor veranda for 30 plus years that got wet and got full sun and only ever showed minimal movement even though they were not oiled or coated. Now machined, they look like new timbers

I'm interested in why you feel there may be a problem as I may need to modify the design as I haven't started building yet.

BTW the table will be under a roofed pergola.

thanks

Originally Posted by Lappa

Thanks for the reply. I followed a design used on some up market outdoor tables I have seen that used recycled timbers eg. Teak.

If they are quality pieces, then I suspect the maker did allow for expansion/contraction (e&c), even if it is not immediately obvious how.

Originally Posted by Lappa

These floor boards I'm using were on an outdoor veranda for 30 plus years that got wet and got full sun and only ever showed minimal movement even though they were not oiled or coated. Now machined, they look like new timbers

Where they were situated isn't that relevant. E&c will be just as bigger problem indoors, in apparently stable conditions.
Anyway, if the timbers were used on a verandah, then I suspect there were gaps between, which alleviated the problem.

Originally Posted by Lappa

I'm interested in why you feel there may be a problem as I may need to modify the design as I haven't started building yet.

BTW the table will be under a roofed pergola.

thanks

What you are building seems to me to be in principal much the same as what I have drawn in the attached file.

The problem is that wood expands across its width, but expands very little along its length (in fact so little we usually just ignore it). Therefore, the boards c,d,e,f,g will be continually expanding and contracting across their width. The problem is that they are trapped by the frame - and boards a and b will not be expanding long their lengths. With nowhere to go as they expand, c/d/e/f/g will simply split apart the frame. The forces involved are immense and no m&t joint can stand up to it. It doesn't always happen - sometimes I've don't it wrong and been lucky - but usually it does.

What you need to do is design to allow the e&c to occur harmlessly. Usually it involves leaving gaps somewhere. I've assumed the boards on your table top will be close butted, being old floorboards. Also, its not really desirable to leave gaps on a table top, unless you want grooves that fill up with rubbish. Another solution is to use a floating panel - where the panel is free to expand into rebates in the enclosing frame. There are all sorts of clever, googleable ways of doing this.

cheers
Arron
e&c.jpg

Thanks again Arron. I dug up and read through a reply from the Heritage Dept I contacted when I was looking at renewing my veranda floor with t&g bosrds and it stated the gaps between boards could be minimal as it had room to expand both in length and width due to the two open sides. I can see your point re e&c.
Looking at the floating idea you suggested (thanks), it would appear that there has to be a visible gap somewhere, either between the boards or around the perimeter if the boards are pinned, so I guess I'll have to choose which one is less unsightly

thanks again for your advice.

Peter

There are ways to eliminate the visible gap. I think Alexs has done a post or two on here of a method he uses whereby the panel is higher then the frame. The panel has a rebate into which a tongue on the frame sits (and can slide). You dont get a gap but you do get a step at the edge. Do a search under Alexs - probably in the boxmaking forum.

Also read about breadboard ends. They are an attempt to get past this problem, and may be adaptable to your needs somehow.

Otherwise, yep, its just a lmitation you have to work with. Most commercially-made furniture would have a veneered ply or mdf panel in the middle, which can be treated as effectively zero e&c. This makes it easy, and partially explains the move to flush, clean-lined furniture over the last 60 years or so.

Great project - I'm looking to do a table top one day, so given it some thought, but still need the courage (and a couple more tools).

Have you considered all top boards to be parallel (lengthways) with the cross pieces underneath at a second level? The table I'm looking at replacing is from old floorboards, with the door type frame you've drawn completely underneath as supporting structure, visible only around the edges. The top looks about 32mm thick, but is only 16 for most of it.

Originally Posted by wpeter5401

Great project - I'm looking to do a table top one day, so given it some thought, but still need the courage (and a couple more tools).

Have you considered all top boards to be parallel (lengthways) with the cross pieces underneath at a second level? The table I'm looking at replacing is from old floorboards, with the door type frame you've drawn completely underneath as supporting structure, visible only around the edges. The top looks about 32mm thick, but is only 16 for most of it.

My wife liked the design she saw in a shop so who am I to argue besides, I liked it too

After great advice from the forum guys, I've slightly changed the design to take care of any expansion. I'd like some feedback. I've done a mock-up to test the procedure.
The gap between the boards will be 1.2 or 1.6mm (nail widths) which, with the 10 boards (81mm wide) will give an overall expansion tolerance of 13 to 18mm approx.
The head height for expansion in the "groove" is 1 mm.

image.jpgimage.jpg

Let me know what you think, members.

Peter

Thats probably way more gap then you need.

If you think about a hardwood floor, the rule is to leave a 10mm gap around the perimeter of each room - and expansion gaps only over 6m continuous. Although thinking about it, I dont know how much you can read into this because floors are usually laid with a slight looseness to accommodate movement, and with new timber floors its usually shrinkage that is far more significant.

I think I remember something about every sixth board being laid a little loose - when doing a new floor.

Anyway, I guess if I was making it and had to estimate the needed gap (and it is only an estimate based on a few similar things Ive done - so dont rely on it) Id be aiming for 4mm over the entire 800mm.

Arron

Thanks Arron,
I remember, when I was looking at tongue and groove for the front veranda, a 1% figure mentioned and the guy from the Heritage Dept said allow a "credit card" between boards.
Guess I'll have to find some thinner nails for spacers

Actually, its 1% for each 4% change in moisture content.

I didnt mention this because I've no idea how likely a 4% change in moisture content is - or is 8 or even 12% possible. No idea.

Yeah, I know, we are overcomplicating it.

Cheers
Arron

Originally Posted by Lappa

Thanks Arron,
I remember, when I was looking at tongue and groove for the front veranda, a 1% figure mentioned and the guy from the Heritage Dept said allow a "credit card" between boards.
Guess I'll have to find some thinner nails for spacers

When I need consistent spacers I use laminate samples. Make sure you use the double sided samples rather than the single sided as they tend to curl a little over time. The thicknesses can vary slightly but if you grab a few of the same sample they are great.

It depends on the change in humidity, the species and the section of the wood. For Jarrah my tables tells me that a change from 5% to 50% humidity would produce about a 2% dimensional change regardless of the section. Slightly more for Flat as opposed to quarter but if you allow for 2.5% you should be ok. So for each 100mm section allow about 2.5mm. If you live in a place such as Sydney where you get extended periods of humidity, or worse Darwin then allow somewhat more but in Adelaide or Perth you should be able to get away with less.

John