Thread: Wood movement (slabs)

I hate slabs, and this is part of why. Slabs crack from differential shrinkage (as you've mentioned), but localised stresses in the tree, drying splits that walk, the presence of heart etc can all be responsible for cracks and splits too.

Ripping and rejoining is the best way around this problem, and while theres some guys out there who might get offended by this comment - dammed near every professional cabinet works I know does exactly that on really wide ones... rip and rejoin. Because done right it's almost invisible and it's far more stable in service which means greater customer satisfaction down the road. Mind, there are few things better then a good slab tabletop done well... it's just there's not a lot of really good wide ones out there either.

Which leads me to saying that actually cutting slabs is pretty pointless. Just cut and number them (1A, 1B, 1C, 2A, 2B etc etc) as planks, and rejoin them later. Easy to handle, easy to store, easy to KD if you so desire. I've been trying to convince people to do that for years but alas - people just love the dammed things.

Nick Mosey

An opening disclaimer - most of my woodworking experience has been in the US - New England - and I only have about 6 months acquaintance with Australian timber, & that in the subtropical rainforest climate of Lismore..

You may have found cracking starting at the cut ends - the exposed end grain dries out faster than the rest & sometimes very rapidly, shrinking as it does so, causing cracks. Sealing the end grain (wax or paint) will help. It's also important to store with care - both faces of a slab should be more or less equally exposed or the piece will cup. Wood with irregular or highly figured grain will also tend to crack around the figuring - uneven & irregular stresses on drying.

Stickering any stack is highly advisable & watch out for sun coming through the workshop window (I speak from bitter experience).

As John G said for stability & to minimise cupping ripping & rejoining is - but the allure of the bookmatch can be irresistible (it has been for me). Certainly cutting out the centre pith is highly desirable.

Good luck.



At the end of the day we all will eventually be forced to accept that one reason for the existence of wood is to keep our egos well in check.

Originally Posted by John.G

I hate slabs, and this is part of why. Slabs crack from differential shrinkage (as you've mentioned), but localised stresses in the tree, drying splits that walk, the presence of heart etc can all be responsible for cracks and splits too.

Ripping and rejoining is the best way around this problem, and while theres some guys out there who might get offended by this comment - dammed near every professional cabinet works I know does exactly that on really wide ones... rip and rejoin. Because done right it's almost invisible and it's far more stable in service which means greater customer satisfaction down the road. Mind, there are few things better then a good slab tabletop done well... it's just there's not a lot of really good wide ones out there either.

Which leads me to saying that actually cutting slabs is pretty pointless. Just cut and number them (1A, 1B, 1C, 2A, 2B etc etc) as planks, and rejoin them later. Easy to handle, easy to store, easy to KD if you so desire. I've been trying to convince people to do that for years but alas - people just love the dammed things.

Thanks for the replies. I'm assuming that when you say rip and rejoin that you have multiple slabs cut directly beneath one another and are re aligning the boards so that the rejoined slabs contain only quartersawn or backsawn boards? As illustrated in this thread. https://www.woodworkforums.com/showth...=slabs+popular

Say for instance i have the one dry slab. If i was to rip this into boards and join the boards back in exactly the same way they were taken from the slab, would this be a wasted exercise?

I guess at the end of the day it comes down to what the customer is after. If the customer wants a single slab table then they must be warned that cracks will eventually appear. If they are after a crack/defect free top then the rejoining of the slab is a must.

If i have a slab that is dry and has no cracking in it, can i expect cracks to appear over time even if i have lacquered both sides? I guess it would come down to what is going on with the climatic conditions.

I always see slab tables that have had the cracks and defects filled with epoxy. I understand that with the defects this is fine but wouldn't the cracks open up over time and the epoxy need regular maintenance? Once again assuming that the slab has been lacquered both sides and the table is for indoors.

Originally Posted by Nick Mosey

Nick Mosey

An opening disclaimer - most of my woodworking experience has been in the US - New England - and I only have about 6 months acquaintance with Australian timber, & that in the subtropical rainforest climate of Lismore..

You may have found cracking starting at the cut ends - the exposed end grain dries out faster than the rest & sometimes very rapidly, shrinking as it does so, causing cracks. Sealing the end grain (wax or paint) will help. It's also important to store with care - both faces of a slab should be more or less equally exposed or the piece will cup. Wood with irregular or highly figured grain will also tend to crack around the figuring - uneven & irregular stresses on drying.

Stickering any stack is highly advisable & watch out for sun coming through the workshop window (I speak from bitter experience).

As John G said for stability & to minimise cupping ripping & rejoining is - but the allure of the bookmatch can be irresistible (it has been for me). Certainly cutting out the centre pith is highly desirable.

Good luck.



At the end of the day we all will eventually be forced to accept that one reason for the existence of wood is to keep our egos well in check.

Thanks as well Nick, very helpful.

Originally Posted by Sambo1111

Thanks for the replies. I'm assuming that when you say rip and rejoin that you have multiple slabs cut directly beneath one another and are re aligning the boards so that the rejoined slabs contain only quartersawn or backsawn boards? As illustrated in this thread. https://www.woodworkforums.com/showth...=slabs+popular

Say for instance i have the one dry slab. If i was to rip this into boards and join the boards back in exactly the same way they were taken from the slab, would this be a wasted exercise?

That's exactly the process I'm talking about. Never a wasted exercise... not only will the joints give a place where it can move if it needs to, it also makes removing the cup from a large slab less wastefull.

I guess at the end of the day it comes down to what the customer is after. If the customer wants a single slab table then they must be warned that cracks will eventually appear. If they are after a crack/defect free top then the rejoining of the slab is a must.

If i have a slab that is dry and has no cracking in it, can i expect cracks to appear over time even if i have lacquered both sides? I guess it would come down to what is going on with the climatic conditions.

I always see slab tables that have had the cracks and defects filled with epoxy. I understand that with the defects this is fine but wouldn't the cracks open up over time and the epoxy need regular maintenance? Once again assuming that the slab has been lacquered both sides and the table is for indoors.

Theoretically, if it's properly sealed both sides then moisture can't get in (or out) so movement should be a none issue. There would still be a period of equalisation through the timber though... unless the timber was seasoned in a controlled environment prior to sealing there will always be some degree of gradient across thicker pieces.

Excellent,

I am of the opinion that a lacquered finish doesn't completely seal the timber it just slows down the effects of changes to the climatic conditions?

Originally Posted by Sambo1111

Excellent,

I am of the opinion that a lacquered finish doesn't completely seal the timber it just slows down the effects of changes to the climatic conditions?

Correct. No finish short of an inch thick plastic coating will truly "seal" the timber.



One other option for slabs when used in situations such as counter-tops & coffee tables is to rip stopped trenches in the undersides and mount them onto a suitable carcase. In a 2" thick slab I'd rip the trenches about 2" apart at around 1 1/2" deep, leaving a 1/2" thick solid top.

This does NOT prevent cracking, although it certainly reduces the risk... however it does prevent the majority of cupping, twisting and warping.