I have come across this myth several times – that sapwood should be avoided at all cost. In some cases it might indeed be true – but sapwood does not deserve to have a bad reputation. At least not if said reputation is flat out false! But is it? This article will try to answer that.

Here are some examples of people perpetuating “truths” about sapwood:

• “For me it has to come off, especially as it doesn’t run the full length of one plank. Also it’ll be a bit softer on the edges where it’s going to get some damage in use.”
• “Sapwood has no place on a table top imo, it’s softer and prone to be damaged easily especially on the edges.”
• “Sapwood usually is softer, weaker and more prone to decay (both rotting or bugs) than heartwood.”
• “However sapwood is not as hard or mechanically resistant so bear in mind if the pieces are going to be stressed or not in any way.”
• “Many experienced woodworkers actually remove the sapwood and use only heartwood for their furniture projects. But this doesn’t mean that the trimmed sapwood is junk that’s good only for the woodpile. While the sapwood will never be as strong, rich, or beautiful as the heartwood, it still has its uses. Just be certain to dry the sapwood thoroughly and to use it in projects where a little bit of movement will not cause problems. Also plan to seal it thoroughly, with ​paint or polyurethane, to prevent it from soaking up environmental moisture.”

While it is true, as this article will demonstrate, that sapwood does decay a lot easier than heartwood and that there is a risk of uninvited guests living’ it up in the sapwood, the strength properties is what people gets wrong. So, it is time for a rabbit hole deep dive in the topic of sapwood!

In this article I’m mainly going to focus on white oak. There might be species where things may be different, but for the most part this article applies to all common species in Europe.

What is sapwood?

When you hear sapwood, you might think of the part of a plank that has a very different color than the heart wood, but the reality is a bit more complex:

The Bark/Periderm – the outer shell of the tree. It consist of dead tissue, and its function is to protect the tree from the elements, bugs, fires and other potential dangers.

The Phloem – a thin layer of living cells beneath the bark. Its function is to transport food and water around the rest of the tree.

The Cambium – next layer of the cake is the cambium; living tissue that produces new phloem on one side and new xylem on the other

The Xylem – this is the part of the tree we want, and it is made up of both sapwood and heartwood. This is the largest part of the tree.

• Sapwood – typically the sapwood in a tree has a lighter color than the heartwood, but the difference might not be too pronounced in some species. This is new wood and is used to move water throughout the tree up to the leaves. Excess food is often stored here as well. Because of the high starch content, the sapwood is prone to bug attacks and rapid decay.

• Heartwood – As the tree ages and a new ring of sapwood is grown, the inner cells are no longer in use and they die off and become heartwood. This creates a strong structural pillar in the core of the tree, allowing it to grow taller each year. If the outer layers of the tree remain intact, the heartwood will not decay, even though it is dead and “retired” sapwood. Because the pores are no longer needed in this part of the tree, they are filled with organic matter along with chemicals called extractives – which changes the look and other properties of the wood so it becomes stiffer and more durable. When the sapwood dies, starch is released to promote the production of new wood cells, which is why the starch content in heartwood is low.
This is – for the most part – the piece of the trunk we woodworkers want!

The reason heart wood does not rot is because substances called tilose coats the “veins” and protect the wood against fungus and bacteria, that is, against rotting. What we call tylosis, is the extension of the cell cytoplasm covering the lumen part of the wood’s veins. Tilose also makes the wood smell like itself by releasing its unique odor – the tree’s eu de perfume! Which is what we woodworkers gets to enjoy when we work; the sweet, vanilla like scent of freshly cut white oak, the caramel-like undertones of teak (which mixes with the scent from the oils)…

Ratio of sapwood vs. heartwood

The proportions of conductive sapwood and non-conductive heartwood in wood species are very different from each other. We can see this by observing the cross-sections of different woods.

• The ratio of sapwood is high in species such as maple and ash.
• In oak, the ratio of sapwood is relatively low. As demonstrated by the image in the start of the article, an old oak tree has about 25-30mm (around 1”) of sapwood.
• Finally, there are poplar, fir, and willow trees where there is not much distinction between the sapwood and the heartwood section.

The quality of wood is easiest to understand by examining the sapwood and the heartwood it contains. Excessive amount of sapwood is considered poor quality because it is still active and has the potential to rot. Traditionally, woods with high content of heart wood has been used for construction, but today the situation has changed. Because of the developments in wood engineering, precise load calculations can be made. This reduces the cross-section of wooden elements, and construction wood is obtained from trees with a high rate of sapwood, grown in afforestation areas.

Tangent: inducing heart wood content in fir trees?

Research has been done to see if we can increase the heartwood content artificially. Here in Norway, an experiment started in 1995 where 99 fir trees was treated in different ways; they cut the top off, reduced the crown of the tree, ant they removed ¹/₃ of the bark layer up to 4 meters in height. Eight combinations of treatments were performed. These trees were on average 120 years old. In 2002, 33 of the trees were harvested by a student for his thesis, while the remaining 66 trees were harvested in 2006.

Interestingly, by reducing the crown of the tree, the width of the annual rings were reduced. For furniture, tight grown wood is preferable over, for instance, rapid-grown sitka spruce with very wide annual rings. Such poor quality is suitable for construction, but you would loose a lot of hair trying to do hand cut dovetails in such poor quality wood. In my support table series, I used that kind of material for the table legs. I cut lap joints, and it felt like cutting the joints in a sponge! Never again…

However, cutting off the top of the tree proved to be a bad idea, as 25% of the trees where this was done had some rot and 1 in 12 trees had died. The “mortality rate” was 100% where the top was removed in combination with removing the bark.

By inflicting wounds in the bark of fir trees, the trees will produce wood with higher content of rosin. This artificially induced heart wood has a slightly different chemical composition than that of natural formed heart wood. It is less resistant to decay than naturally formed heart wood, but way better than sap wood.

The conclusion of the experiment was that it is possible to increase the amount of heartwood in fir trees, but the result does not match the effort needed – it takes about an hour per tree to remove the bark. Not a profitable thing to do, really. If it takes three trees per cubic meter of processed wood, three hours worth of work plus social expenses must be added to the price mer cubic meter.

ln the image below you can clearly see the ring of sap wood in the big trunk pieces that came from a huge white oak I cut down. The bark/cambium can also be spotted as a dark brown ring.

Properties of importance

In a research paper from University of Zagreb, Croatia, the properties of white oak sap wood versus heart wood was investigated (link below). What they found was the following:

While sapwood is less durable because, among other factors, it has less tannins in it and contains more water, it has basically the same strength properties as heartwood.

The mechanical properties were not significantly different between heartwood and sapwood. The density, bending strength and MOE (Modulus Of Elasticity, namely compression strength and Brinell hardness) the differences between sapwood and heartwood were not significant. A significant difference was only found between the means of compression strength of sapwood and heartwood within the white oaks group, where the red oaks faired a little better.

The main factor that made any difference is wood structure, not whether it is sapwood or heartwood.

Conclusion

The only real reasons to avoid sapwood in our projects are color differences, and durability if the piece will be exposed to the outdoors. There is also the risk of non-pleasant critters and bugs that may thrive in sapwood but not heartwood, however for some species, like oak, this usually won’t be a problem. Kiln dried woods will not have this issue, as those tiny buggers won’t survive the ordeal.
For air dried lumber, once the moisture content drops below 20% the bugs won’t be a problem although you cannot be 100% certain that bad things can’t happen.

This means that any inclusion of sapwood does not really matter in unseen areas, such as the underside of a table top, a shelf, on the inside of a chest of drawers, and so on. If you do not want any color differences in the visible surfaces, you must of course avoid sap wood on the faces that will be seen. That being said, it is possible to stain the sapwood to better match the rest of the plank, and this can be advantageous in some cases. For oaks, the sapwood takes treatment a LOT easier than heartwood, which explains why staining the sapwood to match the heartwood works so well.

Full disclosure: prior to writing this article, I too was under the impression that sapwood should be avoided because it was of poorer quality. From now on, unless I want to avoid the light color, I will be able to use a lot more of the boards I have on storage! Just look at the image above – that is a lot of waste!

Knowledge is indeed still important..

Sources:

• Mechanical properties of sapwood versus heartwood, in three different oak species
• EcoChoice blog
• Høgskolen i Nord-Trøndelag (Norwegian text only): treatment of fir trees on root: an attempt to increase the amount of heartwood.