Q: Our vertical cedar board siding became uniformly cupped with outside edges pushing outward along with knots pushing outward. Does the direction of cupping and knot displacement suggest where the moisture came from? Thank You — Dan
A: Wood expands and contracts as its moisture content rises and falls. Increases in wood moisture content can result from exposure to liquid water — for example, rain or storing the wood on wet grass. Drying can be hastened by exposure to airflow and sunshine.
Wood moisture content also rises and falls with changes in the air’s relative humidity. That’s why gaps sometimes appear in wood flooring in cold climates (especially in leaky houses with dry air) and expands again in summer. Wood moisture content always tries to reach a state of equilibrium with the RH of the surrounding air.
These changes in lumber dimension are almost entirely across the grain, not along the length. Wood cups when one face becomes wetter and therefore wider than the other face, due to uneven wetting or drying. The convex face — curving outward at the center of the board — is the wetter face. All these effects are reduced if the wood is well sealed and finished on both sides and all edges, slowing moisture migration into and out of the wood.
Where the outside edges of the siding are pushing out away from the sheathing, as on your home, that indicates that the inside face of the wood is wetter than the outside face. This could be due to excessive wetting of the inside face or excessive drying of the outside face, or some combination of the two.
If the cedar siding was wet when installed (higher than the wood’s equilibrium moisture content for your area) it may be that the outside dried more rapidly than the inside, leading to cupping. The outside would tend to dry faster due to exposure to sun and wind.
Or it may be that excess moisture from the building is wetting the backside of the siding.
If you can describe your wall system from inside to out, as well as your climate and the use of the building, I might have a better idea of the cause. Also, is the siding cupping on just part of building, such as the south-facing wall, or on all sides? And when was the siding installed, how is it finished on each side, and when did you first notice the problem? It would be helpful if you could send a photo — may provide additional clues.
Do you know if the wood is flat-sawn vs quarter-sawn? Quarter-sawn wood is a lot more stable lot more expensive. Some people say that the rings tend to straighten out in flat-sawn wood as it dries. While this may be true to a small degree, the effects of uneven wetting and drying are much greater.
If uneven moisture is the culprit, it’s possible that this is a temporary problem that will resolve when wood moisture levels even out. – Steve Bliss, BuildingAdvisor.com
buildingadvisor says
The Western Red Cedar Lumber Assoc. is probably your best source of info on this topic. They have a technical services department that could answer your specific questions. Also, you’ll find finishing and installation guidelines in their publication How To Install Western Red Cedar Siding,
For a more technical resource of wood performance, the go-to book is the Wood Handbook from the USDA Forest Products Lab. It’s pretty technical and may not provide the answers you need, but may provide some insight.
As an experiment, it might be worth spraying one board with water with a hand spray bottle to see if you can increase the moisture level of the board’s exterior and reduce the cupping. How much moisture gets through the finish is hard to predict, so you may need to experiment. If this works, it might point the way to a solution short of replacing the siding.
However, once wood has badly cupped or warped, it may resist flattening due to permanent compression of the wood fibers called “compression set.” Then replacement with kiln-dried siding, acclimated to site conditions, and sealed and finished on all sides and edges may be your best alternative.
buildingadvisor says
The Western Red Cedar Lumber Assoc. is probably your best source of info on this topic. They have a technical services department that could answer your specific questions. Also, you’ll find finishing and installation guidelines in their publication How To Install Western Red Cedar Siding,
For a more technical resource of wood performance, the go-to book is the Wood Handbook from the USDA Forest Products Lab. It’s pretty technical and may not provide the answers you need, but may provide some insight.
As an experiment, it might be worth spraying one board with water with a hand spray bottle to see if you can increase the moisture level of the board’s exterior and reduce the cupping. How much moisture gets through the finish is hard to predict, so you may need to experiment. If this works, it might point the way to a solution short of replacing the siding.
However, once wood has badly cupped or warped, it may resist flattening due to permanent compression of the wood fibers called “compression set.” Then replacement with kiln-dried siding, acclimated to site conditions, and sealed and finished on all sides and edges may be your best alternative.
Dan says
All photos were taking after we had a flood in December, 2016, My question is this: Does cupping direction tell us if the moisture came from inside or outside? The front of our home faces due west and all boards facing west consistently cupped (arched) toward the home while those boards facing south and under the roof have little to no cupping. Also note that when the boards began acting up the finish became milky, then knots began pushing out shortly after cupping occurred. Our siding is 10″ channel lap cedar. Photos below show cupping and loose knots.
Thank You for your time. — Dan
buildingadvisor says
Thanks for the photos. As I said in the earlier email, cupping of wood siding (or flooring or other boards) is mainly due to uneven moisture content on the two faces of the wood. The wetter side expands relative to the dry side and cupping results. This suggests that the back face of the siding has a higher moisture content (MC) than the outer face. This could be confirmed by an inspector with a moisture meter.
While the south side of a house gets the most overall sun exposure, the west side gets the strongest direct sun in the summer and swing seasons. That’s why west-facing windows tend to overheat houses more than south-facing windows. The same direct sun may be cooking your siding, drying the exterior face, and contributing to the cupping.
You mentioned a flood. Don’t know the details, but this could have certainly added excess moisture to your siding as the evaporating water moved outside, through the walls, exacerbating the problem.
Since the problem started with the flood, the most likely scenario is that the flood raised the moisture content of the siding, especially the interior face, and the western sun dried the outer face, leading to the type of cupping you have experienced.
If the source of the moisture was the new siding being too wet, you would have seen the problem earlier.
Most siding nowadays is flat-grained vs. vertical-grained, making the wood more prone to cupping and warping, as well as loose knots. Yours looks like flat-grained in the photo. Vertical-grain wood is very expensive. Other factors that reduce cupping and warping include:
• Proper drying and storing at the mill and lumberyard
• Allowing the wood to acclimate to local conditions (equilibrium moisture content) before installing
• Sealing all wood surfaces, faces and edges, before installation
• Narrower vs. wider siding plank
• Nailing with ring-shank nails into framing or solid blocking, not just into the sheathing
• Avoiding excessing interior moisture levels
• Proper air barrier and vapor retarder in the walls
• Ventilation space behind the siding
Not sure what finish you used or why it turned milky, but excess moisture would be the first suspect.
As for the loose knots, these are a fact of life with flat-sawn, knotty grades of wood. As wood expands and contracts from gaining and loosing moisture, knots tend to loosen – as do nails.
In summary, it sounds like the interior face of your siding is wetter than the exterior face. The shrinkage on the outer face may been increased by exposure to the sun exposure on the west wall. The most likely source of the excess moisture was the flood.
It’s possible that the problem will resolve itself if the wetting was a one-time issue and the wood moisture levels are allowed to equalize. It’s also possible that cupping could be reduced with better nailing, using ring-shanked stainless-steel siding nails drive a min. 1-1/4 in. into solid framing or blocking. Driving nails on each edge, however, increases the risk of splitting if the wood shrinks further.
Dan says
We did have a flood in our lower level on December, 2016, our water heater failed. Our insurance company sent in a restoration company that did not properly dry our home causing major problems. The finish started acting up in the spring. Soon after, knots started protruding outward with some falling out and by mid summer all west facing boards had cupped. I attempted to straighten but the wood cracks well before it straightens, not an option. Is there a source (book/article/manual) that I can use as a reference regarding how cedar reacts to moisture?
Thank You,
Dan
buildingadvisor says
The Western Red Cedar Lumber Assoc. is probably your best source of info on this topic. They have a technical services department that could answer your specific questions. Also, you’ll find finishing and installation guidelines in their publication How To Install Western Red Cedar Siding,
For a more technical resource of wood performance, the go-to book is the Wood Handbook from the USDA Forest Products Lab. It’s pretty technical and may not provide the answers you need, but may provide some insight.
As an experiment, it might be worth spraying one board with water with a hand spray bottle to see if you can increase the moisture level of the board’s exterior and reduce the cupping. How much moisture gets through the finish is hard to predict, so you may need to experiment. If this works, it might point the way to a solution short of replacing the siding.
However, once wood has badly cupped or warped, it may resist flattening due to permanent compression of the wood fibers called “compression set.” Then replacement with kiln-dried siding, acclimated to site conditions, and sealed and finished on all sides and edges may be your best alternative.