Wood board cupping caused by moisture differences, wet basements, crawl space moisture, or indoor flooding: here we explain how to look at which way a board has cupped to determine where moisture is or has come from.

This article series explains the causes of cupping in wood boards & wood board right side up advice for steps, decks, ramps, concluding which side of boards should face up or down (bark side down or bark side up in some cases) when building a deck or exterior wood stairs.

Wood Cupping In Response to Which Side of Board is More Wet

For decks constructed high enough to allow air circulation below the deck flooring, the moisture level of the bottom of the deck or step boards is less changeable than the deck's upper surface.

While both sides may become quite wet in rainy or melting-snow conditions, the upper side of the deck or stairs in many locations gets direct sunlight more variations in moisture level than its under-side.

In the ramp and entry platform shown above and that we built at an older home in Poughkeepsie, New York, there is no sun exposure on the under-side of this wood ramp but nonetheless the underside of the entry deck and ramp are exposed to air, drying, and perhaps more stable moisture levels than the upper surface that is further exposed to rain and snow.

In contrast, in the case of the ground-level deck shown below, the under-side of the deck boards may remain more wet than the upper-side, thus reversing the level of moisture exposure and risking that the deck boards will cup in the concave pattern on the walking surface- DF.

The ground level entry deck shown above, at a green lot cabin built in Two Harbors, MN, in 2006, using treated wood for both framing and the deck boards is shown in-use in 2013. Below we see details of the condition of this deck in 2022, sixteen years after its original construction.

The lack of ventilation under this deck is a factor in the rot developing on several of its boards (yellow arrows)..

More about the life & hazards of this entry deck is

at DECK DESIGN, LOW HEIGHT

FPL experts agree that on an exterior wood deck, ramp or stair the under-side of wood boards tends to vary less in moisture level than the upper or exposed side. In addition to cupping problems the exposed deck side is more prone to checking and of course photo oxidation wear. - DF

Below we show another board in this same entry deck, one with notable cupping that holds water, inviting greater algae growth and possibly rot.

So I [SB] stand corrected and will install future decking bark side up – ... or down ... according to the wood experts, or more likely avoid the issue altogether and use bark-less composite materials.- Steve Bliss

See DECK BOARD COMPOSITE PRODUCTS

Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, materials & components, & project management through complete construction.

Interior Wood Floor Cupping In Response to Which Side of Board is More Wet

When one side of a board contains more moisture than the other, the wetter side tends to expand and "arch" forming a convex shape while the opposite side of the board, the more dry side, tends to form the under-side of the arch or the concave shape.

Convex-Cupped Interior Wood Floors

In our photo at above left we are using a non-invasive electronic signal type Tramex moisture meter to check the moisture level in an interior wood floor.

At 18% the moisture is a bit high in this region;

It would be useful to also check conditions below this floor. At above right you can see convex-cupped finish wood flooring by shining a flashlight along the floor surface.

Similarly, in our page top photograph of cupped wood flooring the boards are arcing upwards at the board center and down at the board edges:

we think that there is more humidity in this room-side of the floor in all of these convex-cupped flooring photos, or that the boards may have been exposed to flooding or a very wet floor upper surface.

Our photo provides a close-up image of significant convex wood floor cupping following upper surface soaking of the floor.

Since documentation of this condition was required we used a combination of light and a straight-edged object (a pencil) whose shadow pattern makes clear the degree of convex cupping of the wood flooring.

 

Concave-Cupped Interior Wood Floors

Conversely, when we see a wood floor installed over a wet or damp basement or crawl space, we may see the opposite pattern: the boards will be cupped with their concave side facing upwards towards the room.

This situation is illustrated in our photo at left. There are two common reasons we find concave cupped wood flooring indoors:

The the wood floor is constructed over a wet basement or crawl space

The wood floor was exposed to flooding that trapped water remained for some time between the finish floor and the subfloor below.

These conditions can cause extreme wood floor damage, even to the extent that the floor expands and explodes upwards as we demonstrate

at WOOD FLOOR DAMAGE
and

also FLOOR DAMAGE DIAGNOSIS

The US FPL document discussed at BARK SIDE UP ARGUMENT continues with explanation of other reasons for placing wood with bark side up and pith side (tree center side) down, of which the second notes variations in moisture content across deck boards:

...
The bottom [under-side] of the deck is not prone to checking because its moisture content is less changeable than that of the surface.

 

...

Continue reading at ANSWER to BARK SIDE UP or DOWN or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Reommended Articles

• BARK SIDE UP or DOWN on DECKS & STEPS?
  • BOARD CUPPING CAUSES
  • ANSWER to BARK SIDE UP or DOWN
  • BARK SIDE UP ARGUMENT
  • BARK SIDE DOWN ARGUMENT
  • WOOD CUPPING vs WET SIDE
  • CUPPED BOARD HAZARDS
  • SHELLING BOARD SURFACES
• BUTT JOINT CURLING in FIBER CEMENT SIDING
• DECK & PORCH CONSTRUCTION - home
• FLOOR DAMAGE DIAGNOSIS
• SLIPPERY STAIRS, WALKS, ROOFS
• WOOD FLOOR DAMAGE
• WOOD FLOOR MOISTURE

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WOOD CUPPING vs WET SIDE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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