Why hardwood floors gap in winter, and installing floors in heating season

Every winter we get calls from homeowners who’ve noticed dark lines opening up between their hardwood boards. In most cases nothing is wrong with the floor. Wood takes on moisture from the air when it’s humid and gives it back when the air is dry, and each board swells or shrinks across its width as it does.

A GTA winter is about as dry as indoor air gets. Cold outdoor air holds very little moisture, and once the furnace heats it, the relative humidity inside the house can drop well below what the wood was made for. The boards lose moisture, shrink a little and pull apart along their edges. By late spring, when the windows open and the air turns humid again, most of those gaps close up.

The National Wood Flooring Association says gaps and splits are considered normal when they appear and disappear with the seasons. Wider planks show it more, because each board has more wood to shrink, and a floor installed during a humid summer will usually show more winter movement than one installed in the shoulder seasons.

When we look at a floor that’s misbehaving, the shape of the boards usually points to the cause.

The NWFA suggests keeping a home between 60 and 80°F, roughly 16 to 27°C, and between 30 and 50 per cent relative humidity all year round. Health Canada gives the same 30 to 50 per cent range for indoor air, mainly to keep mould in check, so the target suits the house as well as the floor.

You can’t manage what you don’t measure. An inexpensive digital hygrometer tells you the relative humidity in a room, and it’s worth having one on each floor of the house, since a finished basement and a second-floor bedroom can read very differently. Check it a few times a week once the heating season starts, and again on the coldest days of January and February.

Read your flooring warranty too. Many manufacturers set their own humidity and temperature conditions for the life of the floor, and a claim for gapping or cupping can be refused if the house was kept outside them. If you don’t have the paperwork, the product name is often printed on the boxes or on spare boards left from the installation.

Most GTA houses with forced-air heating need some help to stay near 30 per cent in deep winter. A humidifier mounted on the furnace treats the whole house and needs its pad or filter changed and its water supply checked each fall. Portable humidifiers work for a single room but need daily refilling and regular cleaning so they don’t grow mould.

There is a limit. On very cold nights, warm humid air condenses on the coldest surfaces in the house, which are usually window glass and the bottom corners of the frames. If you wake up to water running down the inside of the windows, the humidity is too high for that outdoor temperature, and it’s time to turn the humidifier down a notch. Condensation that sits on frames and sills will damage them long before a dry floor suffers.

The goal is a steady middle, not the top of the range. A house that sits at a stable 30 to 35 per cent through the winter is far kinder to wood floors than one that swings from 20 per cent to 50 per cent depending on who last touched the humidistat.

Winter is a perfectly good time to install hardwood, as long as the house is at normal living conditions. That means the heat is on and set where you’ll keep it, the humidity is in range, and wet work like plaster, drywall compound or fresh paint has finished and dried. A floor that goes in during a construction-site week, with doors open and heaters roaring, will move once the house settles down.

Before we install, we measure the moisture content of the flooring and the wood subfloor with a moisture meter and compare the two readings against the manufacturer’s requirements and the NWFA installation guidelines. If the readings are too far apart, we wait. A few more days of acclimation is cheap compared with gaps or cupping in the first year.

How long the boards acclimate, and whether they stay in their boxes or are opened, comes from the product’s instructions. Every wood floor also needs room to move at the walls, so we leave the expansion gap the manufacturer calls for around the edges, hidden later under baseboards and shoe moulding. Filling that space to make the edge look neat is one of the most common causes of a floor that buckles in its first humid summer.

Solid hardwood is one piece of wood from top to bottom, so it moves the most as the seasons change. It’s still a great floor for main and upper levels over a wood subfloor, particularly in narrower boards, and it can be sanded and refinished many times.

Engineered wood has a real wood top layer bonded to cross-layered plies underneath. Those layers resist each other’s movement, which makes engineered planks more stable in width. If you want wide planks, or the floor will go over concrete or over radiant heat that the manufacturer approves, engineered is usually the better choice.

Neither type ignores humidity, though. Engineered floors can still gap or cup when a house is far outside its range, and their warranties often set humidity conditions of their own. We go through the pros and cons of each, along with vinyl plank, laminate and tile, on our flooring installation page.

Winter deliveries bring their own problem. Rigid vinyl plank and laminate that ride in a cold truck, or sit in an unheated garage overnight, are too cold to install straight away. Cold planks can be brittle at the locking edges and will expand as they warm up. We bring the boxes into the rooms where they’ll be installed and let them reach room temperature for the time the product’s instructions give before a single plank goes down.

Floating floors move with temperature as well as moisture, which is why they need expansion space at the walls, door frames and around fixed objects like kitchen islands. Floors in front of large south-facing windows or patio doors can get much warmer than the rest of the room on a sunny winter afternoon, so the instructions often limit run lengths or call for extra breaks in those areas.

If the room isn’t heated all winter, such as a three-season sunroom or a cottage that’s shut down, check the product’s temperature limits before choosing it. Some floating floors are approved for unheated spaces and many aren’t.

A wood subfloor is checked with the same moisture meter as the flooring. Readings that are high in one area, often near an exterior wall, a bathroom or above a damp basement, point to a moisture source that needs fixing before a new floor covers it.

Concrete takes more care. Basement slabs and condo floors can carry moisture up from below for years, and fresh concrete in a new addition or garden suite needs a long time to dry, especially in cold weather when drying slows right down. Flooring manufacturers specify how a slab must be tested, commonly with relative humidity probes set into the concrete or a surface moisture test, and what readings their product will tolerate. We run the test the product calls for rather than relying on how dry the slab looks.

If a basement has a history of dampness, the flooring choice comes after the water problem is solved. Our basement renovations page explains how we check for water before finishing a lower level.

A few minutes in late October saves a lot of worry in February.

• Put a hygrometer on each floor of the house and note the readings
• Service the furnace humidifier: new pad or filter, water line on, humidistat set
• Lay boot trays and absorbent mats at every entrance before the first snow
• Wipe up melted snow and road salt quickly, and avoid wet-mopping wood floors
• Check the felt pads under chairs and tables, since dry wood dents and scratches easily
• Turn the humidity down if windows start running with condensation
• Call for a check if gaps are wide, uneven or still open by midsummer

Planning a new floor this winter, or wondering whether the one you have needs attention?