With older wood tongue-and-groove flooring, gaps will inevitably develop between the boards. The problems with this are numerous:
It is unsightly and it makes it difficult to clean because these gaps become dirt magnets. Not only that, they can become a trip hazard if the boards begin to cup or curl--a common occurrence with old wood flooring.
Unless your flooring is covered with carpeting, these gaps will cause your room to feel drafty.
While this is normal airflow coming up from your basement or crawlspace, it causes your heating system to work harder to pump out more heat, costing you more money.
Why This Happens
This is a common problem with older wood flooring as the wood shrinks over time.
The problem is magnified when the boards were not tightly laid in the first place. Another exacerbating condition is when the floor becomes water-damaged. Water-logged wood will first swell then shrink as it dries out.
Floors that have excessive heat below (as in the case of a furnace room) are more prone to developing gaps.
What To Do
This is a tough one. Unfortunately, fixes are not easy or pretty.
Floorboard gap repair falls into the category of fixes that are like throwing good money after bad. In many cases, you will spend countless hours on filling gaps, only to be dissatisfied with the results and contemplate laying a new solid hardwood or engineered wood floor.
However, if you absolutely want to repair, your best option is to fill the gaps with a solid, rather than liquid, material. Options, from best to worst:
1. Fill With Wood Strips
This method involves using a table saw to rip strips out of spare floor boards you might luckily have laying around. If no matching floor boards are available, wood of the same species can be used.
Apply glue to the sides of the strips and gently pound the strips into the gaps with a mallet or hammer. Plane or sand down high spots.
Ripping ultra-thin wood can be difficult and dangerous for the DIYer. Exceedingly thin strips involve placing your hand even closer to the sharp blade. Using push sticks is one way to keep your hands further away from the blade, but this means you have less control over the wood feed.
Even if the danger factor is solved, it is difficult to sufficiently control the feed so that you get a wood strip of continuous width.
This method is best left in the hands of experience floor installers.
A commonly prescribed method is also an old one: filling the gaps with rope. Some guides, including Tom Silva at This Old House, recommend using either jute or plain cotton rope.
First, lightly scrape out the gaps with a screwdriver or a five-in-one tool, then vacuum out the debris. Then, lightly apply carpenter's glue to lengths of rope (pre-sized and pre-tested) and push into the gaps.
After the glue has thoroughly dried, re-stain and seal.
One problem with this method is that gaps are not all of equal width. This means you need to purchase several different ropes of differing thicknesses.
Also, gaps between the floorboards may not run the entire length of the boards. Thus, you will have a rope filler that stops in the middle of the floor boards.
3. Patching With Wood Filler
Can you fill in the gaps with wood filler?
This method involves forcing in the filler as far as possible with a putty knife, then sanding to the level of the flooring after the filler has hardened.
This is not a good option. The surrounding floorboards will shift and eventually start to pull away from the filler, cracking it.
Some remodeling guides recommend pulling up the floorboards and re-installing them, closing up the gaps in the process.
On paper, this sounds like a viable idea. But if you have tongue-and-groove flooring, all you will accomplish is breaking off the tongues and destroying your floorboards.