Ask StopGap – Do I have tongue and groove floorboards?

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If you have just sanded your floorboards you are probably looking forward to having your beautiful original wood floor on show. Finding you have cold draughts coming through the gaps in your boards may seem disappointing at first but it’s an issue which is usually easily remedied by fitting StopGap.

Although StopGap is great for sealing gaps in most floorboards, it isn’t suitable for tongue and groove flooring.

In this edition of Ask StopGap, we’re going to be finding out the difference between these two types of floor, and how to tell which of them you are dealing with.

Floorboards or tongue and groove?

Normal floorboards are straight-sided planks of wood, usually 18-24mm in depth. In older houses, they are laid suspended on joists which run at right angles to the boards and fixed in place with nails or screws. This picture shows the ends of the floorboards (with StopGap fitted). You can just see the joist underneath,


Tongue and groove boards differ in that they have a ridge along one side – the tongue – and a groove along the opposite side. One end also has a tongue and the other a groove. When they are laid, the tongue is inserted into the groove to lock the boards together. Over time though, even these boards can shrink, causing gaps and therefore draughts. As you can see below, the gaps in tongue and groove boards are not straight, which sadly means that StopGap is not suitable for use with them.

t&g on floor

How to check which floorboards you have

Simply poke a credit card into the gap and see how far down the side of the board it will go (don’t let go!). If you have tongue and groove, it will only go about 6-8mm before it hits the tongue. If you have normal boards, it may hit the joist 18-24mm down, or if you aren’t directly above a joist, it will go further, past the board and into the space beneath.

If you do have normal boards, StopGap will solve your draughty floor problem. As you can see in the picture above, once fitted, it sits at the bottom of the gap resting on the joists, hidden in shadow.

If you find you have tongue and groove, you will have to use more traditional methods of filling the gaps. These include using sawdust mixed with PVA glue, lengths of string, decorator’s caulk, or a flexible filler. This article at the Guardian is a good starting point for information on how to go about it.

Thanks for tuning in, I hope you found this post useful.