Mats Bengtsson Home Improvement and Do It Yourself

Base measurements on a defined flat and square and middle

Mats Bengtsson mib over the years

Base home improvement on a defined flat, a defined square and a defined middle

When working with home improvement, or when doing wood work, one of the major problems is that nothing is exactly as is should be. The house might have been built a little out of square, or it might have settled since it was built. The wood might have moved a little since it was measured, and thus do not carry the same measurements or flatness as before.

The above are not possibilities, instead they are assured realities for things based on living material as wood:

  • Wood swells and moves based on changed humiditiy in the air
  • Houses built with living material like wood also moves
  • Old houses built with brick or light concrete blocks are often out of square.

So when you aim to lay a floor, the chance is the angle between the two walls are not square. If you aim to build a wall mounted cabinet, chanses are that neither the walls nor the floor and the roof are parallel. Thus, start with giving yourself a defined starting point, by deciding where, and making it a reference. Which reference you select has an importance, but deciding for a reference is way more important. Always use the decided reference, or a few predefined other references (like square and middle) as starting point for further measurements. Also, use repetition instead of measurements to produce good looking results. You need to limit yourself to some base references to keep measurement errors from multiplying. Errors would quickly multiply and aggregate to much bigger errors if you did not do that. If you based measurements on something which itself contained variations, and then based new measurements on references to that, and then based further measurements on those, the total deviation coming from variations, measurement errors, angle errors, ... would quickly become big enough to affect results.

Examples of selecting a reference

For the example of the floor above, decide if you want it in a certain angle to the door, or to a certain wall. With that decided, create a reference you can trust based on that decision, and do all measurements based on that reference. For the bookshelf in the example above, measure the roof and floor. Then decide and build a flat and horisontal stand for your bookshelf, creating a trustable reference:

  • That stand might have an unflat underside, following the floor (taking out the differences in the floor using a scribing method)
  • It might have a different height from the floor in the left and the right side, again following the floor but rmaining horisontal on the overside.

When having such a stand, your bookshelf will stand horisontal, and if built using square angels, thus also be vertical. You can then base all measurements and decision on measurements referencing the built floor stand, and the bookshelf which follows it. This is a lot easier, than trying to build a bookshelf that follows the different angles between floor and roof, floor left wall, roof and left wall, floor and right wall, roof and right wall, not to mention a floor which is slightly uneven, ...

This very same principle can be applied to all measurements and constructions. It makes all work so much easier, simply by knowing what to trust, where to make adjustments, and how to avoid errors multiplying until becoming too big:

  • When you need a straight board, start with making one side straight. Then mark it as the straight side and from then use it as reference for all future measurements. If you need to rip saw the wood, do it based on the straight edge, and the sides will become parallel.
  • When you need to make a square cut of your wood, you should do it based on your marked straight edge. Then mark that side and the new edge as square, and use that square and those edges for all future references. If you always start from those, possible errors in angles will not multiply. They will exist, but they will exist based on your defined starting points. If you instead sometimes took one side as straight, sometimes the other, and there was a slight unparallelism, also angles and measurements would quickly deviate from the small errors you started from.
  • When you need something in the middle, work equally from both sides and it will be in the middle:
    • If you are routing a mortise in the middle, route it from both sides. The mortise will be a slight bit wider than if your routed it from one side. But only if it would not have gotten exactly in the middle, and the extra width will be only the error from your original measurement of the middle.
    • If you are routing a tenon,route it from both sides, assuring the tenon ends up exactly in the middle. Increase the amount of material being routed away in small steps, until the tenon fits the mortise. When it does, the tenon is both of the right size, and placed in the middle.


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