From the ground, without access to the roof, using the same trigonometry a surveyor uses. What matters is deciding which height you mean before you take a single reading.
A building has several, and planning rules, quotes and material orders each want a different one.
Where the ground falls away, planning definitions often specify measuring from the lowest ground level adjacent to the building, but that varies by jurisdiction. If the number is going into an application, check the local definition rather than assuming.
Stand back from the building far enough to see both the base and the point you want, at a comfortable angle rather than craning upward. At least half the building height away is a reasonable starting distance.
Brace the tool or set it on a tripod. Take an indirect measurement: shoot the top point, then the base point, without moving between them. The tool solves the vertical distance.
Repeat from a second position and compare. Two independent readings that agree is a much stronger result than one reading you hope is right, and it costs another minute.
If you know the roof pitch and the building width, the ridge rise above the eaves is half the width times the pitch ratio. A 30 foot wide building at 6/12 pitch rises 7.5 feet from eaves to ridge. Add that to a measured eave height and you have the ridge without shooting at it. Measuring pitch is covered here.
Everything about it is the awkward case for a laser measure.
It is outdoors in daylight, so the dot is hard to see and often invisible at the distances involved.
The target is far away, pushing toward the limit of usable range, which is well below the rated range in sun.
The target may be unhelpful. A dark roof, a reflective metal ridge, or glazing all return poorly.
This is where a camera viewfinder earns its cost, because you stop trying to see the dot and aim with a crosshair on screen instead. A green beam helps with placement at moderate distances. At real range, the camera is the answer. The outdoor picks are here.
Building heights are easy to get badly wrong and hard to notice, so verify by a second route.
Count the storeys. Residential floor-to-floor heights are reasonably predictable, so a two storey house plus a roof lands in a fairly narrow band. If your reading falls well outside it, something is wrong.
Use the shadow method on a sunny day. Measure the building's shadow and the shadow of something whose height you know. The ratio of the shadows equals the ratio of the heights, and it needs no instrument at all.
Count brick courses, which run to a consistent module on a brick facade.
Any of these confirms you are in the right region. Agreement between two independent methods is what makes a number trustworthy.
Your measurement is fine for quoting scaffolding, ordering siding, or working out whether a ladder reaches.
It is not fine as evidence. If the height determines whether a development is permitted, whether a boundary structure complies, or whether a right to light is affected, that needs a surveyor with instruments that produce a defensible record. The cost of a survey is small against the cost of building to a wrong number. The general height method is here, and siding, which needs the gable heights, is here.
If the question is really how to choose a laser measure rather than this specific tool, our front page is the better starting point.