Get a helper to hold the end of the tape.

Hook the tape under your foot.

Get a laser measurer.

ED

 

Get a pole of known length...say 6' (or eye height)

Measure from the bottom of the header to the top of the pole.

 

measurements will not give you an accurate read for the header sag, because it is just as common for sills to sag under patio doors. a string pulled tight will give a more definitive look. with helpers, place the string in the upper corners just in front of the frame so the string can remain straight.

 

To take that measurement the 2 sliding stick method has worked for decades. When the top and bottom sticks touch the target put a pencil mark where the two overlap. A Quick Grip clamp is handy to clamp the two together if you like. Now measure the sticks combined distance.

 

Run your string across the header and the bottom track--as often as not,water has gotten under the door and puffed up the flooring,causing the bottom track to bow up in the center---

 

OhMikes string suggestion is almost certainly the simplest, most accurate way to measure sag of a header. If the string is tight, over a few feet it is essentially straight, and you can easily measure the difference between the header and the string. If the middle of the header is lower than the ends, the header has most likely sagged.

Couple of words of caution. The header may have been bowed to begin with, so you cannot conclude that the header sagged excessively just because the middle is lower than the ends. Also note that any header will deflect under load in the middle. If your real concern is whether the header is going to fail structurally, you need to calculate the expected deflection under load, and compare to the actual deflection. If actual deflection is greater than anticipated deflection, you need to assess your deflection assumptions. Likely the header is not as stiff as expected, the load is greater, the size of the header is different than you assumed, or the span is incorrect.

 

I'm gonna need a formula for that or something, Daniel... :jester:

 

You can find formulas or even on line calculators for deflection in an mechanics textbook, but there is an easier way to handle this. If you measure the deflection at the center of the beam, and divide the length (you need to use the same units, typically inches) by the deflection, you will get a value known as l/d. For example, if your header is 10 feet long (120 inches), and the deflection at the center is 1/2 inch, your l/d = 120/.5 = 240. Typically the minimum allowable value is 180 for structural members. If you are supporting a floor that is going to be tiled, you want a minimum of 240. If you are using sensitive tiles, like large porcelain tiles, you want a minimum of 480. If you are using natural stone tile, you want 720.

The l/d value is usually called the stiffness of the beam. So you measure the deflection using the string technique, divide the length by the deflection, calculate the stiffness, if it is not large enough, you worry about what to do next.

 

I am wondering if the sag you're looking at really is a sag. You will have to remove the finish material (sheetrock, eg). Is the door operating? Is there a crack in the sheetrock/plaster? It could be trick of the eye. That is, if the frame is next to less than straight ceiling corner or deck, frame could look curved. Header and sill are parts of the rough framing and you can't decide on their condition with precise measurements.

You can remove the trim and see if the space between the door frame and the header is roughly even.

If you are after the demolition, do you have at least 4x8 or double 2x8 header? Is the header material in good condition? For 5-6' slider door, I would use 4x10 header with double jack studs/single king.