At Home In The Woods

[brin]

How flat is flat?
This question is meant for those who work in the construction trade.

I used a string to find a straight 9 foot 2x6 board. I took the board around to various spots on the subfloor to see how flat the floor is. We intend to install hardwood and ceramic tile flooring everywhere on the main floor.

I found a spot where the subfloor had a hump. Can someone tell me how to determine whether or not a hump is an issue? How much is a problem? How do you measure it? Can I use a straight 9' board and a ruler?

I could use some input from people with experience in the trade. I talked to a friend who is a general contractor but I couldn't get a definitive answer from him.

Thanks for the feedback.
Obed

Obed..Have you thought about those laser levels they sell at the big box stores for hanging pictures ? You could set one of those down on any flat surface and any bump would break the beam and you would know where the bump or rise starts and then move the level and shoot the laser again and find the next point and so on all around the suspected bump and you would have it isolated and know it's dimensions...could do this on all floors and walls.

 

[EddieWalker]

There are allot of answers to that question. There flat enough, and perfectly flat. In most homes, flat enough is within 1/8th of an inch over 8 ft.

The problem you run into with wood subfloors is the joists under them have a bow to them. In your case, that would be the bow in the trusses. As you add more weight to them, and over time, that bow should level out. There is too much bow to a flore when it's noticable. Most floors have something wrong with them, but it's impossible to feel or see the angle or bow to them. It comes from working with wood and using bubble type levels, which are open to a certain degree of accuracy.

The real question is how much is it off and how high is the high spot? Is it noticable when walking on it? Why did you check it? The fact that you checked it means that it's a problem. Clients dont' start checking things unless they have a reason.

By any chance, is this where the shims were used to build up the low spot from the concrete below?

Eddie

 

[Obed]

There are allot of answers to that question. There flat enough, and perfectly flat. In most homes, flat enough is within 1/8th of an inch over 8 ft.

Eddie, how can I use my straight 9ft board to see if this hump is within 1/8 of an inch over 8 ft?

I don't know if I would notice this hump just walking over it. I took my board around the floor just checking to see how flat it was and found this spot. However, this spot might be close enough. I just trying satisfy myself of that fact. I know most clients wouldn't do this but I'm not "most clients". I've seen enough poor quality home construction examples in the past to make me carefully inspect the quality of all construction jobs that I pay for.

Obed

 

[mikefromnh]

Take a level and put one end on the high spot and one end in the low spot. Slowly raise the low end till it reads level and measure distance between level and floor. Then divide by the length of the level to determine the slope per foot. (it takes at longer to read then to actually do.)

The longer the level the better, I have a six footer for installing floors that I use. An 1/8" in six feet is not something I would worry about.


No matter how straight your 2x6 is you won't get an accurate measurement -- it will only tell you if it is way off or not.

 

[tommu56]

The way the measure the concrete flat work at work is string a line over the surface spaced about 1'' above it and level and measure the high and low spots relative to the line their tolerance is +1/4 and - 1/4 from that line but that is concrete and for setting machinery.

For tile it is most important that floor not flex.

tom

 

[Obed]

I went out tonight and looked more at the hump in the floor. The first thing I did was try to determine if I would have noticed it by walking on it or looking at it as was suggested by some people in this thread. I don't think it would be noticable by just looking at it or walking on it.

I then measured the hump the best way I could using my 9 ft straight edge board. Keep in mind I'm checking for flatness, not necessarilly for level. I already have a pretty good idea on how overall level the floor is due to the water level measurements we made some days earlier. I'm trying to find out if I'll have problems later with hardwood or tile installations or teetering furniture.

I placed the middle of the board on the hump, pushed one end down so it touched the floor, then measured how high the other end was off the floor. I then divided this number by 2 to get the height of the hump relative to the the height of the floor at each end of the board. The conclusion was the hump is 3/16 of an inch higher in the middle of the board than it is 4 1/2 feet away at the ends of the board. I then moved the board to find that one end of the board in the original measurement was in a low spot about 1/8 inch deep. Thus, I was measuring at the most severe spot I could measure.

Next I went to inspect the trusses on which the subflooring was sitting. I placed my board accross the bottom of the trusses to see how flat they were sitting. To my surprise, the bottom of the trusses were fairly flat. It turned out that the truss under the hump was about 3/16 inch taller than the neighboring trusses (24 3/16" vs 24").

In the end, I don't think I will notice this hump. Trusses aren't manufactured absolutely perfect. We'll frame the rest of the house and check floor flatness again and address issues at that time. This hump was the worst spot in the floor that I found. I believe I'm satisfied with the leveling at this stage. I think I'll sleep well tonight.

Obed

 

[Duffster]

I went out tonight and looked more at the hump in the floor. The first thing I did was try to determine if I would have noticed it by walking on it or looking at it as was suggested by some people in this thread. I don't think it would be noticable by just looking at it or walking on it.

I then measured the hump the best way I could using my 9 ft straight edge board. Keep in mind I'm checking for flatness, not necessarilly for level. I already have a pretty good idea on how overall level the floor is due to the water level measurements we made some days earlier. I'm trying to find out if I'll have problems later with hardwood or tile installations or teetering furniture.

I placed the middle of the board on the hump, pushed one end down so it touched the floor, then measured how high the other end was off the floor. I then divided this number by 2 to get the height of the hump relative to the the height of the floor at each end of the board. The conclusion was the hump is 3/16 of an inch higher in the middle of the board than it is 4 1/2 feet away at the ends of the board. I then moved the board to find that one end of the board in the original measurement was in a low spot about 1/8 inch deep. Thus, I was measuring at the most severe spot I could measure.

Next I went to inspect the trusses on which the subflooring was sitting. I placed my board accross the bottom of the trusses to see how flat they were sitting. To my surprise, the bottom of the trusses were fairly flat. It turned out that the truss under the hump was about 3/16 inch taller than the neighboring trusses (24 3/16" vs 24").

In the end, I don't think I will notice this hump. Trusses aren't manufactured absolutely perfect. We'll frame the rest of the house and check floor flatness again and address issues at that time. This hump was the worst spot in the floor that I found. I believe I'm satisfied with the leveling at this stage. I think I'll sleep well tonight.

Obed

That is about the only drawback to floor trusses.

 

[tommu56]

Actually if you were using lumber put in correctly they might be similar results if you put the crown up like its suppose to be because the joists might be that far out.

tom

 

[Pops15]

Tom's suggestion of using blocks and a string will work very well. Use a couple of 2X4 scraps. Nail them flat to the floor at each end of the house. Stretch a string taught over the blocks. Measure the distance between the subfloor and string at each truss. You don't want any more than 1/8 to 3/16 variance within 8'.
You can check your walls for straightness by nailing blocks to the inside of the top plate and stretching a string to gauge off of. Just be sure your corners are plumb before you check for straight.
A cation about levels...... many of them are highly inaccurate! You can check their accuracy by turning them end for end and turning them over. If you get the same reading each time then you're good to go. If you get different readings the level isn't accurate. Check the plumb vials the same way.
Pops

 

[radioman]

yup -- string method is the only relaible method for finding issues with straightness. I don't bother with blocks. I just drive a nail in area of wall or ceiling I am working with about an inch or so on one side. I drive a nail on the other side at the exact same height on other side. I grab my chalkline which is usually the only string I carry and wrap around first nail and wrap on second nail and pull tight and then tie off. I always check the height of the string right at the nail on both sides to ensure they are idectical hieght. just pick a number- an inch is easier to work with. Then you can measure off from the string along the string at various points to make sure its the same as the controlled hieght. Its not worth the hassle to make sure its plumb- but check for straightness first of anything you are working with. Once you are satisfied its straight then you can check for plumb if it matters for floors, countertops, cabinents.

Even if you can sleep at night with the hump issue- it will cause you headaches when installing cabinents and stuff. you will require alot of shimming from each end of hump. A hump on the end is easier to work with, not in the middle.