# Setting Prehung Doors-Clearance for Carpet?



## Spencer

-Door comes with a 7/8 undercut (bottom of door to bottom of jamb.)
-8 lb pad
-good quality frieze carpet
-1/2" desired between door and carpet

How high are you guys shimming up your door? Main thing I need to know is how high carpet is going to finish out at????


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## tedanderson

I don't think that you can figure that out until the carpet is layed... unless you are doing a new multi-unit building. Otherwise this is going to vary from one room to the next. 

The last time I had new carpet installed, I ended up removing all of the doors and trimming them down on a case-by-case basis. I don't like for the doors to drag on the carpeting but I do like it when they are as "tight" as they can be to the floor.


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## Jaws

tedanderson said:


> I don't think that you can figure that out until the carpet is layed... unless you are doing a new multi-unit building. Otherwise this is going to vary from one room to the next.
> 
> The last time I had new carpet installed, I ended up removing all of the doors and trimming them down on a case-by-case basis. I don't like for the doors to drag on the carpeting but I do like it when they are as "tight" as they can be to the floor.


If you dont have a return in each room, being to tight could be a problem for the A/C.

Like T/E said, its hard to tell from one room to the next if you didnt pour the slab or build the foundation.


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## Spencer

Jaws said:


> If you dont have a return in each room, being to tight could be a problem for the A/C.
> 
> Like T/E said, its hard to tell from one room to the next if you didnt pour the slab or build the foundation.


1/2" will help with the HVAC. I would rather have them just over the flooring but I know that doesn't help with return air.

This is new construction, I don't see any reason why doors would have to be cut after installation. I'm not following the second part of your post. We must not be on the same page...

Hopefully the carpet guys will chime in and give me an idea what the finish height would be on the carpet with pad.


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## TAHomeRepairs

Only once, I made the assumption... HO starts off wanting berber so I set everything at about 3/4. last minute they changed to a thick fiber carpet with double padding. 13 slabs and 4 bifolds later, I never assume. Plan on trimming and hanging after carpet.


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## Jaws

Spencer said:


> 1/2" will help with the HVAC. I would rather have them just over the flooring but I know that doesn't help with return air.
> 
> This is new construction, I don't see any reason why doors would have to be cut after installation. I'm not following the second part of your post. We must not be on the same page...
> 
> Hopefully the carpet guys will chime in and give me an idea what the finish height would be on the carpet with pad.


Down here, its slab on grade. If some hack builder hires a chit concrete guy there will be waves like you wont believe. Code is 3/8 in 32"  I wouldnt pay for a slab with 3/8 in it untill it was capped. 

Makes for a ***** of a time setting doors and making base LOOK straight when its wavy.


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## barry1219

Jaws said:


> If you dont have a return in each room, being to tight could be a problem for the A/C.
> 
> Like T/E said, its hard to tell from one room to the next if you didnt pour the slab or build the foundation.


Bingo!!!...you have no idea how much of a factor this can be.!


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## rusty baker

Doors shouldn't be hung until after the carpet is installed anyway.


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## Spencer

> Only once, I made the assumption... HO starts off wanting berber so I set everything at about 3/4. last minute they changed to a thick fiber carpet with double padding. 13 slabs and 4 bifolds later, I never assume. Plan on trimming and hanging after carpet.


Are you talking about hanging the doors on the jamb or actually setting the entire prehung and then running casing and base? 

I set the prehung unit and then remove the door until after carpet. What I am looking to do is shim up the jamb to the desired height so that I will have 1/2" after carpet is installed and won't have to cut the doors. 

Are you guys saying you hang them low and just plan on cutting off the bottoms? 

I think I used to shim up the jamb about 3/8 - 1/2" but I'm really just looking to know what carpet typically builds up to so I can figure it out myself.



> Down here, its slab on grade.


You're setting on wood here unless you're setting in the basement. Not very many homes are built on slab. I would say around 10-15%. Basement is your cheapest square footage around here.



> Bingo!!!...you have no idea how much of a factor this can be.!


What are some of the adverse effects of letting the doors tight to the flooring when the return air is drawn from a central location? I am just curious, I know its bad, but don't know enough to know exactly what problems could arise form it...


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## Texas Wax

New construction in your neck of the woods - HVAC is all Hi-Low in every habitable room. Register high, return low by function for heating requirements. As a general rule.

HVAC in Texas is cooling and uses a central return at the HVAC unit for the entire house or Zone it covers. As a general rule. Meaning that gap at the bottom of the door is how the air will be circulated out of the room, when the door is closed. Makes a BIG Difference. Still trying to comvince the better half to leave the bedroom door open LOL

Holding the jambs up 1/2" -3/4" where there is carpet is our rule in new construction. Door slabs are usally 1" up from bottom of jamb on prehungs. So with 1/2" pad and up to 1/2" carpet, that'll leave a 3/4" gap, below the door. Now they cheap out on the carpet. Usually carpet comes in between 1/2" and 3/4" with pad...

Still a crap shoot. Shoot low and gamble on cutting doors - Jambs to high, a real problem, unless you can verify the carpet height -is my advice.


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## tedanderson

Spencer said:


> What are some of the adverse effects of letting the doors tight to the flooring when the return air is drawn from a central location? I am just curious, I know its bad, but don't know enough to know exactly what problems could arise form it...


It makes the system less efficient and causes it to work harder to achieve the desired temperature. The air handler with the ducting is essentially one big breathing air pump that has to inhale as much air as it blows out. 

If you've ever been in a house where the A/C seems to be blowing very softly through the vents, chances are that there are some rooms that have been closed off. This effect is not as noticeable with newer systems but back when my family had a heat pump back in the early 80's the house was always cold and drafty until a technician told us that we have to keep all of the doors open in order for it to work. 

We had gaps under the doors but the system was a little more finicky than they are today.


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## Spencer

tedanderson said:


> It makes the system less efficient and causes it to work harder to achieve the desired temperature. The air handler with the ducting is essentially one big breathing air pump that has to inhale as much air as it blows out.
> 
> If you've ever been in a house where the A/C seems to be blowing very softly through the vents, chances are that there are some rooms that have been closed off. This effect is not as noticeable with newer systems but back when my family had a heat pump back in the early 80's the house was always cold and drafty until a technician told us that we have to keep all of the doors open in order for it to work.
> 
> We had gaps under the doors but the system was a little more finicky than they are today.


I just had a new geothermal system and duct work installed in the house I am remodeling. My system pulls all the return air for the 1st and 2nd floor from one big return duct at the top of the open stair. I can definitely see how closing all the doors in the upstairs would be detrimental.


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## PrecisionFloors

Everything here is slab on grade. All the doors are set at about 1/2" give or take. Typically works out well with builder grade carpet and pad. Heavy frieze and 8lb pad would probably require 3/4.


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## Marven

If this is a high end house, get your laser out and set all the top jams at the same height. Key off the front door if it is in. You may have to shorten a jam or two to do this. Cut off bottom of doors as needed.

If equal height of doors is not critical, set the hinge side of doors 1/2 " off floor to tuck in carpet. Adjust the gap around the door and let the other side jam fall where it may. If you need more room for carpet under door, trim the door.


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## TAHomeRepairs

Spencer said:


> Are you talking about hanging the doors on the jamb or actually setting the entire prehung and then running casing and base?
> 
> .



actually in this instance, the whole house was built out of CMU interior and exterior walls. The jams were steel and built into the CMU. So all I did was install bifolds and slabs. but that forever changed my thinking on doors before carpet.


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## Spencer

PrecisionFloors said:


> Everything here is slab on grade. All the doors are set at about 1/2" give or take. Typically works out well with builder grade carpet and pad. Heavy frieze and 8lb pad would probably require 3/4.


I think i'll go for setting the jamb side up 1/2. 1/2+7/8 undercut = 1-3/8

1/2" for pad and 1/2 for carpet. Should put me about where I want to be. Can't add any more on so I'll play it safe.


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## txgencon

Texas Wax said:


> ...
> 
> HVAC in Texas is cooling and uses a central return at the HVAC unit for the entire house or Zone it covers. As a general rule. Meaning that gap at the bottom of the door is how the air will be circulated out of the room, when the door is closed. ...


I haven't seen an upflow unit with just one return installed in 30 years in my area. Everything is attic mounted horizontal flow with returns in every habitable room and the master closet if over about 80 square feet. Maybe in "starter" homes...


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## greg24k

Industry standard 1"- 1 3/8" depends who makes the doors. Just did a whole house, all doors were 1 1/4"


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## txgencon

I think he's actually talking about the clearance between the jambs and the floor. With so many people doing the stained concrete thing, I'm not sure you should leave more than a 1/16" clearance. They can butt carpet to the jamb.


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## TAHomeRepairs

Spencer said:


> I think i'll go for setting the jamb side up 1/2. 1/2+7/8 undercut = 1-3/8
> 
> 1/2" for pad and 1/2 for carpet. Should put me about where I want to be. Can't add any more on so I'll play it safe.


I would think for most instances, this should be fine. I don't do any new construction, but all that I see, there are returns in each room, so you shouldn't even have to worry about return air gaps anyway. Are there return air ducts in the rooms in question?


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## loneframer

In the rough, 1" under the door isn't enough. I like to see a minimum of 1 1/4" and prefer 1 1/2". Shim jambs up accordingly.

After carpet, tile or hardwood, that leaves 1/2" to 3/4" under the door for return air. That's not a lot. On a 32" bedroom door, 16 to 24 square inches of return? That's about the equivalent of a 5-7" wide stud bay in a 2x4 wall.

I'm no HVAC specialist, but in a master bedroom with 4 or 5 supply registers, I'm thinking that's light for return air.


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## thom

a 32" door set 3/4" above the carpet will give about 23 square inches of return air. A single 6" supply duct will provide about 28 square inches of supply air. So, your close if you have only a single supply register in a room if you leave 3/4" between the door and carpet. 

If the room is larger, say a master suite with 2 or 3 or 4 supply ducts running into it you will probably need to install a jump duct between the bedroom and the hall. 

Not providing adequate space for return air will create a high pressure area in the room which will result in air exfiltration which equals heat/cooling loss. This exfiltration is matched by infiltration from another place to compensate for the loss, this time drawing in unconditioned air. 

Whether a house has individual room hvac returns or a whole house return is a function of both local standard and market segment of the home.


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## Spencer

TAHomeRepairs said:


> I would think for most instances, this should be fine. I don't do any new construction, but all that I see, there are returns in each room, so you shouldn't even have to worry about return air gaps anyway. Are there return air ducts in the rooms in question?


One smaller return on the main level and this large return at the top of the open staircase on 2nd level. There is also a return air duct to a 600 sq ft bonus room above the garage and turn air in the basement. I have a 4 zone system which I love.

This was a remodel of an old farm house with 8x8 sills so running a bunch of return through outside walls was impossible. We had to work with what we had. No big deal on main level because it is completely open but if all the doors were shut upstairs and door were tight to the floor there would only be about 10% of the area left open that was being supplied for. Definitely need to make sure there is at least 1/2" even though I would prefer less.


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## TAHomeRepairs

If those are returns(not supply), those doors can sit right on the carpet and all is well.


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## Spencer

thom said:


> a 32" door set 3/4" above the carpet will give about 23 square inches of return air. A single 6" supply duct will provide about 28 square inches of supply air. So, your close if you have only a single supply register in a room if you leave 3/4" between the door and carpet.
> 
> If the room is larger, say a master suite with 2 or 3 or 4 supply ducts running into it you will probably need to install a jump duct between the bedroom and the hall.
> 
> Not providing adequate space for return air will create a high pressure area in the room which will result in air exfiltration which equals heat/cooling loss. This exfiltration is matched by infiltration from another place to compensate for the loss, this time drawing in unconditioned air.
> 
> Whether a house has individual room hvac returns or a whole house return is a function of both local standard and market segment of the home.


That was a perfect explanation. Exfiltration from high pressure in the room and infiltration to compensate for the lack air elsewhere.


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## Spencer

TAHomeRepairs said:


> If those are returns(not supply), those doors can sit right on the carpet and all is well.


Yes, those are return air.

I have 3 bedrooms and 2 baths upstairs. Each room has a supply coming in through the ceiling but the only return air is in the hall above the stairs.

Care to elaborate? Are the able to pull enough air from the rest of the space that the bedroom door being shut won't matter. Seems like the exfiltration from high pressure could still be an issue.


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## TAHomeRepairs

Spencer said:


> Yes, those are return air.
> 
> I have 3 bedrooms and 2 baths upstairs. Each room has a supply coming in through the ceiling but the only return air is in the hall above the stairs.
> 
> Care to elaborate? Are the able to pull enough air from the rest of the space that the bedroom door being shut won't matter. Seems like the exfiltration from high pressure could still be an issue.


No you need the return air. The doors need the gap to allow air flow from the rooms to the returns. Often I see returns on each individual room, thus you no longer need the door gaps. With your situation, you will need the space under the doors.


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## txgencon

I've seen older homes with central returns where doors that stayed closed a lot would have additional soiling of the carpet where air was forced under them.


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## TAHomeRepairs

txgencon said:


> I've seen older homes with central returns where doors that stayed closed a lot would have additional soiling of the carpet where air was forced under them.


Yeah and if it doesn't have enough gap under, look at the dust on the door stop once you open the door.


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## rusty baker

Returns must be based on regional weather conditions. In my area, there is one big return for the entire house. This is the same on old houses and new construction.


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## TAHomeRepairs

rusty baker said:


> Returns must be based on regional weather conditions. In my area, there is one big return for the entire house. This is the same on old houses and new construction.


Maybe, but the only houses I've worked in less than ten years old are 3500 sq ft +, so maybe they just have better designed systems. The cookie cutters may still have a single central return.


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## TLP

How ambient air moves from one area to another depends on CFM (cubic feet per minute, not square feet), air density, temperature, dew point, not just pressure. The flow also depends on whether the cavity is a venturi where pressure and velocity have an inverse relationship to each other and/or the duct is diverging or converging relative to air flow like a HVAC boot. The square volume under a door is the least efficient at passing CFM, especially with carpet on the boundry layer causing turbulence. The tight gaps around the door can be just as efficient if they are a venturi shape and less turbulent.

You're making alot of crude assumption by only looking at pressure through a square area duct door cavity and the bottom of a room or door. I wouldn't get to pre-hung upon it unless you really know what you are talking about. Just make sure there is a gap under the door so the carpet does not rub it, that simple.


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## Bills86e

*Should I think?*

This question is easily answered. Set the jamb on the high side, PLS.

The door and hardware consultant has the responsibility of the undercut
and to sends this information to the manufacturer. Always read the Specs before you start a job. It may specify setting elevation by the 
General contractors benchmark. As an installer you don't have after/ forethought, but are expected to read the Specs. CYA



CYA = Cover your Ass


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## Spencer

TLP said:


> How ambient air moves from one area to another depends on CFM (cubic feet per minute, not square feet), air density, temperature, dew point, not just pressure. The flow also depends on whether the cavity is a venturi where pressure and velocity have an inverse relationship to each other and/or the duct is diverging or converging relative to air flow like a HVAC boot. The square volume under a door is the least efficient at passing CFM, especially with carpet on the boundry layer causing turbulence. The tight gaps around the door can be just as efficient if they are a venturi shape and less turbulent.
> 
> You're making alot of crude assumption by only looking at pressure through a square area duct door cavity and the bottom of a room or door. I wouldn't get to pre-hung upon it unless you really know what you are talking about. Just make sure there is a gap under the door so the carpet does not rub it, that simple.


No offense but you sound like a typical engineer. 

Your first paragraph was very interesting but really didn't contribute anything to solving the problem. Second paragraph makes it sound like I shouldn't worry about things like this unless I have a degree in aerodynamics.

So where is the return air coming from if I put the door down just above the carpet? Its not coming from around the door, the door is tight to the doorstop. It doesn't take an "engineer" to know that the underside of a door is not the ideal air return. Unfortunately in the real world sometimes that's all we get.

I was just installing a bedroom door last night and the air kicked on. Pulled the door right up tight to the stop and man could you feel the air moving under the bottom. And that's without casing on. Still helps to have a healthy gap.


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## Spencer

Bills86e said:


> This question is easily answered. Set the jamb on the high side, PLS.
> 
> 
> 
> 
> 
> CYA = Cover your Ass


Goes both ways. I don't have a door stretcher for when some woman comes and and doesn't like how high I set their doors. You can cut it off but you can't put more on.


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## Spencer

Man, I get home to see that all of the posts by the great engineer extraordinaire have been moved to the trash bin. I am so glad that table saw advice thread got trashed. Waste of time. Hopefully he got directed to the DIY Forum. 

Bravo Mods. Bravo and well done. :clap: :thumbup: :clap:


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## Big Shoe

I'll make this simple. Try blowing into a bottle. :whistling

It is code here that all rooms have return air. Older homes do not. You need to have a gap at the bottom for the air to move efficiently. IMO


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## Spencer

Big Shoe said:


> I'll make this simple. Try blowing into a bottle. :whistling
> 
> It is code here that all rooms have return air. Older homes do not. You need to have a gap at the bottom for the air to move efficiently. IMO


Old school common sense at its finest. :thumbup:


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