# Filling gap under exterior door threshold



## Bob... (Jan 29, 2013)

I just replaced an old sliding glass door with an Atrium unit/center hinge. All went well, but the wide threshold leaves a 3/4" gap under. How should I fill this gap? I'm thinking just pack some non-ag concrete under it, but want to do it right. Thanks for any suggestions.


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## turbo4 (Jan 30, 2021)

I usually cut and plane a piece of treated 5/4 for those places. Regular wood may rot under there. Plus you need something uniform and solid to support the threshhold. Either flush with the threshold or it can protrude somewhat.


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## Bob... (Jan 29, 2013)

turbo4 said:


> I usually cut and plane a piece of treated 5/4 for those places. Regular wood may rot under there. Plus you need something uniform and solid to support the threshhold. Either flush with the threshold or it can protrude somewhat.


The concrete under the threshold is uneven about 1/2" under (as though someone had packed concrete under the original sliding glass door). So placing a board under wont' work, unless I rip it to fit smallest thickness of the space, which will compromise support. I tried chiseling, but it causes more unevenness..


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## turbo4 (Jan 30, 2021)

Some type of cement mix may be your only option then. You would have to pack it good so theres no space under.


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## Bob... (Jan 29, 2013)

turbo4 said:


> Some type of cement mix may be your only option then. You would have to pack it good so theres no space under.


Do you have any suggestions as to the most efficient way to pack it? Is a knife the only viable way? I guess I'd have to make it thin enough to pack well, but thick enough not to 'run'. Should I place a board across the gap opening when I finsih packing to keep it from bleeding out?


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## Carpet (Jan 1, 2018)

I'd cut 1/2" thick PVC and put it back there, and add slim foam rod to fill in gaps. Then, pack in concrete or mortar up to the edge for aesthetic and to prevent water getting back there. I think the PVC would hold up to weight as it's resting on a surface. The only other option would be to stuff concrete into it like turbo4 said.


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## Bob... (Jan 29, 2013)

I finally got around to dealing with this this week. I used a Rapid Set Cement All and 4" knife and just kept packing it under. I used blue tape on the threshold and step. It came out nice and solid. Thanks for your comments and input.


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