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Mortar Slope for a Basement Window


Itneverends42's Avatar
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02-05-13, 03:47 PM   #1  
Mortar Slope for a Basement Window

Hi - I'm in the process of replacing an old metal frame basement window. I have removed the old frame and excess mortar and now have a nice rough opening to work with. I'm planning to install a vinyl slider and would like to install it right in the concrete with tapcons. Ideally I'd like to have the biggest window I can fit as it will be the only window in a finished basement. My concern is if the bottom of the window is right on the concrete, will that allow me to mortar enough slope to correctly shed water? Is there a correct 'pitch' that should be used? From the outside, the window would be set into the rough opening about 3 inches.

If it helps I'm going to install an American Craftsman 70 series slider. These have weep holes on the outside bottom which may make installing the mortar trickier.

Any help is appreciated! Thanks!

 
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02-05-13, 03:58 PM   #2  
I see this is your first post, so welcome!

Its always nice when the opening can be prepared prior to installing the window, so that the mortar is cured. I know that's not always possible.

I'd plan to set the window on a 3/4" wood (or PVC) sill, laid flat on the cement, so I would measure accordingly, making a 1" deduction on your height and 1/4" deduction on your width (provided the opening is plumb and level = nice and square) Keep the wood about 3/4" back from where the exterior surface of the window will be and use that as a straightedge for the highest point of the mortar.

Install the window last and after the mortar is dry, caulk the bottom of the window to the mortar, being careful to not block the weep holes.

If you have to do the mortar last, after the window is installed, that's not a big problem, just be sure you get the mortar back into the void under the window and use the window itself as your straightedge as you slope the mortar. Setting the window up 3/4" in the rough opening is the main thing. Caulking the window after the mortar has cured is also important.

 
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02-05-13, 05:01 PM   #3  
Thanks for the reply!

I'll plan to use either 3/4" PT or PVC for a sill. When you say keep the sill 3/4" back from the exterior surface of the window, this means the front 3/4" of the window will actually sit on cured mortar, correct? Just want to make sure I understood correctly!

 
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02-05-13, 08:42 PM   #4  
Yes, that's right. That ensures that the slope of the mortar comes up underneath the outer 3/4 - 1" of the window, which is where the window has the most leak potential from rain and so forth. The window will actually be suspended over the slope of the cured concrete, since it will actually be sitting solidly on the level surface INSIDE, while the cement drops away from under the window on the OUTSIDE. There will be a small gap there, maybe 1/8" - 1/4" or so. If foam backer rod fits into the gap, that would be a good choice prior to caulking. You will be depending on your sealant to keep out any water there. I'd recommend OSI Quad, Vulkem, NP1, or a similar polyurethane sealant for all sides of the window.

You will like using the PVC if you so choose. Lays flat, doesn't warp, cuts nice, never rots.

 
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02-11-13, 08:59 AM   #5  
Just ordered the window yesterday taking into account the 3/4" sill for the rough opening.

The foundation wall is filled block. However, the two top most blocks the sill will rest on are only filled with stone. I was going to remove the stone and to fill them in with something and skim the top so it's nice and level. I was going to use regular mortar. Is that ok or should I use an actual concrete mixture? (If this question belongs in another section, let me know).

Thanks!

 
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02-11-13, 07:29 PM   #6  
a bag of ready mix cement would work better to fill a large void, imo.

 
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02-12-13, 03:56 AM   #7  
I wouldn't remove much of the rock. IMO if you have a half inch space over the rock and can force mortar mix down into the rock - that would be good enough.


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