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Author Topic: Concrete foundation and sill  (Read 1059 times)

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Offline Chris Nutbean

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Concrete foundation and sill
« on: January 18, 2017, 09:48:21 am »
I am close to starting my timber frame shed build which will sit on a concrete foundation. I am assuming some sort of damp proof gasket/membrane will be required between the foundation and sills. What is regarded as best practice for this type of build?

Offline ljohnsaw

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Re: Concrete foundation and sill
« Reply #1 on: January 18, 2017, 10:27:20 am »
 popcorn_smiley

My inspector referred to it as a "sill gasket".  I will be needing that when the snow melts in July so I await recommendations...  I shouldn't have issues with termites (10' foundation wall), but I was also planning on putting a wide flashing down that will extend about 1½" inside and out.  My sill will extend about 6" on the outside of the block.  This should also prevent vermin from scaling the walls and getting into the house.
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 64' - using it all to build a timber frame cabin.

Offline Chris Nutbean

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Re: Concrete foundation and sill
« Reply #2 on: January 18, 2017, 11:29:06 am »
I have had a look on Google for Sill gasket and a few options are available. However they are bright pink or pale blue. I think that your idea of a flashing may well be the way ahead for me also.

Offline Jim_Rogers

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Re: Concrete foundation and sill
« Reply #3 on: January 18, 2017, 11:45:05 am »
Yes, some type of water proof membrane should be between the wood and concrete. That way the wood won't wick up moisture from the concrete. A commonly used thing is "ice and water shield" paper, cut to fit.

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Offline ljohnsaw

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Re: Concrete foundation and sill
« Reply #4 on: January 18, 2017, 12:12:04 pm »
However they are bright pink or pale blue.

I saw the pink stuff somewhere and said, "really?".  They probably make it like that so the building inspectors can see it without really trying... ;)
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 64' - using it all to build a timber frame cabin.

Offline robmelby

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Re: Concrete foundation and sill
« Reply #5 on: January 18, 2017, 12:33:04 pm »
 Fellows, we call that product sill seal around here. It comes in different widths according to the size of your sill plate. It compress and fills voids between your masonry wall and your sill plate. We use it on every foundation we work on. Its not pretty but is an industry standard. 50' rolls . 

Offline ljohnsaw

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Re: Concrete foundation and sill
« Reply #6 on: January 18, 2017, 01:16:34 pm »
Well, the Sill Seal (and others) come in "convenient" 3½" and 5½" widths. :-\  My wall is 12" thick and my sill "plate" is 14" wide (x 8" tall).  So I would need to run 3 strips.  I think I might be doing the Ice and Water Shield route that Jim mentioned.  Or, I suppose I could buy a roll of packing material that looks like the same closed cell foam (pink), only it comes in 12", 18" and 24" widths...
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 64' - using it all to build a timber frame cabin.

Offline Heartwood

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Re: Concrete foundation and sill
« Reply #7 on: January 18, 2017, 05:59:53 pm »
The pink fiberglas sill seal is meant for variations in the concrete from dead level for up to an inch; it fills the gaps to prevent air infiltration. So does the thinner blue/pink foam sill seal, but it's meant for dead level and straight foundations with no more than 1/4" variation. Ice and water shield does indeed separate the wood from the concrete (a good idea) as the others do, but won't help with air infiltration, if that's an issue. You could always caulk or spray foam after. For a continuous sill we usually use the blue (or pink) foam stuff.

Offline ljohnsaw

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Re: Concrete foundation and sill
« Reply #8 on: January 18, 2017, 06:11:00 pm »
Well I found some on-line places that sell white "packing" closed cell foam in 12" (and other) widths.  They have many thicknesses including ¼" and ½".  The only problem is how much you have to buy.  For $48, I can get about 4 times what I need of ¼" and I could double it up to make ½".  If I bought the ½", I  don't know what I'd do with all the leftover!  My block walls are pretty darn flat and plumb - within 3/8" overall (26x49) checking with a transit, me sticking and my son reading.  However, when I plotted it out, I think my simple David White transit wasn't dead level.  The measurements showed the whole building with a very even but slight tip!
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 64' - using it all to build a timber frame cabin.

Offline ScottCC

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Re: Concrete foundation and sill
« Reply #9 on: January 18, 2017, 06:59:56 pm »
Heartwood has it exactly right with The Who What and Why.  The concern I'll have for you is when you put a piece of flashing specifically sticking out it never finishes well, tends to get bent up and then ultimately cut off.  I like to use oversized drip edge.
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Offline ljohnsaw

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Re: Concrete foundation and sill
« Reply #10 on: January 18, 2017, 07:08:54 pm »
Heartwood has it exactly right with The Who What and Why.  The concern I'll have for you is when you put a piece of flashing specifically sticking out it never finishes well, tends to get bent up and then ultimately cut off.  I like to use oversized drip edge.

Won't (shouldn't) be an issue for me.  The flashing will be 10' above the floor and grade.  No one should be knocking into it!  I plan to bend it down at a 45° angle inside and out with about 1½ to 2" exposed.  That should keep the field mice at bay.  Maybe even the carpenter ants - maybe...
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 38" cut Bandmill up to 64' - using it all to build a timber frame cabin.

Offline Chris Nutbean

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Re: Concrete foundation and sill
« Reply #11 on: January 19, 2017, 03:56:47 am »
Taking into account the very valid points in this thread versus what materials are available to me here in the UK (Grace - Ice & water shield = £300 per roll). I will look for a black closed cell foam for the bottom layer and place a damp proof course on top. I believe this should be enough to prevent air infiltration, wicking up moisture from the slab and by not being pink negate the need for a decorative trim or flashing.
I will get in touch with manufacturers of closed cell foam regarding compression limitations, expected longevity and the all important cost. Thank you for your suggestions, I will post an update on my progress soon.

Offline danreed76

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Re: Concrete foundation and sill
« Reply #12 on: January 19, 2017, 07:59:00 am »
For a moisture barrier between our sills and our foundation wall, we used a copper flashing tape that ended up being a little over $1 US/ft.  We then put the narrower 6" sill gasket on top of it, where it was completely concealed by the sill plate.

The flashing tape was thin enough that it conformed to the shape of the top of the wall once the sill gasket was compressed.

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Offline Don P

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Re: Concrete foundation and sill
« Reply #13 on: January 19, 2017, 08:35:54 am »
In theory the termite shield is not meant to stop termites so much as to make the mud tunnel visible if they do try it. That takes a perfect seal at joints and bolt penetrations and visibility of the shield everywhere. Think about things like stairs and porches, plenty of hidey holes. Codewise, never use wooden grade stakes in a pour, they will tunnel through them. The 4" solid top course or block grouting at the top of a wall is to prevent them from tunneling up inside of hollow block. The job I'm on had wooden grade stakes in the footing and hollow block up to a white oak 8x8 sill. The insides of the blocks were full of mud tunnels. The sill, floor and several corner posts were hollowed out. Interestingly we were watching the little varmints escaping through decaying mint roots from the very healthy patch out front. I apologized to the lady of the house after the unfortunate bobcat accident took out the patch... about 2' deep  :D.

In the real world, if termite pressure is heavy, mechanical methods like those have little chance of success. The well is right by this old house so chemicals are out. They will have to be vigilant.

I will look for a black closed cell foam for the bottom layer and place a damp proof course on top. I believe this should be enough to prevent air infiltration, wicking up moisture from the slab and by not being pink negate the need for a decorative trim or flashing.

Chris... I see you all mention a damp proof course and rising damp pretty often, can you enlighten me?


Offline Chris Nutbean

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Re: Concrete foundation and sill
« Reply #14 on: January 20, 2017, 04:55:24 am »
Don,
For my shed build I am using 6"x 6" English oak which will sit on an 8" concrete slab.
The climate here in the UK has more than enough rain to cause damp problems in most building projects and in the case of timbers will eventually result in wood rot.
Rising damp occurs when water from the ground is able to ‘wick’ upwards through the bricks/concrete slab or the mortar of a building – much like the way that oil rises up through the wick of a lamp. This will likely result in rotting of the timbers.
To help eliminate/reduce this problem when I dug the foundation I filled it with a couple of tons of sub-base which was then compacted and then this was covered with a sheet of damp proof membrane. The concrete slab was poured on top. So in theory I should have potentially eliminated most of the moisture that can rise through the slab. However, the sides are still exposed and condensation can also form between the concrete and the timbers. Hence the need for the timbers to sit on a damp proof course which is very similar to the damp proof membrane that I used under the slab.
I am hoping that by getting this right now will mean that my shed will be around for a very long time without suffering from preventable problems.
It's a shame that the foam sill gasket recommended in this thread is pink as this would have been the perfect solution for my build had it been black. That is why I have opted to combine some black closed cell foam and a damp proof course to create a similar product.

Offline Don P

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Re: Concrete foundation and sill
« Reply #15 on: January 20, 2017, 06:48:47 am »
Ah, so pretty much the same as we do. I'm watching it rain this morning so need to call off this morning's pour. It is a thickened edge monolithic slab we are going to set a Lucas mill on but is set up typical. The perimeter is bearing on soil for a foot or so wide then inside that the clean gravel base covered by poly to make a capillary break. Any wood in contact with the concrete is treated or naturally decay resistant... I use treated with the sill seal underneath and then stack untreated on top. I usually hide the sill seal under the treated sill, it is simply a gasket and need not be seen, I don't consider it my damproofing although it could be seen that way. I've gotten it in pink, green, white, have not seen black. I usually try to run the siding down below the sill 1/2" or so with the angled termite shield coming out under that. The foam is not seen usually.

As an aside, we used to use aluminum pretty often for that termite shield, until they changed the treated lumber formulation. There is enough free copper in the ACQ treatment that it pinholes aluminum in a very short time. I try to find the zinc-y-est galvanized I can. Somebody mentioned copper, that is ideal, no galvanic action.

Offline danreed76

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Re: Concrete foundation and sill
« Reply #16 on: January 20, 2017, 07:38:41 am »
I'm watching it rain this morning so need to call off this morning's pour.

For a long time I thought there was a law of physics in there somewhere.... schedule a footing inspection or a concrete pour, then sit back and wait for the rain!  We finally gave up and planted a garden to get the rain to stop  long enough to pour concrete  ;D

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Offline nativewolf

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Re: Concrete foundation and sill
« Reply #17 on: January 20, 2017, 08:50:18 am »
I'm watching it rain this morning so need to call off this morning's pour.

For a long time I thought there was a law of physics in there somewhere.... schedule a footing inspection or a concrete pour, then sit back and wait for the rain!  We finally gave up and planted a garden to get the rain to stop  long enough to pour concrete  ;D

So you're the one that caused the drought    :D

Offline Chris Nutbean

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Re: Concrete foundation and sill
« Reply #18 on: January 20, 2017, 09:20:00 am »
Don,
You have opted for a more elaborate foundation than my build. I think it is referred to as a frost protected foundation here. Due to the small size of my build ( 13' x 10' ) and the stable soil, I was able to opt for a straightforward floating slab.
I won't really have the means to hide the sill seal as the top 5" of the slab will be visible. I have also put a 45 degree angle around the top 1" of the slab so I can site the wood to within an inch of the true edge.
My shed will not be clad/boarded with wood, instead the frame will be on show and I will use 6" cordwood to make the walls.
I hope the weather improves to enable the pour!