How to protect exposed half-lap sill joinery?

[sheneman]

Hello!

I am building a small 9x14 timber framed structure.  I am using a pier foundation with 8x8 sill plates on the piers.  The timber frame is actually going to be infilled, so the timbers will be visible from the inside and exposed on the outside as well.  There will be large overhangs on the roof, but driving rain and snow can and does happen.

I am concerned with the exposed half-lapped sill plates.  In general, the exposed timbers will be fine if they have a chance to dry, but I worry about water getting trapped into the half-lap joinery and causing rot.   I can make the joinery super tight, but water from driving rain and snow will still undoubtedly make its way into the half lap where it cannot easily air dry.  The sill plates are made from untreated Douglas fir and Grand fir.

I've thought of soaking the ends in a penetrating oil.

What else can be done?  Should I try to somehow seal, cover or protect the exposed joinery in some other way?  Caulk the joinery so water won't find its way inside?   I don't want to use paint, but I'd be open to using clear protective oils or stains.

[Jim_Rogers]

sheneman:
First of all, another time when you're going to do a sill corner, you may need to pick a different joint.
A half lap isn't the best joint for this corner, in my opinion.

The way the old timers would have done it is to have one long sill going the complete distance of the frame. And a shorter cross sill connecting the two long sills.

As shown here:



This means that there is only one timber with end grain, and that would be on the gable ends of the building. This prevents some water shed off the roof on the eaves sides to be splashed back up onto the end grain of the timber, should there not be any siding to shield it.

To prevent water from entering your joint now, you'll have to either coat the ends with something that will repel the water or some how or other protect it.

Maybe some one who builds log homes and has had experience with treating exposed log ends can give you some better advice.

Good luck...

Jim Rogers

[shinnlinger]

I'm with JIm,  I built a frame once with a lapped sill like you did and I made a mental note...never again.

As for sealers, I have used melted parfin (double boiler, trust me) and mixed it 50/50 with turpentine...or was it kerosene???   with good results in the not so noticeable but beads the water department...

[sheneman]

Thanks, guys!  Next time, I will try your sill plate joinery.  It makes a lot of sense.

-Luke

[routestep]

Get some Timbor or Boracare(sp) and put it in the lap joint. That should slow down rot just in case water does find a way in.

[highrail]

He is using a pier foundation.  Jim, is there any problem with support for the short side sill using that join?

highrail

[shinnlinger]

Do you mean you want to pier the long side and hang the short side?

If so, I would guess you are probably OK doing so with a structure that small, BUT What kind of load to you anticiptate the structure getting?  You can run a load calculation for a 4x8 on the flat and see if you are comfortable with the numbers, or you can set 4 more piers, 2 under each end of the short side sills  and weigh the cost/hassle/appearance vs risk.

[TerryKing]

Get some Timbor or Boracare(sp) and put it in the lap joint. That should slow down rot just in case water does find a way in.

These are a combination of ethylene glycol (auto antifreeze) and borate.  You CAN make it yourself for a lot less money...

-- The whole article on this is here:   
http://terryking.us/boatbuilding/RotEthyleneGlycol.html

This is aimed at boatbuilding and I've used it extensively for that.  But I also use it (plain ethylene glycol antifreeze) successfully on exposed porch floorboards, outhouse flooring, and on parts of the 32x32 foot timber framed barn I built 5 years ago in Vermont.  I especially hit the lap joints in the sills (8x8 hemlock) every year. I also soaked all the lap joints, and all the post pockets with glycol a couple times before assembly.  And I have  2 removable floor sections for a truck-repair pit that accumulate a lot of sawdust, dirt and water over a winter.. Once a year I have blown them clean and soaked the mating areas with glycol.  Clean as a whistle after 5 years.. And there is no green or growing stuff on the sill joints either.  I just use a hand sprayer, and wash my hands afterward.. Not highly toxic like other chemicals. 

"Works For Me"....

----------------( extract: How-To part from Dave Parnell )-----------------
      BORATE WOOD PRESERVATIVES:
      COMMERCIAL AND HOME-BREWED       

          Tim-Bor®: Solid sodium octaborate; dissolves in water to make
          approx. a 10% solution containing 6.6% borate (B2O3); about
          $3/lb. plus shipping.
          Ship-Bor®: Same as Tim-Bor®; $19.95/lb. plus $2 shipping.
          Bora-Care®: 40% solution of sodium octaborate in ethylene
          glycol; 27% borate content; $70/gal. plus shipping.

          Home-Brew Water Solution of Borates:

              Based on U.S. Navy spec. of 60% borax-40% boric acid (this
              ratio gives the maximum solubility of borates in water);
              65% water, 20 %borax, 15% boric acid; 15.8% borates; borax
              costs 54 cents/lb. (supermarket), boric acid costs about
              $4/lb. in drug stores (sometimes boric acid roach poison,
              99% boric acid, is cheaper in discount stores); equiv. to
              Tim-Bor® or Ship-Bor® at 30 cents/lb. To make this
              solution mix the required quantities and heat until
              dissolved. The boric acid, in particular, dissolves
              slowly. This solution is stable (no crystals) overnight in
              a refrigerator (40°F.), so can be used at temperatures at
              least as low as 40°F.

          Home-Brew Glycol Solution of Borates:

              This is equivalent to Bora-Care® diluted with an equal
              volume of glycol to make it fluid enough to use handily;
              50% glycol antifreeze, 28% borax, 22% boric acid. To make
              a stable solution you mix the ingredients and heat till
              boiling gently. Boil off water until a candy thermometer
              shows 260°F. This removes most of the water of
              crystallization in the borax. This solution is stable at
              40°F and has a borate content of 26%. With antifreeze at
              $6/gal. and borax and boric acid prices as above, this is
              equivalent to Bora-Care® at about $15/gal.
----------------( end extract: How-To part from Dave Parnell )-----------------

Dave is a retired DuPont chemist who has done a lot of personal research on this subject.

-- Toxicity?? We all know ethylene glycol antifreeze is toxic to us and all mammals.  So make sure there is no remaining liquid left for animals. It absorbs pretty quickly into wood. Here's Dave Parnel's take on toxicity:
---(copy)---
          Glycol's toxicity to humans is low enough that it has to be
          deliberately ingested (about a half cup for a 150 lb. human);
          many millions of gallons are used annually with few
          precautions and without incident. It should not be left where
          children or pets can get at it, as smaller doses would harm
          them, and they may be attracted by its reported sweet taste
          that I have confirmed by accident. The lethal dose of borates
          is smaller than of glycol, but the bitter taste makes
          accidental consumption less likely.
---( end copy)---
Note: There is a movement in USA to require a bittering agent to be added to auto antifreeze.

Hope this is useful to some people...

[Jim_Rogers]

Terry:
Welcome, and thanks for all that information.

Jim Rogers

[Jim_Rogers]

..... is there any problem with support for the short side sill using that join?

If the short side sill is only holding up the flooring from the outside edge to the next floor joist, I would say no, that there isn't any problem with that joint. However if there is more load bearing posts being supported by that cross sill then there maybe a need for a pier under the sill directly under the post.
It really hard to say without seeing the entire frame design.

[moonhill]

You could make the pier under the corner large enough to support both sills.

Tim

[shinnlinger]

I like Moonhills's idea for an enlarged pier, but one thing to consider when making/pouring peirs is you dont want alot of exposed area on the top of the pier for rain to hit and splatter on the sill. 

In my head, when I hear pier, I think Sono tube, and if so you can go big, but it will have a large flat splatter area and increase costs/excavation.

Obviously though you could make custom forms with a tapered edge or set big stone or precast square piers.

Terry,

I have yet to digest your info, but it seems like a great idea.  If you want to be real cheap, you can ussually go to junk yards and get all the anti freeze you want for nothing.  They need to drain it from cars before it mixes with other fluids and becomes hazardous waste.  They ussualy have hundreds of gallons sitting around.

[sheneman]

The piers for this project are actually 8" sonotube piers.  They have a piece of angle-iron coming out of the concrete, and welded to this angle iron is a custom-welded L-shaped (90 deg angle) steel plate.  This plate provides a bearing surface for both sill plates at that joint.  We bolt both the sill plates onto the steel L-shaped plate.

This design raises the sill plates off the concrete,  strongly ties the sill plates to the foundation, and provides a surface for both sill plates at each corner to rest on.  There is no weight on the joinery itself.   The angle-iron was placed a bit off-center in the concrete such that the concrete piers are well-protected by the sill plates, etc.

-Luke

[shinnlinger]

Sound neat....If you are worried about rust, you could wedge some PT scraps under there too...

[ARKANSAWYER]

 




  In either joint there is as much end grain exposed to the outside.  I put my siding down so as to cover this area.  I also put linseed oil in the joint and on the ends.  But I try to get the siding to cover them when possible.  If I am leaving exposed I would most likly seal the thing as well as possible.  You can seal the joint with silicone calk.