Trim around windows on walls with siding

[Stephen Tashiro]

On the web, I see two different methods of putting trim around windows and doors on walls that have siding. One method is cut the siding very precisely so it buts into the side of the trim. The other method is to install trim that overlaps the face of siding, so that when you look at the side of the trim there are gaps due to the bevels in the siding. Is one of these methods preferable to the other ?

[Bill McNiel]

Stephen,
I have always butted the siding to the window frame, run the trim over the siding and then caulked the edge which seals the gaps. This provides the best waterproofing IMHO.
Regards - Bill

[Chris Kiely]

Cut a rabbet into the trim piece and your siding tucks behind the trim, and you also get a nice fit to the window.

[Eric Schmid]

I'm working on a house that has both overlapping and butted siding/trim interfaces. There is dry rot behind both. I've worked on many houses with the same issues. I prefer a butted interface, mainly because it looks better (in my opinion).

Water will get behind siding and trim, so the house wrap and flashing at the windows and doors is important. Wood on wood contact points are where most problems occur. Sealing/painting the sandwiched pieces individually and before assembly will go a long way towards prolonging the life of the wood.

Furring the siding out with 1/8" non-organic vertical strips or using house wrap with drain channels will add another level of protection.

[Neil Gaskin]

In my area most all trim is installed and siding ran to the trim with flashing. Depending on the type of siding and trim and the amount of expansion and and contraction will determine the space between two left for caulking. Running tight doesn't allow for caulking to operate properly. I have seen PVC trim was a pre-installed rabbit. We don't typically use the detail around me. Your siding and trim are your first barrier against water infiltration but should not be thought of as the only. The rain screen mentioned below is best especially with wooden siding. The real important part is the whole installation from building wrap, window installation details, and how everything integrates together along with proper flashing done properly with enough spaces left for caulking between materialsetc. I wouldn't recommend installing trim over the siding for a few reasons.between materials

[Todd Mason-Darnell]

What type of siding?

[Peter Kelly]

On the web, I see two different methods of putting trim around windows and doors on walls that have siding. One method is cut the siding very precisely so it buts into the side of the trim. The other method is to install trim that overlaps the face of siding, so that when you look at the side of the trim there are gaps due to the bevels in the siding. Is one of these methods preferable to the other ?Door / window trim and corner boards first, then siding tightly butted to casing. Can't imagine how you'd make it work the other way around.

[Ken Combs]

Pick the one that looks the best to you.

The 'trick' to siding and exterior trim is not so much the application of them as it is the sealing and flashing of the openings before starting trim/siding.

Best thing ever invented for this is the newer self-adhesive flashings. Properly done they will protect the building from damage no matter how the trim and siding is done.

That said, I like the look of thick window/door/corner trim with butted siding. Add flashing under all butt joints and and a premium caulk IN, not on, each. By that 'IN' I mean, put a bead on the end of the siding, before butting it to the trim or adjacent piece. That seals the joint from the inside out and the caulk is protected from weathering.

[Wayne Lomman]

I always butt the siding to the trim. As the other guys have said, the flashing must be 100% before starting the timber work. My house is 50 years old and still no rot. Good old tassie oak. Cheers

[Larry Edgerton]

Pick the one that looks the best to you.

The 'trick' to siding and exterior trim is not so much the application of them as it is the sealing and flashing of the openings before starting trim/siding.

Best thing ever invented for this is the newer self-adhesive flashings. Properly done they will protect the building from damage no matter how the trim and siding is done.

.

I disagree.

If water gets in behind the siding where is it going to go? How will the membrane possibly help? At best it redirects the water down to below the window where it will rot out the wall there. For any flashing to work it has to direct the water to the outside, something the bitutane membranes around the window do not do, it stays in the wall.

The larger problem is the the normal transfer of moisture through the wall is impeded. In cold weather moisture tries to go from the inside to the outside. When it hits the backside of the membrane in condensates and stays there.

I was forced by the code department and the homeowner to use it on a house I built 15 years ago. I had wood siding and wood trim. I got a call about a problem with a window and went to investigate. What I found was that the moisture trying to get through the wall had collected on the backside of the membrane and rotted out the wall. The back side of the membrane came off with osb stuck to it, and in this window, a kitchen window so more moisture, the wall was rotten to exactly the width of the membrane. they were Pella windows with steel flanges, and the flanges were totally rusted out. The siding had not leaked. Upon further investigation we found the same thing happening to lesser degrees on all of the windows, worse on the windows in bathrooms and the kitchen.

For a very informative read on why many of the things we are doing according to code are wrong I would suggest that you buy a copy of John Listerbureks book that best suits your climate. I use "Building in cold climates" as I am close to the Canadian border. It explains how moisture transfer happens and gives solutions that you will not see in the code book. When we started to tighten up our building we created a whole bunch of new problems. Shoddy construction methods actually make it worse, as contractors leave places for water/moisture laden air to get in, but no place for it to get out.

On the trim. I do not like a rabbit over the trim. it makes it too hard to get a good caulk seal, and looks like vinyl. In a retrofit I will lay down a bead of Lexel against the bevel siding and push the trim into that, then caulk the outside with Big Stretch to finish. I like a sill to direct water out over the siding, but if the design will not allow at least have a rabbit on the bottom of the bottom piece to lead the water out onto the siding.

.

[Ken Combs]

I disagree.

If water gets in behind the siding where is it going to go? How will the membrane possibly help? At best it redirects the water down to below the window where it will rot out the wall there. For any flashing to work it has to direct the water to the outside, something the bitutane membranes around the window do not do, it stays in the wall.
The flashing is only part of the building system. All siding should have a rain screen drainage system behind it. wind driven rain can penetrate into unlikely places!

The larger problem is the the normal transfer of moisture through the wall is impeded. In cold weather moisture tries to go from the inside to the outside. When it hits the backside of the membrane in condensates and stays there.
That's not the fault of the membrane. Rather is speaks to the need for a vapor barrier on the inside. Either use closed cell foam, or a vapor tight sheet over the studs and under the finish material. As a last resort, there are vapor tight paint products. controling indoor humidity is also good. That is especially true if there are unvented heat sources in use.

I was forced by the code department and the homeowner to use it on a house I built 15 years ago. I had wood siding and wood trim. I got a call about a problem with a window and went to investigate. What I found was that the moisture trying to get through the wall had collected on the backside of the membrane and rotted out the wall. The back side of the membrane came off with osb stuck to it, and in this window, a kitchen window so more moisture, the wall was rotten to exactly the width of the membrane. they were Pella windows with steel flanges, and the flanges were totally rusted out. The siding had not leaked. Upon further investigation we found the same thing happening to lesser degrees on all of the windows, worse on the windows in bathrooms and the kitchen. see above, the humidity control/vapor barrier is especially important in those areas. Even it the moisture doesn't make it to the back of the membrane, it can condense inside the wall. the exact point of condensation depends on outside temp, inside temp etc. The point is, it shouldn't even be inside the wall.

For a very informative read on why many of the things we are doing according to code are wrong I would suggest that you buy a copy of John Listerbureks book that best suits your climate. I use "Building in cold climates" as I am close to the Canadian border. It explains how moisture transfer happens and gives solutions that you will not see in the code book. When we started to tighten up our building we created a whole bunch of new problems. Shoddy construction methods actually make it worse, as contractors leave places for water/moisture laden air to get in, but no place for it to get out.Don't have the book, but have read a lot of his articles. Good info, but it does conflict with other published recommendations. Manufacturers have their own, other engineers also. I like to take data from all the sources and sift through it to pick what sounds right to me.

On the trim. I do not like a rabbit over the trim. it makes it too hard to get a good caulk seal, and looks like vinyl. In a retrofit I will lay down a bead of Lexel against the bevel siding and push the trim into that, then caulk the outside with Big Stretch to finish. I like a sill to direct water out over the siding, but if the design will not allow at least have a rabbit on the bottom of the bottom piece to lead the water out onto the siding. all good practices. The brands may differ by region, but quality products are available everywhere.

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See my comments in red above. I should have mentioned originally that no one thing is a sure fix. Rather, each product and method have to work together as a building system. In the old days we didn't have to worry about any of that. Buildings had enough air leaks to dry out!

=

[lowell holmes]

Stephen,
Did you find a consensus here? :)

I don't have wood sided walls, so I have no experience. I do have wood trimmed plate glass picture windows. The white pine trim rotted regularly. Even to the point that I removed the windows and made a new window frame. I will not bore you with the details, but I do have suggestions.

My wood of choice for wet applications is quarter sawn white oak. Provide channels for moisture to escape. I do caulk, but I caulk and counter caulk. Try to avoid leaving any pockets that moisture will accumulate in. You might monitor the flashed connections to see if any penetration is there so you can deal with it.

I would consider making the window trim from quarter sawn white oak.

Living in a wet area (Galveston County), my back door threshold had to be replaced two times because it deteriorated because of moisture. I replaced it two years ago with white oak, and being bare, it has maintained it's integrity. It is as solid as the day I put it in. It is a rock solid threshold.