Mounting block material. Is redwood okay?
DamionL
| Posted in General Questions on
Hi all, I need to make some mounting blocks and I bough some pvc trim but it’s only 1” thick. I need something thicker so I would like to know what other durable materials are acceptable? Could I mill some 2x redwood? It’s locally available and seems durable for outdoor use.
Attached is a photo of the siding. I plan to flash the penetrations with prosoco joint and seam filler, space the block 1/8” off the siding to continue the air gap, and caulk all the edges of the siding to the block using Sashco big stretch.
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You can get PVC "trim" in dimensional lumber sizes too, such as a 2x4 that would be 1.5" thick. You can also laminate several pieces of PVC trimboard with the solvent welding process. I much prefer PVC for small blocks (looks like your application is for an electrical box?) since it tends to hold up very well over time.
Bill
I may try to laminate 2 pieces and plane it down to size. I’m just a bit concerned with having open pores on the face due to planing the laminated block to size. Apparently you can close the pores of cut faces with acetone. Also, there are so many color limitations on pvc due to expansion and contraction due to heat, but maybe this isn’t as much of an issue for a mounting block?
Yes, you can seal the pores with acetone. Solvents melt the PVC a bit, then the polymer chains in the plastic can move freely and "retangle" when the solvent evaporates. You can also "heat polish" the material, but that is tricker to get right than just wiping it with some solvent.
I don't see an issue painting a block whatever color you want. Dark colors can be a problem with larger pieces of PVC trim because the hotter it gets, the more it expands. This is especially an issue with very long pieces (like fascia boards). With a small block, any expansion is going to be pretty limited simply due to the small relative size of the piece, so I wouldn't worry about it. You can, BTW, get bloack PVC trim board material if you want.
What I would do if laminating two pieces is to cut the pieces, then glue the two cut faces together. This will leave you a mill finish on the uncut sides, so no issue with open pores, and the solvent welding ("gluing") of the two pieces will seal everything in the mating area between the two pieces.
BTW, you can order solid PVC block from plastics distributors like US Plastics in Lima, OH. You can get it in various thicknesses, and it's solid material (no pores at all). I've machined some special mounting blocks for electrical fixtures out of this material in the past -- you can work with it with regular woodworking tools.
Bill
Thank you for the advice Bill. The reason I was going to plane both faces is because even though pvc is supposed to be stable, the pieces have a small bow, so I was planning on milling both faces flat and to thickness, then glue. Do you think having a milled and acetone surface on the face will be an issue? Also is the process to just lightly sand the cut surfaces with 220 grit sandpaper then rub some acetone and wipe off?
You won't have a problem with the solvent-sealed surface. All the solvent is doing is a chemical version of heating the plastic up enough to flow, then when the solvent evaporates off, that's like the plastic solidifying again.
Plane or mill the surfaces flat, then sand them smooth, then wipe on some solvent and let it evaporate off. When you're done, you should have a pretty smooth surface. If it's not as smooth as you want, try another solvent step. It doesn't hurt to do the solvent step several times if necassary.
Bill
Damion,
Yes redwood will work, although you will need to make the block bigger than if you used PVC just so it doesn't break.
You don't want to leave a gap behind the block, but rather there should be a head-flashing going back to the WRB to divert water to the exterior. You can seal the sides and bottom, although with corrugated siding using flashing there is preferable too.
I've sometimes used hardwood like maple for small blocks. Hardwood tends to be more durable than softwood for this sort of thing, and it's still easy to paint. I much prefer the PVC material though simply for longevity since I find all of these mounting blocks seem to be much more prone to rot and deterioration than the regular siding and trim.
Bill
Malcom,
My rationale for my process is because first of all the wall wrb is peel and stick and I cannot remove the fiber cement panels because the overlaps are glued and the panels break easily. So getting behind the panels or under the wrb to install a head flashing would be cumbersome if even possible.
Instead the plan was to treat the mounting block similar to the corrugated panels which theoretically act as a rain screen. First of all the penetration will be flashed to the wrb, so that should cover any water leaks. By spacing the mounting block off the wall, any water that gets behind the mounting block should run off similar to if the corrugated panel was still there. Caulking all 4 sides seals the block to the siding, hides the cut inaccuracies, and hopefully acts as an extension of the current siding by keeping water out but letting water that gets in shed down or dry back out through the fiber cement panels.
Not an ideal situation but I feel like it’s the best option with my limitations.
Damian,
I thought they were metal panels. What you propose makes sense given the limitations.
I avoid PVC when I can and prefer to use real wood. Redwood will work fine, just oversize it as Malcolm recommends because the corners will be vulnerable to breaking. I space them off the wall a bit as you suggest, and--importantly--flash them above just as you would a window, and provide a rabbet below. Big Stretch is amazing but mechanical laps are even better. I have been installing and/or specifying them for over 25 years and don't know of any that have failed, either painted or unfinished. (That doesn't mean that none have failed, but if done well they last a long time.)
Planing Azek makes a huge mess--the dust has a high static charge--and leaves a very rough finish. I've never tried the solvent approach but imagine it would work well. I have used multiple coats of primer, sanding between coats, and that works as well.
Thank you Michael, please see my reply above to Malcolm as to why I wasn’t planting on installing flashing. Just curious what other types on wood you specify and why you avoid pvc?
Damion, that makes sense, but if the top of the block is going to be exposed, water will sit on it and eventually the wood will rot. Sloping the top would help.
I'm in the northeast and often use eastern white cedar wall cladding with white, red or yellow cedar blocks. When the cladding is painted, I often use Lifespan pine--a radiata pine that has a light rot-resistance treatment and is pre-primed. Sometimes I just use (or ask the builders I work with to use) plain white pine that is primed on all sides.
PVC has a ton of health and environmental problems, it's a non-renewable resource and it's no fun to work with. When cutting outside, the microplastics created go everywhere and last forever. I use bio-based materials whenever I can, and when I can't, I prefer metal to petroleum products because it can be recycled easily. But I have used PVC as house trim and for light blocks.
When PVC trim was still fairly new to the market I wrote this article on using it as house trim. If you click on the PDF you can see light blocks (and outlet blocks) of Azek with copper flashing: https://www.finehomebuilding.com/2008/01/17/a-balcony-deck-built-to-last.
I've had problems with my SawStop saw when ripping Azek/ cellular PVC down, presumably for the same reason of static charge. It doesn't trip the blade brake or anything like that, but it seems to fault out the motor controller and shuts the saw off after ripping a few feet, even in bypass mode. Love that saw, but it sure doesn't like cellular PVC.
PVC is a dielectric material, an excellent electrical insulator. As such, it can hold a lot of static charge, and moving a bunch of it through the air at high speed (like blowing PVC sawdust in one direction) is a great way to build up that charge! I've had similar issues when vacuuming dust through long lengths of polyethylene hose/tube materials.
About the best you can do to keep the static charge down is to do the work on a very humid day.
Bill