Real estate


Choosing the right deck materials

Can you mix PVC decking and pressure-treated wood, the owner of this deck asks.

Q. I have three wide stairs leading from my main deck up to the back-door landing. While I was removing the rotted pine-trim boards, I found pressure-treated ones backing the full width of each step. Can I install the new PVC trim boards directly on the pressure-treated boards? Seems like the space between would always be wet. Should I prime the pressure-treated first? Many thanks for your advice.


A. Yes, you can install PVC riser and trim boards directly over unprimed pressure-treated wood. Your stair stringers (the main structural supports) have a solid board connecting them, which is great for decking and trim-fastening support.


The PVC will be unaffected by the moisture on the pressure-treated wood; however, there are a few procedures that should be followed when installing PVC trim. Some contractors are not aware of or do not follow these steps:


Get The Weekender in your inbox:
The Globe's top picks for what to see and do each weekend, in Boston and beyond.
Thank you for signing up! Sign up for more newsletters here

1. Do not use finish nails, brads, or staples. Fasteners MUST have a head.

2. Fasteners should penetrate the wood substrate a minimum of 1½ inch.

3. The best is the Cortex fastener system by FastenMaster. This system has a special adapter that sinks the screw at the correct depth to receive a PVC plug. It conceals the fastener heads without caulking, sanding, or painting.

4. Install fasteners no more than 2 inches in from the end of boards.


5. Install two fasteners for boards less than 12 inches wide. Wider boards require more fasteners.

6. Install fasteners every 12 to 16 inches on-center.


1. Glue joints and secure with a fastener to prevent separation.

2. Use PVC adhesive that is fully cured in 24 hours.

Expansion and contraction

Here is where contractors fail to follow the installation rules. There is nothing worse than getting a call back a year later because trim boards open up a quarter of a inch.


1. PVC trim does expand with temperature extremes and moves differently than wood.

2. A general rule of thumb is that it will move inch per 18 feet. Since you are doing only stairs, this doesn’t apply to you.

3. If you ever install long sections, such as 16- to 20-foot fascia boards, consider using a solid sub-fascia (to provide a stronger base) and construction adhesive and fasteners to reduce movement.

4. Installing and gluing one joint tight and allowing 1/16 of an inch on the other joint to move is another option, or this joint could be “shiplapped” (cut so there’s overlapping) to hide the movement. In both cases, apply a flexible caulk.

Q. Our concrete outdoor steps need replacing. We’d like to use pressure-treated wood, but were told to utilize composite material for the treads, baluster, and handrails. To keep costs down, we’d like to use composite only on the treads. Is it OK to do it this way? What are your thoughts on using Trex?


A. I’m not sure why someone told you not to use pressure-treated wood for the whole project. While it’s not my first choice, it is certainly durable enough. Yes, it’s absolutely acceptable to mix and match materials, and Trex is a fine product.

Dear Rob

A few weeks ago a reader asked whether electric-heat cables can be installed on vulnerable pipes in a wall (“Tips to keep your pipes from freezing this winter,” Oct. 23). My answer focused on that question, and a few readers suggested great alternatives.

One is to install a circulation pump and pipe loop to keep the water moving. I did this a few years ago in a client’s unheated telescope observatory. The telescope was cold-water cooled, and we decided that instead of heating the pipe, we could put a thermal sensor in the area to turn on the circulation pump. It worked like a charm and is still running today. It required a continuous pipe loop.

Another approach is a hot-water recirculation setup using the existing cold pipe as the return loop. This approach can be done without the considerable hassle of exposing pipes and wrapping them in insulation and heating tape. Parts needed: a thermostatic bridge shunt, bronze pump, outdoor thermal sensor, and timer.

Thanks, readers.

From Pat Sluz in New Hampshire: Thank you so much for your “right on” advice on our odor issue (“Sewer odor creates a stink in addition,” Oct. 16). We followed your advice, and sure enough, there was no P-trap. A shoutout to Jim Wetherbee of Wetherbee Plumbing & Heating in Milford, N.H.; he isolated the area, cut just a 12-by-12-inch hole in the garage ceiling, and installed one. As you — and Jim — suggested, the plumbing was probably done by the previous homeowner; Jim noticed a few other odd things that “no licensed plumber would do.”

For safety reasons, he suggested we have an electrician check the wiring in that section of the house to be sure no other cost-saving shortcuts were done that may be a fire hazard. We did that, and sure enough . . . Thank you again and again for the great advice.

Rob: Thanks for getting back to me, Pat. I’m so glad it worked out.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to or tweet them to @robertrobillard.