How do I build a shed under my deck?

April 13, 2013 5:52 PM

I want to enclose underneath my deck but need to deal with frost pushing up on the walls.

I have a deck off my house that overhangs my driveway. It's about 8' off the ground and the driveway is paved right under it. I'm in the process of replacing all the flooring and railings on the deck and plan to install an under-deck waterproofing system. I'd also like to add walls and a door so I can use it as storage.
I know how to frame a wall. My concern is about where the bottom of the wall meets the pavement. Is it unwise to attach the baseboard to the pavement? I'm concerned about winter heaving pushing up on the wall and ultimately up on the deck. I don't want to dig up the pavement and pour a footing for the whole thing.
Anybody have any thoughts?
I'm considering just raising the baseboard a few inches and leaving a gap at the bottom but then the floor will get wet when it rains.

posted by Thrillhouse to Home & Garden (4 answers total)

Hang your wall from whatever is supporting your deck. Build a grade beam (I'd probably use concrete for unheated space but pressure treated wood would work too) underneath the suspended wall leaving a gap between the wall and beam. A couple inches is probably more than enough. Bring your outside finish (and your inside finish if you have one) down below the gap. You'll want to cover the gap by a generous margin to allow for settling. You can fill the space with a compressible insulation like fiberglass bats. And I'd caulk the wall - beam joint with a butyl caulk to resist the ingress of mice and insects. If things are actually moving relative to each other you may need to re-caulk on a regular basis.

Ideally you'd want the bottom of the exterior finish at least 6" above ground to protect the framed wall and the finish material from water or 12" if you have termites so your grade beam would be taller than that.

Obviously don't attach the wall to the grade beam.

Having said that does your pavement currently move? If you don't see movement of the pavement relative to the support column of the deck you may be over thinking this.
posted by Mitheral at 6:49 PM on April 13, 2013

I get the feeling that Mitheral has done more of this sort of construction than I have, but I'd be inclined to have the weight of the walls sitting on the pavement, with an expansion-tolerant joint up at the top rather than between the wall and a "grade beam." I don't understand what's gained by suspending the walls from above.

I'm considering just raising the baseboard a few inches and leaving a gap at the bottom but then the floor will get wet when it rains.

Not sure this is what you meant, but "baseboard" generally refers to a piece of trim, not something structural. Did you mean 'bottom plate?' Anyhow, I would not count on any seal between walls and pavement to keep water out. If you want a dry floor and the pavement slopes toward the enclosed area, you'll have to put in an elevated floor above the pavement level.
posted by jon1270 at 2:36 AM on April 14, 2013

Your right that either will work. However by hanging the wall from the columns you only need a movement tolerant joint at the bottom. If you place the walls on the ground you need a movement tolerant joint at the top and at the support columns for the deck. With the joint at the bottom the wall ends up rigidly attached to the building on three sides. Also imo building the former is easier. All you have to do is place a spacer between your grade beam (say a 2x4 on it's flat) and the wall framing and then drive it out once the wall is fastened. Finally if you end up putting electrical or even water services in the wall you won't have to deal with wire/pipe going thru an expansion joint.
posted by Mitheral at 8:33 AM on April 14, 2013

Thank you, Mitheral. That makes perfect sense. That's what I'll do.
posted by Thrillhouse at 11:40 AM on April 14, 2013