2 Questions - One Code, One Structural

karma_carpentry

Hello. I have a couple of deck questions that would be pretty basic for anyone who's done this before, but I just haven't come across them. I guess that's what the forum is for.


QUESTION 1:

I'm coming up with a proposal for someone who wants a deck above a low-slope rubber roof. I have 8" of clearance to the roof but I need a little more.

What's the maximum 'step-up' that is allowed by code, or by common sense, from the house floor onto the deck ? I know code might be different in different locations but I want to get an idea anyway. Or alternatively, what's the max height of a threshold?

In the MA code book, I see that a landing on the outside of an exterior door cannot be lower than 8-1/4" BELOW the threshold on the exterior side (780 CMR 3603.11). Okay that's common sense - it's a step. But what about a step up? Or suppose I install a new tall threshold, which is probably the better way to do it (for looks and safety) - can I put in a 1-1/2" tall threshold? Or 2" tall?

I see a code point (780 CMR 1017.1.1) that says floor surfaces on both sides of a door cannot be more than 1/2" difference -- but with an exception "This requirement shall not apply to exterior doors... which are not on an accessible route." This is an exterior door that is not on an accessible route. It's not a means of egress either.

And anyway, the first code section refers to the height of the threshold, and the second on to the height of the interior floor surface. Which is relevant?

I know this is a long post but I'll explain why I'm going to such pains over this. Normally I make deck surfaces slightly lower than the interior floor, or else on standard step height below it. (I have always used the interior floor height, NOT the threshold height, as the reference point.)

This deck is 14 by 16 feet, over a low-pitch double-hip rubber roof. The roof is very springy and not adequate to bear load, so I want to span the 14 feet with joists, but I need to use 2x10's at 12" OC while I only have clearance for 2x8's if I make the deck level with the interior floor.

I ran the calcs and got a max span of 14'3" for No2 HemFir 2x10 at 12"OC with live load of 60 and deflection of 1/240.



QUESTION 2:

What's the best technique for laying a floating deck on a rubber roof below it?

This isn't for the current proposal, but I am really curious about other people's experience and preferred techniques for it.

Do you lay sleepers right on the rubber? Do you use any other material for a little standoff? One person told me he uses a ripped section of Azek to make keep the PT lumber drier. Does anyone cut the sleepers to make 'feet' and give less contact with the roof surface?


Thanks in advance for any help on these issues.

Peladu

My shot at question 1: I would not build a transition that would be considered a trip hazard. My Opinion is anything under 4" step is a trip hazard. 4" is the lowest/highest differential I would go.

Question 2: Never done it, almost had to do it, but I was forced to walk.

jakcarpentry

I live in FL and stair code reqiures a step to be no larger than 7 3/4".

I have done a few decks using Trex over existing porches and/or roofs. Usually the foundation has been framing and a tile sub comes in and applies a sealer (i.e. Mer-Kote) over the sleepers. Once the mer-kote is applied, I can start laying Trex. Not sure if this could be done over an existing rubber roof.

Also, not knowing the HO's budget, you could consider using steel rather than wood joists to make your span using an 8" wide material.

Hope some of this gives you some ideas.

Stone Mountain

I've done dozens of rooftop decks. I don't know if I should tell my secrets though. LOL

I glue some blue styrofoam onto the roof membrane (always allowing for drainage with 2" gaps in the foam in the direction of the flow)
Then I rest the sleeper on top of the foam. Next, I bolt the header to the wall if there is room. When putting the wall plate on ,I always use flashing and sealer to prevent water ingress around the bolts.

The styrofoam does two things:
1] because it's inert, it won't rot. It also prevents the wood from being in contact with standing water, which minimizes deterioration. All substructure should be pressure treated wood.
2] It spreads the weight of the wood. Especially if it's a rubber or asphalt membrane, you don't want the joist bearing down on the surface. It can make an indent in the material, leading to potential wear. With styrofoam underneath, ( I use strips about 4" wide), you don't have the problem.

With regards to the step UP, it makes logical sense that the rise be not more going up than it would be going down.

__________________
Residential Deck Specialist: Vist our Website
www.fenceanddeck.ca

karma_carpentry

Thanks for the tips, everyone. Peladu, that makes a lot of sense about the trip hazard. Maybe I will go for a full healthy 4" step up. I can see the logic of either having no rise, or else a substantial and noticeable rise.

I also have thought of painting the leading edge of the step in a bright yellow color to make it more visible.

Stone Mountain, the styrofoam idea sounds good to me. Like I mentioned above, someone else I know uses Azek in the same way. Azek might be more durable, but it's also not as soft as styrofoam in terms of preventing wear of the rubber.

Interesting about the drainage gaps. I have thought that I would always place the sleepers in the direction of the slope, so they don't obstruct any water flow. But I guess in some circumstance it might be necessary to place them against the flow.

karma_carpentry

Just want to follow up. What I came up with is a design change - to make the deck split-level. Near the door will be level with the floor, framed with 2x8's tied into a ledger against the house. The other end of those will hang on a ledger that hangs below a doubled 2x10 joist at the step-up.

Thus, I can frame a completely solid deck and still have no step-up from the house.

Anyway, it was an outswing door and I wasn't looking forward to reversing it!

A design change that made all the difference.