Solution for rooftop decks

by Dave
(Vancouver)

Typical failure

Hi all -

I live in a townhouse complex in Vancouver in which all units have a rooftop deck located above a flat section of roof. What we are seeing are a number of decks failing with boards that seriously twist, rendering large sections of deck virtually unusable.

Some background:

1- The decks are built over black roofing membrane and get a decent amount of sun throughout the day (heat?)
2- There is a slope to the roof for drainage so the sleepers get quite thin at the high points - less than 1/2" depth (bad design)
3- The boards (2 x 6) tend to relax during the wet winter months and twist during the dry, hot summer months.

An initial thought was to replace the boards with 1x6 or 1x4 and add the extra inch to the sleepers. This will keep the finished deck flush with the door sills and a metal cap at the front end. We also thought about securing stainless steel or something across the end of the decks to try to resist the twisting.

We are looking for a long term solution and so I thought I'd post here to see if anyone had any thoughts.

Thanks!
Dave

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Apr 24, 2010
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A few ideas that might help...
by: Editor - Rich Bergman

What you have described implies potentially quite a few problems. Looking at your pictures it appears that easch joist is cut progressively thinner than the next so that by the time you get to the very last joist it is about 3/4" thick. This means there is no way that thin little sliver of a joist will offer you enough wood for the deck screw to hold down the deck board.

Certainly there is a lot of heat being absorbed by a black rooftop surface. It is like your deck boards are being baked and then soaked during winter. I can only imagine movement would be even worse if the decking material was pressure treated pine.

For sure you can not continue with deck boards attached to such a thin piece of wood. Maybe you could cut the deck boards back to the next joist and lay a single row of paving stones over the thinner joists as they won't move, they are heavy and they are not affected by the weather. May not look so good though.

A little trick might be replace the joists with thicker joists all around so that you actually raise the level of the deck everywhere or just in the area where the joists are so thin. This might mean sloping the deck upwards towards one end but if done nicely it won't be noticeable.

You might consider using different decking boards. Ipe is a great performer and doesn't move much but is much more expensive. Composite decking is less than Ipe but is very heat sensitive and requires enough clearance so it doesn't over heat. There are alternate deck materials like extruded aluminum decking but this is more expensive also.

Another idea for your climate is to remove the decking and lay down a new set of joists that is a little thicker and designed to accommodate the slope, sheat it with t&g plywood and cover with duradek or a vinyl like that. But you still have heat issues I would think given its location.

And the last thing that might help but it might not look so good is to cut some stainless steel bars say 1.5" wide and 1/8" thick with counter bored holes, two per board and screw it down over the ends of the boards that are lifting. Stainless has an urban appeal and might prevent the boards from moving independently when they heat up.

If any other builder has some tips to help this fellow out please share your expertise.

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