A deck on the roof can be a wonderful thing, with unimpeded sunlight and a view to the farthest horizon. If your house is a good candidate for one, a roof deck will enhance it like no other feature.
But however desirable it may be, you can't just stick the deck up there. There has to be plenty of structural support underneath.
"In principle, what you are doing is adding a floor to the house," says Kip Humphrey of Baltimore. His company, Deckcraft Inc., builds more than 100 decks a year.
That means that the structure has to support the "dead load" -- the weight of the deck's building materials -- and the "live load" -- deadload plus the weight of the furniture and people that will use it when it's finished.
A roof deck, Mr. Humphrey says, should be treated like a first floor -- designed to carry a live load of 60 pounds per square foot.
Since roofs are very rarely designed to support that kind of weight, some other structure will have to be built to support it.
Mr. Humphrey doesn't like to support any of the deck on the roof. He prefers to use the sides of the building -- for instance, the side parapets of a typical row house -- because they are more successfully waterproofed than posts sticking through the roof.
If you plan to use the parapets to support the deck, make sure the parapets are solid, Mr. Humphrey warns. He suggests hiring a structural engineer to evaluate the structural integrity of the building and to recommend additional support if warranted.
In rare cases where there have been solid masonry side walls but no parapets, Mr. Humphrey has called in a mason to extend the side walls above the roof. If there aren't any side walls to build up, he uses some combination of posts and beams supported above and below the roof.
Here are some other recommendations from Mr. Humphrey:
*The roof should be in excellent repair before you start to install anything over it. While you can build a deck so it doesn't inhibit maintenance, it's easier, and less expensive, to fix the roof beforehand.
In some cases of flat or low-pitch roofs that need repair, Mr. Humphrey suggests considering a rubber-membrane type replacement, which requires somewhat less maintenance than the more common asphalt built-up roof. (The flashing will still need attention.) One drawback to rubber-membrane roofing is that it can be damaged by prolonged exposure to sunlight, but if the roof is mostly covered with a deck, such damage is minimized. Another concern with roof decks is damage caused by ice buildup. Rubber-membrane roofs seem to hold up better under ice; shingle roofs fare the worst because ice can get underneath the shingles, Mr. Humphrey says.
*Roof decks should be built in sections, so parts can be lifted out for repairs to the roof underneath. Mr. Humphrey builds roof decks with removable 4-by-4-foot "panels."
*Any support posts must be well flashed so there will be no leaks in the roof. If the deck is supported on posts, Mr. Humphrey uses short lengths of six-by-six completely flashed and sealed as a base. Then the four-by-fours that form the structure are fastened to the six-by-sixes with metal post anchors.
*If wind could be a problem, Mr. Humphrey suggests building, on the lowest side of the deck, an "apron" that extends from the floor of the deck to within an inch of the roof surface. (Some roofs in urban Baltimore, he says, get fierce updrafts that can carry leaves and debris up under the deck.)
*If you're anchoring the deck structure to a wall, find out what kinds of anchors are acceptable in your locale. For instance, some building codes don't accept sleeve anchors.
*If your house is in a historic preservation district, you may still be allowed a roof deck as long as nothing is visible from the street. (Such a restriction could be a problem if you need screening to give the deck some protection in full sunlight.)
*Roofs aren't normally made to be looked at, much less inhabited, and parts of them can be pretty ugly -- the pipes and vents that jut up, the antennas, satellite dishes, ventilators and compressors that dot it, the view of your neighbor's inexpensive patch job.
Mr. Humphrey suggests built-in planter boxes to soften and screen the immediate view -- warning, of course, that the extra weight of the boxes, soil and plants has to be taken into consideration in designing the support structure.
*Use the best lumber available for roof decks, because it is less likely to have problems such as cracking and splitting. Mr. Humphrey recommends No. 1 grade pressure-treated wood,which has fewer knots, for the basic structure.
*One currently trendy feature for roof decks, Mr. Humphrey says, is a block and tackle system for hauling things up and down. Do you really want to climb a narrow spiral stair with a 40-pound bag of topsoil under your arm?
Speaking of spiral stairs, before you get too heavily into your roof-deck design, decide how you're going to get up to it -- and how you'll persuade your friends to go up there. Most people don't relish the idea of climbing a straight ladder through a skylight, or following a fire escape over the edge of a three-story building. Access needs to be comfortable, or you won't get the best use of your "extra room" in the sky.
Next: Answers to readers' letters.
Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.
If you have questions, comments, tips or experiences to share about working on houses, write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore, Md. 21278. Questions of general interest will be answered in the column; comments, tips and experiences will be reported in occasional columns.