Independent Consulting Engineers NV (ICE) is an engineering company which has as one of its main activities the engineering design of building structures and building elements: in general, engineering that makes sure that the building element in question does not fail in case a major disaster strikes.
ICE has been established on the island in 1982. In its 35 years of existence, several major natural disasters have hit the island, and ICE is proud to be able to say that virtually all the projects they have done over the years have performed very well through these disasters.
On September 6, our island was hit by a major hurricane, Irma, the strongest ever recorded in the Atlantic Basin, with extensive damage as a consequence. We have seen all over the island severe damage, especially to metal roofs. The question arises whether it is possible to build a traditional metal sheet roof that can withstand the hurricane force winds without sustaining significant damage.
Metal sheet roofs in general
One has to realize that any metal sheet roof will always be more vulnerable than a concrete roof. If the roof is hit by a heavy object (we have seen damage by a flying washing machine!), chances are that the metal sheeting and possibly the structure under it will sustain damage, more so than a concrete roof would sustain.
On the other hand, it is also clear that in most cases, the concrete roof will be more expensive than the metal sheeting roof. With proper engineering design, detailing, and care during construction, it is quite possible to build a metal sheet roof that will be able to come through a storm as Irma with little to no damage.
General built-up of a metal roof
We consider below a roof with a built-up as is very commonly used on the island. Different roof set-ups will require specific engineering and might require different detailing.
A standard roof as commonly used on the island consists of a combination of timber roof members, plywood, and metal roof sheeting (see also figure 1 “General View”):
1. Roof rafters: These are the main load bearing elements of the roof. Depending on the span of the roof, they can vary in size: 3”x 6” and 3”x 8” are common sizes. The size of the rafters has to be engineered, depending on the span and configuration of the roof. The rafters have to be anchored in the ring beam.
2. Plywood or T1-11: This is the finishing which remains visible from the inside of the house.
3. Waterproofing: Typically felt, or other – ensures that no water penetrates.
4. 2”x 4” battens: These are fastened to the rafters, the metal roof sheeting is fastened to the 2”x 4” battens.
5. Metal roof sheeting, and metal flashings on roof edges and ridges.
A few general rules in construction of a wood and metal roof:
• NO NAILS, ONLY SCREWS.
In construction of wood structures for hurricane conditions, one should never use nails, even the “twisted” or “ribbed” kinds. As wood dries and works, and nails start to corrode, the force needed to pull out a nail will reduce very rapidly with age of the structure: a structure which would have been fine when new, will be blown apart when a few years old. Screws require a much larger force before they can be pulled out, and will retail a much larger portion of that force after a few years. One of the most frequent reasons for failures of wood roofs we see is the use of nails to fasten the roofs. Use of nails to fasten the metal sheets to the structure below is even worse and a recipe for disaster.
• DO NOT TRY TO SAVE A FEW DOLLARS BY USING LESS OR SMALLER SCREWS.
The cost of screws in the total picture of the construction of your house is minimal. We often see failed roofs where the reason is that somebody tried to save a few bucks by using less or smaller screws.
• ANY ROOFSHAPE CAN BE PROPERLY BUILT AND ENGINEERED TO WITHSTAND HURRICANE WINDS.
Flat roofs, sloped roofs, gable roofs, complicated roof shapes can all be built to be hurricane resistant. It is a myth that certain roof shapes are more resistant than others. It is correct that each shape requires specific engineering. A Gable roof with steep slopes is engineered completely differently from a relatively flat gable roof for example.
Typical construction detailing for roof construction
Below we will go through the different elements of the wood and metal sheeting roof and mention the most important aspects of proper detailing:
Rafters have to be anchored in the reinforced concrete ring beam. The ring beam on its turn is connected to the foundations below by columns or reinforcement steel in the masonry block-work. Anchoring prevents the rafter from being lifted free from the structure below.
We typically use a detail as given in figure 6: The rafter is tied down by a continuous steel bar, typically ½-inch, going through a hole drilled in the rafter, and tied to the ring beam by stirrups which form part of the ring beam steel reinforcement. Important here is that the stirrups that are tying the continuous rod down are closed and part of the ring beam. Stirrups should be not more than 20 cm (8”) apart.
Details where rafters are held down by a piece of steel bent over the rafter, or a piece of steel sticking out of the ring beam and twisted around the continuous ½-inch rod have proven not to be sound.
Rafters have to be sized depending on the span of the roof, or in case of more complicated roofs depending on the overall engineering of the roof. When standard sizes of plywood sheets are used, rafters should be placed 24 inches (61cm) apart.
Fascia board (see figure 2): Typically a fascia board will be installed at the end of the rafters. In case 6” rafters are used, the fascia board should be a 2”x 8” board. The board should be attached to each rafter end with each time two screws, 5” long. The fascia board will be a structural member, adding strength to any roof overhang.
Roof overhangs: without specific engineering, roof overhangs of more than 30cm (12“) should be avoided.
2. T1-11 or Plywood
Also here goes: NO NAILS. Plywood or T1-11 should have a thickness of at least 5/8“ (16 millimetre), and should be screwed directly onto the rafters. We typically use 2“ (50 mm) screws, maximum 30 centimetres (12 inches) apart, into each rafter supporting the plywood (typically 25 to 27 screws per 4’x8’ plywood sheet). See figure 2.
Waterproofing can be felt or any other, tacked to the plywood: It is held in place by the 2x4” battens
4. Battens (2”x 4”)
The battens are the direct connection between the metal roof sheeting and the structure below, and therefore should be correctly fastened to the rafters. Again: NO NAILS.
We typically use two 5-inch long screws on each crossing between the 2”x4” batten and any rafter below.
Typically 2”x4” battens are placed 80 centimetres from each other. See figure 3. This depends however on the type of metal sheeting material: profile, thickness, stiffness of the sheet, etc., 80cm is an appropriate distance for regular “wave” metal sheeting as seen most on the island with sheet thickness of 0.75 millimetre or 22 gauge.
Closer to roof edges and ridges, the force of the winds are larger, however. We recommend doubling up on the battens in these areas: see figure 1. Be sure 2”x 4” battens are placed near the roof edges parallel to the roof slope. These will be required to fasten the metal flashings – see figure 4.
Insulation sheets can be placed in between the battens (please make sure of insulation – the sheets cannot be put on fire: some materials sold burn very hard and fast. Test material by holding a lighter to it.)
5. Metal sheeting and fasteners
The traditional metal roof sheets as used on the island should have a thickness of at least 0.75 millimetres (sold on the French side or 22 gauge USA). Do not accept thinner sheets as they are too flexible, vulnerable and require specific fastening.
The sheets have to be fastened to the 2x4 battens, with spacing between screws of maximum 25 centimetres (10”). On the edges of the roof, these spacings have to be halved to 12.5cm or 5”. See figures 3 and 4.
Important is that the proper screws are used: Do not use self-tapping metal screws, as their holding power is significantly less than proper wood screws. Screws should be of sufficient length to penetrate at least 3cm into the 2 x 4 batten. Each screw be placed on the top of the “wave” of the sheet and should be equipped with a metal cap (rounded piece of metal, about 5 by 3 cm, to fit on the rounding of the metal sheet): this cap prevents that the screw is pulled through the metal sheet.
We still see metal sheets (mostly imported from Europe or the USA north-east) which consist of flat sheets, with a single raised rib every 25 to 40 centimetres, screwed down in the flat section of the sheets, with relatively small screws, which come with a tiny rubber ring, and a small washer. Under normal conditions, these sheets are often a problem because rainwater will penetrate trough the screw opening when after a few years, the rubber ring has deteriorated, and in hurricane conditions, the screw and small washer are simply pulled through the sheet. Not to be recommended for these areas with our heavy downpours and our hurricane winds.
Flashings are the flat metal sheets covering the edges of the roof and the ridges of the roof. They are important because failure will often give water and wind an easier access to the rest of the roof and the interior of the building.
We typically fasten them into additional 2 x4 battens along the edges or ridges of the roofs, and into the fascia boards on the outside of the roof: see figures 3 and 4.
Flashings should always be fastened into something solid as the battens. Never should they be screwed directly only to the metal roof sheeting: the light self-tapping screws typically used for this purpose will not hold the flashings once the wind blows.
The above gives a brief outline on how a standard, relatively simple roof can be built in such a way that it stands a very good chance to come through a storm as Irma, with little to no damage. A large number of roofs put together in this way, or variations thereon, have come through the storm very well. The additional cost in doing it right versus not right, is minimal, and in most cases certainly not worth the additional risk.
One should keep in mind, however, that for any more complicated or larger roof structure, a little bit of money spent on proper engineering can prevent a lot of trouble and misery on the long run. The costs involved are a small fraction of the cost to build the roof.
Jan Vanden Eynde has been with Independent Consulting Engineers NV since its inception in 1982. ICE has permanently on staff in Sint Maarten a number of highly qualified professionals, with years of experience in hurricane and earthquake resistant engineering. ICE is available for any type of engineering project, from the smallest (“ I want to make an opening in this wall in my house, can I do that or do I have to reinforce something?”) to the largest projects on the island (Porto Cupecoy, Atrium, Aqua Marina Towers).
ICE can be reached at Zaegersgut Road 13, telephone 542-2421, e-mail [email protected]