Reroof and Structural
Q. Is there any information we can get that will explain the proper truss system required for installation of standard weight tiles? Our home has manufactured trusses, spanning at least 11 feet, set at 24″ O.C. The vaulted ceiling in one area has a system of 2 x 12s, also spaced at 24″ O.C. Roof sheathing is 7/16 OSB. Does this all seem sufficient, or would you recommend the trusses be spaced at 16″ OC?
A. Roof tiles are most commonly applied to structures such as you described although you have not given enough information for me to determine the suitability of your system. There are a great many variables that enter into the equation – particularly in regards to your pre-fab trusses which are essentially engineered systems. The 11 foot span on the truss seems quite short – truss spans are typically expressed as a total assembly span so, in this case you may be looking at a 22 foot span which, depending on the type of truss, would not seem too excessive. Your 2″x12″ rafters at 24″o.c. will typically support a standard weight tile roof if the horizontal spans do not exceed 16-20 feet depending on lumber species and grade.
Q. I am a structural engineer here on Maui. We are considering specifying your roof tiles but I need to examine the impact of lightweight vs. normal weight tiles. Could you supply me with estimates of the light and normal weight systems (psf) that I can use to evaluate the impact to my roof framing?
A. Lightweight tiles weigh approximately 6 pounds per square foot and standard weight tiles weigh between 9-10 psf. You should be aware that lightweight tiles are not recommended for roofs that require frequent foot traffic or extremely high winds. Since Hawaii wind speed design is for 105 mph, lightweight tiles would require a wind clip to be installed on each tile.
Q. For concrete tile, stated weight is 950lbs. per square. Does that include underlayment or is it tile only?
A. The published weight of all roofing tiles is for only the tile. Your actual design will consist of the combined dead load and live load factors. Your message does not state where in Northern California you are, but unless you are situated in the mountains where snow load must be considered, your design live load will probably be 20 psf (pounds per square foot). This factor is the same, regardless of which type of roofing material you choose. The dead load is a compilation of all of the roofing and framing components and represents the static weight that is constantly being supported. This includes the roofing material, underlayment, roof sheathing and rafter system. Unless other components such as solar panels or roof mounted mechanical apparatus are added, the dead load for a 9.5 psf roofing tile would typically be around 14 to 15 psf. To determine your combined load, an engineer would add the dead load to the live load to give you approximately 35 psf which is what he would design your structure to carry. There are other factors that may come into play, but that is basically how roof designs are figured.
Q. We have a large area in our kitchen family room that a tile roofing company has said would be at risk if we used lightweight roofing tiles due to the extra weight of the tiles and roof assembly. At risk meaning we have a chance of the roof sagging over time and since we live in Northern California the chance of the roof suffering damage during an earthquake is greater. This does concern me because we have a neighbor who roof is sagging after tiles were put on. Please give me your opinion. The area is a 20 foot span from a support beam to a support wall. The roof pitch is 6/12. The joists are 2×8’s and are 16″ apart. The existing roof is wood shake.
A. You did not state whether your roof system was a cathedral ceiling where the rafters are also carrying a ceiling load or was conventional construction with an attic space separating the ceiling joists from the roof rafters. If your roof rafters are indeed 2″x8″ at 16″o.c. that span 20 feet, then I would imagine that you probably already have some sagging since this would be overstressed even for a wood shake or shingle. The irregular appearance of wood shakes will sometimes hide the sags and bumps of a roof assembly whereas the straight lines of a tile will typically expose them. Depending on when the house was built and the grade of lumber used, the maximum span for that rafter configuration, based on 1997 values of No. 2 grade lumber, would be roughly 18’4″ for a lightweight tile, 17’5″ for a standard weight tile and 18’8″ for a wood shake. Over spanned rafters may be strengthened by adding purlin support if proper support points are available or by doubling the rafters where no support points are available. There may be other issues to consider if reinforcement is required and we would recommend that you have an experienced engineer advise you as to the most appropriate course of action and feasibility.
Q. I have a 17 year old house with asphalt shingles. Can I leave the shingles in place and install your tiles over this surface. Some of the shingles are curled. Would this affect drainage? Do I have to do a big structural analysis of the rafter support system – 2” x4” on 24 inch centers?
A. It is possible to install tiles over existing asphalt shingles if the structure is shown to be capable of supporting the combined weight. Based on the information you provided, I would guess that you have a pre-fabricated roof truss system. Depending on the design, you may be able to reroof without reinforcement but you should have this verified by an engineer or truss manufacturer before beginning. Whether the existing shingle will serve as a proper underlayment depends on the condition. If the shingles are curled, there is a good chance that the shingles are brittle and will likely experience quite a bit of damage during the process of reroofing. It is generally advisable to install a new underlayment prior to the installation of the tile. Since you live in a hot climate, you may also want to consider optimizing your results by installing the tile on a counter batten or elevated batten system that will increase the airflow beneath the tile which will help keep your attic cooler. Other enhancements such as ventilation and radiant barriers may be considered if you want to have a measurable impact on controlling the heat gain into your home.
Q. I have an open beam type of home. Roof is designed with 4″x12″ rafters about 8′ apart and span lengths of approximately 12′. Can this type of roof support lightweight shake tile?
A. Since lightweight tile is less than 6 psf, it is generally used as a direct replacement for shingles and shakes. While 4″x12″ rafters spanning only 12 feet would appear to be sufficient, you did not mention what type of decking is spanning 8 feet. You also did not mention the roof slope or if there is a ridge beam supporting the rafters. Our normal recommendation for open beam structures is to have it checked by a professional engineer when doubt exists since all of the structural components must be matched to the expected loads.
Q. My home was built in 1938. All the rafters are 2×4 construction spaced 24 inches o.c. Is lightweight tile acceptable for a re-roof on this house?
A. Most lightweight tiles are designed as reroof options for houses that were not originally designed for standard weight tile. In most cases, no additional reinforcement is required for lightweight tiles although it is still important to consider the condition of the structure. On a house as old as yours, there are sometimes changes that have been made to the structure and these need to be identified and analyzed to certify their compliance to standards. Rafter sizes of 2″x4″ on 24″ centers is quite common for homes built at that time and they would be capable of supporting a tile roof if they are properly supported and attached. Current allowable, unsupported spans for 2″x4″ at 24″o.c. would not exceed 7’9″ for lightweight tile or 7′ for standard weight. Your building official will likely want to know the rafter spans and other information prior to issuance of a roofing permit.
Q. Can I use your products on a roof with pitch less than 412 I have a 40 year old house with 2X4 rafters 24″oc… what will need to be done regarding engineering reports?
A. Tiles may be installed below a 4:12 slope but special consideration must be made for the selection and application of the underlayment at the lower roof slopes. Also, it is recommended that a counterbatten or elevated batten system be used at lower slopes. The engineering requirements depend on the policies of your local building official. In most cases, an engineering report verifying the capabilities of the structure will be required. The size of the rafters is not as important as the type and frequency of supports and ties. Most roofs are capable of supporting lightweight tiles without enhancement, but if standard weight tiles are used, some reinforcement may be necessary. 2″x4″‘s are commonly used in the current pre-fabricated trusses that support tile roofs on new homes but when 2″x4″‘s were used in roofs forty years ago, it was usually for relatively short spans or on steeper roof slopes. As a rule, 2″x4″‘s on 24″o.c. should not span much more than 7 feet without support.
Q. Need to know if my room with cathedral ceiling will support your light weight tile at 596 lbs sq.
A. Tiles are commonly installed over cathedral ceilings and lightweight tiles are typically considered a viable alternative if you are replacing an existing roof. The specific capability of your roof system must be determined by knowing the span of the rafters and the size and spacing of the rafters.