Sizing a ridge beam

R802.3 Ridge. A ridge board used to connect opposing rafters shall be not less than 1 inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. Where ceiling joist or rafter ties do not provide continuous ties across the structure, a ridge beam shall be provided and supported on each end by a wall or girder.

A ridge beam is different from other types of beams in several ways, including its horizontal orientation and its placement at the peak of the roof. Without a ridge beam, the roof could collapse under the weight of snow, ice, or strong winds. So, it’s important to choose the right size and type of ridge beam for your roof to ensure that it ...Ridge beam sizing is based on the span of the beam between supports, and the amount of roof load supported by the beam. A ridge board is a non-structural member that serves as a prop for opposing rafters to rest against and connect to. Ridge boards can only be used in roofs with slopes from 3 in 12 up to 12 in 12.Wood beam size for 15 feet span. Typically in residential buildings or any other projects, a wood beam size of 3-2×12 or 6×12 (3-nail 2×12) is required for 15 feet span. So if the span of the beam is 15 feet, the depth of the wooden beam should be 12 inches thick and the width should be 6 inches.. Wood beam size for 16 feet span. Typically in residential buildings or any other projects, a ...The Ridge Beam is the horizontal timber beam at the apex of the roof. It has no structural function but is the member against which the high ends of the rafters lean. It ensures that the rafters meet in a straight line and stay upright. Where a trussed roof is used, the rafters are the top chord of the truss and truss action spans the width of ...If you have a 30-foot span, it is important to know just how long a beam you will need for support. There is a good rule of thumb for this: divide your span (in inches) by 20. So, if your span is 30 feet (or 360 inches) you would divide that by 20 to come to 18 feet. Generally, it will be between a third and half of your overall depth.Software. ForteWEB™ Software. Whether you’re an architect, engineer or design professional sizing joists, beams, posts or studs, ForteWEB™ software performs member calculations and identifies solutions for the conditions and geometry you specify. Size members for a specific spacing, depth or the best economical fit.If you have a 30-foot span, it is important to know just how long a beam you will need for support. There is a good rule of thumb for this: divide your span (in inches) by 20. So, if your span is 30 feet (or 360 inches) you would divide that by 20 to come to 18 feet. Generally, it will be between a third and half of your overall depth.

How To Calculate A Ridge Beam Size. Posted on October 3, 2022 by Sandra. Beam pavilion in timber framing roof framing learn how to frame a how to build a hip roof 15 s with ridge board size code requirements rafter framing southwest new. Ridge Beam Vs Board What Is The Difference.Oct 9, 2023 · Ridge beam size ≈ (5 * 10,000 * 20^2) / (8 * 2000) ≈ 125 inches So, the recommended ridge beam size is approximately 125 inches for the given roof span, load, and LVL 2.0E material strength. Please note that this is a simplified example, and actual ridge beam calculations may involve additional factors and structural engineering expertise. I am wanting to size a lvl ridge beam, span of ridge is 18' width of building front to back is 17' 8" roof pitch 5/12" with 2"x10" rafter 16" oc with collar tie 2"x6"1/3 from top width side to side 18 … read moreThe beam calculator uses these equations to generate bending moment, shear force, slope and defelction diagrams. The beam calculator is a great tool to quickly validate forces in beams. Use it to help you design steel, wood and concrete beams under various loading conditions. Also, remember, you can add results from beams together using the ...Ridge board requirements for residential structures can be found in Section R802.3 of the 2018 International Residential Code. This section will outline what size a ridge board needs to be and what purpose it serves. When is a Ridge Beam Required? The International Residential Code covers the prescriptive requirements for…

Nov 5, 2021 ... I believe Chief "sees" these as Wall Ledgers which by default are sized per the Ridge Default size spec's if the roof plane touches a wall,. but ...The benefits of using this calculator include saving time and effort in manual calculations, reducing the risk of errors, and optimizing the use of LVL beams. With the LVL beam span calculator, users can confidently make informed decisions regarding beam sizing and placement, resulting in a more efficient and cost-effective construction process.Mar 29, 2018 ... http://www.homebuildingandrepairs.com/home_additions/index.html Click on this link if you're looking for more helpful videos about your next ...Table 2 Prolam Ridge Beam supporting roof and ceiling 48 KB pdf. Table 1 Prolam Lintel supporting roof and ceiling 85 KB pdf. Table 3 Prolam Rafter for internal use supporting roof and ceiling 80 KB pdf. Table 4 Prolam Verandah Rafter supporting roof without ceiling 111 KB pdf. Table 5 Prolam Verandah Rafter supporting roof and ceiling 112 KB pdf.PFS-TECO RR 0106 PFS-TECO RR 0105 ICC ES ESR-1387. For conditions not shown in this guide or for technical or sales support, contact US Lumber at 1-888-613-5078 or at [email protected]. Distributed by. Weyerhaeuser Rim Board, EverEdgeTM EEITM joists and EverEdgeTM LVL beams are distributed by U.S. Lumber.The Southern Forest Products Association provides size selection tables for various beam spans and loading combinations for Southern Pine dimension lumber and Southern Pine glued laminated timber are available for the following applications: Refer to SFPA’s publication Southern Pine Maximum Spans for Joists & Rafters (download in Publications ...

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Sizing Tables Window and Door Headers 6-7 Headers Supporting Roof 8-9 Ridge Beams 10-11 Headers Supporting Floor and Roof 12-13 Floor Beams 14-15 Floor load Tables TimberStrand® LSL Width: 3½" 16-17 Microllam® LVL 18-19 Parallam® PSL 20-21 Snow Roof load Tables TimberStrand® LSL 522-23 Microllam® LVL 24-25The tables here give maximum spans for rafters and beams. The figures are based on quality materials. For lesser grades, spans should be shorter. Keep in mind that these are maximums—in other words, shortening spans slightly will result in a more solid structure. Find out the loads your overhead must bear, and then determine the rafter sizes.A ridge beam is commonly used when the space immediately below the roof framing is open to the interior, such as with vaulted ceilings. Either a ridge beam or a ridge board can be used with flat, recessed, or tray ceilings that create an attic space between the ceiling joists and the roof rafters.To get started, I need to know only two things. The first is the pitch of the roof. On this project, the roof is a 12 pitch, or 12-in-12. The second thing that I need to know is the total run of the common rafters. As a house is framed, measurements can vary slightly from the design, so I disregard the plans when finding the run of the common ...BC Calc ® performs engineering analysis to help our customers size beams, joists, columns, studs and tall walls for their building projects. It is simple to use, yet flexible enough to analyze a variety of common applications. The user enters the member geometry, adds loads, holes and other relevant data and then selects a product for analysis.

The LVL beam span calculator is a valuable tool for construction projects. By accurately calculating beam spans and sizes, users can ensure structural integrity and compliance with building codes. The calculator’s insights can help users make informed decisions and optimize their projects.Canada Sizing Table Lookup :: Weyerhaeuser. Engineered Lumber. Resources. Canada Sizing Table Lookup. Use these simple calculators to size Trus Joist TimberStrand LSL and Parallam PSL beams and headers. Select the application below you are looking to calculate. Calculators are for use in Canada applications only.When it comes to construction and engineering projects, the use of steel beams is essential for providing structural support. Steel I beams, in particular, are widely used due to t...#1. How to calculate a ridge size. 05-11-2008, 04:18 PM. The Architect on my small addition job has a 5 1/2 x 16" X 25' long--690 lb ridge going in. In the past this …This section will outline what size a ridge board needs to be and what purpose it serves. When is a Ridge Beam Required? The International Residential Code covers the …Ridge beam sizing is based on the span of the beam between supports, and the amount of roof load supported by the beam. A ridge board is a non-structural member that serves as a prop for opposing rafters to rest against and connect to. Ridge boards can only be used in roofs with slopes from 3 in 12 up to 12 in 12.Wood Products. Engineered Lumber. Resources. U.S. Sizing Table Lookup. Use these simple calculators to size Trus Joist TimberStrand LSL and Parallam PSL beams and headers. Select the application below you are looking to calculate. Calculators are for use in U.S. applications only. For Canada switch to Canada Sizing Table Lookup.How to Use an LVL Ridge Beam Size Calculator: Input the roof span in feet (the distance between the supports). Input the total roof load in pounds (the weight the ridge beam must support). Select the type of LVL beam you plan to use from the dropdown list (e.g., 1.75" x 11.875" or 1.75" x 14"). Click the "Calculate" button.The size of the ridge beam required to span 24 feet depends on several factors, including the load it will bear, the spacing of the supporting columns, and the spacing of the beams. A structural engineer can help determine the appropriate size and type of ridge beam for your specific project.What is the distinction between a ridge board and a ridge beam? Ridge beam sizing is determined by the length of the beam between supports and the amount of roof load that the beam supports. A ridge board is a non-structural structure that serves as a platform for opposing rafters to rest and connect with. Ridge boards can only be used on roofs ...Use the rafter length formula: rafter length = √ (rise2 + run2) Substitute the rise from step 1 into the rafter length formula: rafter length = √ ( (run × pitch)2 + run2) Simplify the formula: rafter length = run√ (pitch2 + 1) Substitute the known values for pitch and run to calculate the rafter length.The maximum spans in the table above are for the horizontal distance the rafter can span (NOT the diagonal distance or actual length of the rafter). So if your house is 26 feet wide and the roof ridge beam is centered on the house, you will need a rafter that can span 13 feet. Let's use this example and find a size of rafter that will work.

40ft x 12″ = 480″ (length in inches) ÷ 24″ on-center = 20 rafters. After getting the answer, add 1 for the ‘0’ rafter and multiply it by 2. You’ll get the number of rafters needed for your roof. 40ft x 12” = 480” ÷ 24” on-center = 20 rafters + 1 = 21 rafters per side x 2 = 42 rafters. Get a Free Roofing Estimate.

Section modulus for a wooden beam = (beam width * (beam depth)^2) / 6. So for a 2 by 6 beam with actual dimensions of 1.5 x 5.5 inches this would be: 1.5 * (5.5)^2 / 6 = 7.6. If the section modulus in step 4 is less than the required modulus times 2 (factor of safety) then the beam is not strong enough.These structural members are used as vertical columns, horizontal beams, and arches, header, girder, rafter, purlin and ridge beam in wood frame house. ... Glulam beam size calculator. Glulam beam size calculator: To calculate the size of Glulam beam, convert the span length in feet to inches by multiplying with 12, divide the total span length ...and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams. 5802.3.1 Ceili ng Joist and R aft er Conne ctions. ... mm by 102 mm) (nominal) minimum-size crossties. The connec tions shall be in acc ordanc e with 780 CMR Table 5602.3(1) or connec tions of equivalent capacities shall be provided. WhereWhat Size Ridge Beam For 2 4 6 8 10 Rafter Civil Sir. Ridge Beam Vs Board Trus Joist Technical Support. Rafter Span Tables ... Ridge beams and boards modern structural solutions for a frames builder rafter framing southwest new brunswick service mission rafter spans with ceiling attached how far can a 2 6 8 10 12 14 rafter span civil sir ...When you are preparing to build a shed roof, you must first engineer the rafters - that is, know the species of wood you will use for the rafters, the nominal size of the rafters (2x6, 2x8, etc.), and the spacing of the rafters. To determine the size of the rafters, using span tables, you need to know three things:For a 24 foot span, you will need atleast 12″×18″ size of RCC beam for 2 to 3 story building in which 18″ is the depth of the beam and 12″ is width providing with 4nos of 16mm bar at top, 4nos of 20mm bar at bottom and 2nos of 16mm crank bar of Fe500 with stirrup T8@6″C/C and M20 grade of concrete ratio (1:1.5:3) with clear cover ...Beam spans in these tables range from 15-feet to 45-feet, in 5-foot increments. Girder spans also range from 15 feet to 45 feet in 5-foot increments for each of the beam spans noted. Therefore, beam/girder depths tabulated cover 28 different bay sizes for each of three load cases. Dead loads address the self-weight of the floor/roof framing system.Sep 16, 2021 · We need to calculate the total load per lineal foot (plf or pounds per linear foot) that the ridge beam needs to support in order to specify the correct size of the LVL. The span of the roof is ½ of the width, so 26’/2 = 13’, and the length is ½ of the overall length since we will be using an intermediate post, so 36’/2 = 18’:

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Sep 4, 2021 ... Shed roof build how to calculate ridge beam height for any pitch slope easy way brampton by law compliant. I explain how simple it is to ...9.23.8.3. Built-up wood Beams. (1) Where a beam is made up of individual pieces of lumber that are nailed together, the individual members shall be 38 mm or greater in thickness and installed on edge. (2) Except as permitted in Sentence (3), where individual members of a built-up beam are butted together to form a joint, the joint shall occur ...Ridge Beam Size Definition Span Table Calculator Definecivil. Ridge Beam Sizing Doityourself Munity Forums. H Bldg Cad Ccsite Details Residntal Const 8 Rafters W Ridge Beams Model 1. Lvl User S Technical For Headers Beams Column Lications Residential Floor And Roof Systems. Systematic reconstruction of cellular trajectories across mouse ...From https://www.justanswer.com/ythiJustAnswer Customer: what size ridge beam is required for a 40' length with 16' rafters 16"oc also necessary rafter size ...8. Jan 26, 2010. #1. hello, i'm a newbie with a question regarding the size beam needed to support my ridge beam. picture an easy rectangle house, 14'6" on both sides of the ridge beam, 13.13 rafter angle, 2x6 rafters 24" o.c., shingles,stick built, on slab, in georgia. the lower picture shows where i am going to open the wall (clear span) that ...Ridge beams support the upper ends of the roof rafters (at the peak) and span between structural supports such as posts or columns. If the rafters are simple span members between the ridge and bearing walls (as shown below), the ridge beam will support one-half of the roof area. In order to support the imposed design loads (dead, …Framing in a ridge beam in shed roof construction is not hard. Here's a picture of a gable style shed using a ridge beam. Now if you have done any looking around this site, you'll quickly see that I am not a fan of using a ridge beam in shed construction. I always, always build trusses. Its the way I was trained to build sheds.How do you size a ridge beam roof? Ridge beam sizing is based on the span of the beam between supports, and the amount of roof load supported by the beam. A ridge board is a non-structural member that serves as a prop for opposing rafters to rest against and connect to. Ridge boards can only be used in roofs with slopes from 3 in 12 up to 12 in 12.BC Calc Benefits Summary: Easily size joists, beams, columns, studs, and tall walls. Includes BCI ® & AJS ® joists, Versa-Lam ® LVL, BOISE GLULAM ® beams, and dimension lumber. Collaborate with others and share files in the cloud or send links and project files via email. Customizable project information and user defaults.The process of identifying the correct steel beam weight, size and length can be difficult and time-consuming.But to make your life easier, Buy A Beam comes with a built-in steel beam calculator. As you add steel products to your basket, you can customise their sizes, lengths, widths and any additional processes.Feb 28, 2024 ... I would suggest that your ridge beam in wood would be the better option, that roof shown in your drawing is not going to be heavy as it would be ... ….

span = 128.81 inches ≈ 10.73 ft. Kenneth Alambra. Use this wood beam span calculator to check a wood beam of a particular size and length to see if it can support a given uniform linear load by comparing its allowable and required bending and shear stress.The size of the LVL (Laminated Veneer Lumber) needed to span 30 feet depends on several factors, including the load it needs to support and local building codes. Typically, for a span of 30 feet, you would need 5-1/4″ × 16″ or 5-1/4″ × 18″ size of engineered LVL beams, often custom-made to meet the specific requirements of your project.If you have a 30-foot span, it is important to know just how long a beam you will need for support. There is a good rule of thumb for this: divide your span (in inches) by 20. So, if your span is 30 feet (or 360 inches) you would divide that by 20 to come to 18 feet. Generally, it will be between a third and half of your overall depth.The dormer will still look like it springs from the steep roof below the ridge, rather than them sharing the same ridge. user-3012521 Good design practice. Keeping it simple. 32' * 10' 320sf * (local dead load and snow loads) "7x18 DF Glulam" 640 lb beam with minimal loading values.Get instant results with this quick and easy method to calculate a ridge beam size using simple maths and the FREE download here: https://geni.us/timbercalcu...If you’ve decided to build a gambrel roof, you have to perform accurate calculations, mark up and make a drawing. First of all the center line is marked on the binding top from the end of the building (the position of the ridge purlin). Count half of the ridge beam thickness and mark up the position of the first element of truss system.Design of single span ridge beams in roofs in houses, office buidlings, schools and other structures.222 Catoctin Circle SE, Suite 201 Leesburg, VA 20175 202-463-2766. [email protected] www.woodaware.info. AMERICAN WOOD COUNCIL. The purpose of this informational guide is to provide awareness to the fire service on the types of wood I-joists and how they are used in the construction of residential buildings. Sizing a ridge beam, You can size your ridge by being over conservative, using the 60 psf snow load, unfactored by ASCE-7, a dead loading (from what you've given) of 12 psf, thus totaling 72 psf. With your tributary area given, your beam needs to be sufficient for handling 288 plf for its 14 foot span., A ridge beam is a long piece of wood or metal that is used to support the roof of a building. The span is the measurement of the distance between the two endpoints of the beam, which must be considered in order to ensure that the beam is strong enough to support its intended load. By understanding how to accurately determine the span of a ridge ..., Steel beam. LVL beam. #1. Wood Beam for 20 Foot Span. Keeping in mind our general thumb rule, you can get the size of a beam of a 20-foot span. You must first get four nailed 2×16 (4-2×16) or an 8×16 size to span 20 feet. So in this 20-foot span, your wood beam should be at least 8″ in width and 16″ in depth (4-2/16)., A simple span roof hip beam's load is a distributed load of triangular shape with load "0" at the corner of the roof and max. at ridge connection. I get confused over the tributary area. Assuming 34 psf dead and live load, and assuming that the hip is the diagonal of a 9ft X 9ft square (18ft wide roof with equal slopes) and assuming I'm using ..., 05-16-2008, 01:38 PM. Re: How to calculate a ridge size. If the roof pitch is less than a 3:12 than it does need to be designed as a beam. The pitch could be steeper with-out collar tie or other things too but if its less than a 3 its got to be a beam since there is little outward push and more verticle loading., Table 2 Prolam Ridge Beam supporting roof and ceiling 48 KB pdf. Table 1 Prolam Lintel supporting roof and ceiling 85 KB pdf. Table 3 Prolam Rafter for internal use supporting roof and ceiling 80 KB pdf. Table 4 Prolam Verandah Rafter supporting roof without ceiling 111 KB pdf. Table 5 Prolam Verandah Rafter supporting roof and ceiling 112 KB pdf., How to Use an LVL Ridge Beam Size Calculator: Input the roof span in feet (the distance between the supports). Input the total roof load in pounds (the weight the ridge beam must support). Select the type of LVL beam you plan to use from the dropdown list (e.g., 1.75" x 11.875" or 1.75" x 14"). Click the "Calculate" button., Ridge beam size ≈ (5 * 10,000 * 20^2) / (8 * 2000) ≈ 125 inches; So, the recommended ridge beam size is approximately 125 inches for the given roof span, load, and LVL 2.0E material strength. Please note that this is a simplified example, and actual ridge beam calculations may involve additional factors and structural engineering expertise., Mar 3, 2019 ... http://www.homebuildingandrepairs.com/framing/index.html Click on this link for more information about roof framing, new house construction ..., This chart is for initial rough sizing hip and valley rafters, work should be checked by an engineer before plans are finalized. Length L in feet is shown at right. This chart is for intersections of equal pitch ONLY. Column headings refer to live load. This is based on an article in The Journal Of Light Construction., Oct 12, 2023 · How to Use an LVL Ridge Beam Size Calculator: Input the roof span in feet (the distance between the supports). Input the total roof load in pounds (the weight the ridge beam must support). Select the type of LVL beam you plan to use from the dropdown list (e.g., 1.75" x 11.875" or 1.75" x 14"). Click the "Calculate" button. , Beams must be laterally supported at the top along the length of the beam and at the top and bottom at the ends. The load carrying capacities tabulated are for total load including the weight of the member. BEAM WEIGHT: 35.0 pounds per cubic foot was used to determine beam weight per lineal foot shown in the table., Ridge Beam Calculation Formula. RidgeBeamLoad = (RoofLoadArea * UnitLoad) / BeamLength. Where: RoofLoadArea is the total roof area supported by the beam. UnitLoad is the combined dead and live load per square foot. BeamLength is the length of the beam in feet., A simply supported beam is a beam that has two supports located at each end. One support is a pinned support, which allows only one degree of freedom, the rotation around the z-axis (perpendicular to the paper). At the other end, there's a roller support, which enables two degrees of freedom, the horizontal movement along the x-axis and …, Ridge beam sizing chartBeam sizing Ridge beam vs. ridge board - trus joist technical support timber trussBeam beams. Span joist tji floor joists tables size 16 ceiling roof canada supply vidalondon carpet menards sizing framing garageStandard size ceiling beams Awesome lvl floor joist span chart and viewRidge beam sizing chart., Considering adding an addition with open "cathedral" ceiling (to avoid trusses). Span will be 20' with 2' roof overhang on each end. I'm considering 2 or 3 2X10's laced together as ridge beam (since it will be supporting the load) supported by several vertical 2X4's burried in wall to support the beam. Rafters probably 2X8's or 2X10's on a 18' span resting on beam., BC Calc Benefits Summary: Easily size joists, beams, columns, studs, and tall walls. Includes BCI ® & AJS ® joists, Versa-Lam ® LVL, BOISE GLULAM ® beams, and dimension lumber. Collaborate with others and share files in the cloud or send links and project files via email. Customizable project information and user defaults., Set amid the beauty of the Blue Ridge Mountains, Asheville has become one of the more popular towns in North Carolina. The city’s thriving art scene, historic architecture, and nat..., If you have a 30-foot span, it is important to know just how long a beam you will need for support. There is a good rule of thumb for this: divide your span (in inches) by 20. So, if your span is 30 feet (or 360 inches) you would divide that by 20 to come to 18 feet. Generally, it will be between a third and half of your overall depth., RIDGE BEAM The engineering principles are pretty straightforward for a ridge beam. The sloped roof joists span between two pinned points; one at the envelope of the structure on top of a framed ..., The Southern Forest Products Association provides size selection tables for various beam spans and loading combinations for Southern Pine dimension lumber and Southern Pine glued laminated timber are available for the following applications: Refer to SFPA’s publication Southern Pine Maximum Spans for Joists & Rafters (download in Publications ..., Oct 22, 2020 · A ridge beam is commonly used when the space immediately below the roof framing is open to the interior, such as with vaulted ceilings. Either a ridge beam or a ridge board can be used with flat, recessed, or tray ceilings that create an attic space between the ceiling joists and the roof rafters. , RR Ridge Rafter Connector. RR. Ridge Rafter Connector. An interlock provides alignment control and correct nailing locations. For a rafter-to-face connector, flatten the top flange into the face plane. The RR may be used with rafters sloped up to 30°. 18 gauge., You can use a W12x45 (50 ksi) steel beam for your 28' clear span roodf beam. The beam is 8" wide and 12" deep. It weighs 45 pounds per foot of beam length. You will need to have a local engineer detail your connections, size your support columns, and determine foundation requirements., Using these values in the provided HTML code: Moment of Inertia (I) is calculated as (12^3) / 12 = 144 cubic feet. Required Beam Size = (5000 * 12^2) / (4 * 0.25 * 144) ≈ 156.25 inches. So, the recommended microlam beam size, in this example, is approximately 156.25 inches. In practice, you would typically round up to the nearest available ..., I'm having some problem figuring out the size of my ridge beam (or maybe a parallel cord truss if the beam ends up being to big), However, a structural ridge beam may be required for several reasons, as described in this section. Support for high ends of rafters is typically necessary for the following conditions; (1) No attic floor (ceiling) joists, such as for a sloped (cathedral) ceiling. This condition generally results in lack of resistance to outward thrust force ..., You can use a W12x45 (50 ksi) steel beam for your 28' clear span roodf beam. The beam is 8" wide and 12" deep. It weighs 45 pounds per foot of beam length. You will need to have a local engineer detail your connections, size your support columns, and determine foundation requirements., Jun 13, 2018 · This requires a secure connection of the rafters to the ridge beam at the top. Furthermore, if the vaulted roof is constructed as a hip roof, things get even more complicated. Not only must the ridge beam be supported, but the top ends of the hips must also be supported. And the rafters must have a secure connection to the hips. , The ridge board is always full depth of cut rafters unless a low pitch(<3/12), then designed as beams. 1x boards "ok", check local code. IRC 802.3 The ridge board can be of any length, splice plates are not required here as it is not structural, only holding the rafters in opposite line., Floor Beam Sizing Calculator; Use the drop downs to select your floor load, framing length, and material to calculate the appropriate size of TimberStrand or Parallam beam. Calculator is for use in U.S. applications only. For Canada switch to Canada Sizing Table Lookup., 40ft x 12″ = 480″ (length in inches) ÷ 24″ on-center = 20 rafters. After getting the answer, add 1 for the '0' rafter and multiply it by 2. You'll get the number of rafters needed for your roof. 40ft x 12" = 480" ÷ 24" on-center = 20 rafters + 1 = 21 rafters per side x 2 = 42 rafters. Get a Free Roofing Estimate., Considering adding an addition with open "cathedral" ceiling (to avoid trusses). Span will be 20' with 2' roof overhang on each end. I'm considering 2 or 3 2X10's laced together as ridge beam (since it will be supporting the load) supported by several vertical 2X4's burried in wall to support the beam. Rafters probably 2X8's or 2X10's on a 18' span resting on beam.