A friend asked me to help her build a garage with a wood floor in central Maine. The garage will be 20′ x 32′ with a 4′ frost wall, and a 14′ wide garage door on the gable end. I’m struggling with how to design the wood floor and what materials to use for the floor framing.
Currently we are planning to run a beam down the center of the long dimension and use either 12′ – 2×12 PTL, 12″ o.c.. along each side of the beam, or 20′ I-joists run the full width, again 12″ o.c.. My first question is whether or not we need to use PTL? The building site is fairly well drained and we plan to install a perimiter drain to daylight along the foundation footing as well as a 6 mil vapor barrier on the bare ground beneath the deck. We plan to use full dimensional 2×6 hemlock for the decking. Currently we have no plans to allow for ventilation in the crawl space. Under these circumstances, do you think it’s necessary to use PTL???
My other question is how to design the joists under the garage door opening. Currently my friend plans to store snowmobiles, a tractor, atv’s and a trailer or two in the garage, but no doubt the day will come when she decides to put a car or truck in the garage. I can’t help thinking that the joists below the garage door will need to be reinforced somehow in order to withstand the force exerted when a car or truck is driven into the garage.
We intend to build a ramp for entry through the 14′ garage door. My concern is the rotational forces exerted on the first few joists when someone drives a vehicle into the garage. Would it be sufficient to build a beam of 3 or 4 2×12’s for position directly below the garage door opening? Would that be sufficient to overcome any tendency for the first joist to want to topple inwards when a large vehicle is driven up the ramp??? Or is this not an issue at all???
Thank you in advance for any assistance!
Sco
Replies
Wow ... haven't seen that in a while. I'm NOT an expert, but building science is my general profession. We had wood floor garages all over where I grew up ... lots of steep hillsides and garages w/out a lot of concrete.
PTL ... pressure treated lumber, right? What about the vehicles covered in snow/rain when you pull them into the garage? That would be my biggest concern. As such, I think I would avoid TJI joists as they potentially will get wet regularly ... unless you are designing a waterproof deck.
A few years ago, I saw this builder do a garage over a living space. He did joists like 12" oc then poured a concrete deck on it. Don't know if it was entirely successful ... he seemed like not the best contractor, but wasn't the worst, either.
It seems by all rights all the framing/decking might need to be PTL. I know what it is like to drive through a foot of snow and arrive home and pull the car into the garage ... LOTS OF WATER! Your ground moisture issues seem like largely a non issue in comparison.
Actually, these folks already have a 3 car garage attached to their house where they park their everyday vehicles. This garage will be for their toys - 4-wheeler, motor cycles, lawn tractor, etc. So, theoretically this second garage should stay much drier than the other. On the other hand, your point is well taken - maybe i am worrying about the wrong source of moisture...
If the job will require engineered plans, will go through a plans-check process in order to acquire permits, you will want to examine what the code says in your building jurisdiction.
I know that many rural areas have nothing like this going on, and you can pretty much build what you like. If that is your situation, then you need not be concerned about code, but you still may want to get a local engineer to help with your structure.
We think of cars and pickup trucks as really heavy things, when we think of a wood-joist floorframe they might sit atop, but they are really not that much, if we start comparing them to things like bathtubs full of water, and upright pianos.
A 6,000 pound pickup truck occupying a rectangular space of 6.5 x 18 feet, is equivalent to about 50 pounds per square foot of loading, while mainfloor living spaces in houses are designed to 40 psf today.
Here is a handy link to see what you might need, as far as floor joists go: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
but wouldn't the bigger issue be that the 6,000 lbs is point loaded on four six inch by six inch spots?
A very good piece of advice I once got is not to always ask "Can You (do x or y or z)?" but rather you should be asking "Should you do x, y or z?" The answer to the first question is invariably YES while the answer to the second is most often NO.
Can you build a wood floored garage? Yes
Should you build a wood floored garage?..................................
Here it is back to you.
How was this addressed for you in engineering school?
Is the floor frame structure a loose collection of joists, sheathing, planking, etc.? Or do all those parts, fixed together with their glue, screws, nails, whatever, produce a diaphragm of some type?
It is a diaphram ... but what's your point? A beam is a beam. Point loading vs. evenly distributed loads command much different considerations in structural design. Four 'point' loads of 1,500 lbs each on a diaphram is much different than 50 lb/sqft. 1,500 lbs on a 6x6 area is ... that's right 6,000 lbs/sqft. While the diaphram/structure can hold say 6,000 lbs evenly distributed, it may not hold the point loads imposed by a vehicle. That may simply be the difference between e.g. 1/2" sheathing and 1 1/2" sheathing/flooring.
A piece of paper will hold a pair of heavy scissors laying on it and held in place along two edges. But try to hold the same scissors on the point ... no chance. Diaphrams are very similar.
I remember my first house ... I was young and ... semi-smart :) ... to backfill our basement, we drove a bobcat onto our floor structure - floor trusses and 3/4" plywood ... plus some more plywood to avoid puncturing the subfloor. Boy was that a scary thing to do ... but we got away w/ it.
Are you a structural engineer?
Because if you are, please proceed to tell the O.P. what he or she needs for those point loads. Size the beams, the deck, and size the joists.
And be sure to allow for the vehicle to be parked within some reasonable range of available space. Furthermore, allow for those point loads as they move along when the vehicle is driven in an out of storage.
You might as well, while you have your sliderule out, figure the dynamic loadings from starting and stopping, and add that into your beamstress analysis.
No need to get upset, Gene. I'm just making comments. I have had enough structural training to be a structural engineer ... I don't use that education regularly to enable me to engage in a detailed conversation of structural theory or calcs.
I do know that you design structural elements very differently if they have a point load(s) or a uniform load. A floor diaphram is not much more than a series of beams laying along side each other. You may likely design the point loading for the worst case positions on that beam given your anticipated potential load (e.g. your 3/4 ton truck with toys in the back).
And yes, you may very well have to accomodate the dynamic load of the vehicle in motion ... which doesn't affect the floor much I suspect, but may affect the shear resistance of the supporting structure ... although I suspect that in many or most situations, that dynamic load is satisfied by wind or seismic requirements that may easily be much larger than the dynamics of a vehicle coming to a sudden stop from say 1 mph ... although I don't really know.
Do some reading on another forum
Go here and read the whole thread. All of it. http://www.contractortalk.com/f14/structural-advice-33004/
Wood Floor
I made the mistake of not using pressure treated lumber when I decked in my Garage Floor when converting the space to a kitchen. Even though I had a vapor barrier under the slab and used PT everywhere it contacted cement, the joist were not 18" off the ground and the building inspector in my county would not approve it. I eventually had to cut in ventilation, install a vapor barrier above the slab, and after three months of hagling they reclassify the joists as sleepers. Finally the job was done, but the lost time and aggravation was enough to make me wish I used all PT lumber to begin with.
Might want to check if you are having inspections. Otherwise, the floor should look sweet!
Scott Jarvis
For anyone checking out this post 10 years later, enclosed car trailers have 3/4" plywood with 16" centers for floors. These bounce down the road for many years with no problem.
Although I would put down a vapor barrier and a waterproof sealer.
This is a very interesting observation you make. I was getting the feeling, as I read thru the above posts, that the writers were making this problem more complex than it needs to be. My property includes a large barn with wood floors that are untold decades old. While the decking was originally every bit of 2 inches thick, the framing underneath is nothing special. There are now definitely weak spots; but we believe most of the structure to be 100+ years. Moisture from below, and wear and tear from the top have taken their tolls, and we expect complete replacement in the next few years. In thinking about this project it never occurred to me that I might have to undertake structural engineering to build a serviceable floor. Thanks for your insight.
Cedar. Everything should be lined with cedar. It wouldn’t be cheap & there are far more durable options, but imagine that smell every time you open the car door, like fresh, sexy man-ness. The perfect lining to house your GT3RS and your ’63 250GT Lusso. It is a renewable resource so it wouldn’t even offend your maid’s Prius.
This is of course answered in the context of “best”, “wood”, . . . . there are other materials that might be better that aren’t wood and other woods that may be far more practical
I fellow in my area, which has an arid climate, built a garage on a steep hillside. The floor was framed in wood and properly structured for the load. The crawl space underneath was about 8' deep on the downhill side, with rocky, well-drained, soil. About ten year later he opened the garage door only to find the car down in that deep crawl space. A lack of ventilation below the floor, and possibly above as well, resulted in the floor rotting until it eventually gave way. Getting the car out of the hole with little headroom for hoisting was a challenge.
Well I am one of those engineers someone once referred to above in the chain and yes I have designed a number of wood framed garage floors over my career. I hold my PE in 3 states but none of the ones so far mentioned. Does that make me an expert, not necessarily but I will share some thoughts here that I have used in the design of wood garage floors without getting into specific number since it is not technically legal for me to do so outside my licensed states. First one should always look to the local codes for help and if there is nothing there go to the national codes. for really most of the US since 2000, the International Code Council (ICC) has published the I codes and for a residential design the specific code would be the International Residential Code (IRC) and if it lacks the info one needs or is out of the parameter or restrictions of the IRC then one needs to refer to the International Building Code (IBC).
So to that end one needs to actually look at a couple of things:
1. I bring this up first as it seems very few people ever know about this code requirement and it has been around as long as I can remember back into the 1980's codes UBC for those that remember. That is that in the 2018 IRC Section R309.1: "Garage floor surfaces shall be of approved noncombustible material". This is really very key and important to take into account. I have seen quite a few garage fires or vehicle fires in garages over the years and they are not pretty. Please note also that asphalt paving does not meet this requirement. The best way we have found to meet this requirement is to add a concrete topping layer to the wood floor. With this one gets several advantages:
a. A noncombustible surface.
b. A surface that is slope able to meet the IRC Drainage requirements of Section R309.1.
C. If one adds reinforcing bars to the concrete, it distributes the point loads out that have been mentioned above. This is a big advantage, more on this below.
2. Now to get to the Structural part of this. Back to the IRC this time to Section R3015 Live Loads. There is Table R301.5 in this section which gives design loads for various residential loads and one of those lines is for: Passenger Vehicle Garages. Exactly what we want and it lists the Live Load as 50 psf (Pounds per Square Foot). I believe this was a load given by a previous person and it happens to be right from the code. However, we need to heed the footnote "a" which says: Elevated garage floors shall be capable of supporting a 2,000-pound load applied over a 20-square-inch area or interpreted an area 4.5 inches square.
It is interesting to note that the ASCE 7-16 uses 40 psf for a passenger vehicles only in Table 4.3-1. In section 4.10.1 it lists the point load of a 9 passenger vehicle or less as 3,000 lbs though and acting on a 4.5 sq. in. area. While the IRC does not make this real clear, the ASCE does make it clear that the point load is to be calculated separately without the uniform load at the same time. So most of us engineer type use this same distinction and do not impose both loads at the same time. For engineers either of these two methods can and are used to design garage floors to.
Then we need to also figure a dead load on this floor for the wood framing and the concrete. In general 12 psf is used for a dead load of the wood framing and for the concrete it is all in how deep one pours it. One can figure 150 pound per cubic foot. So if you take a 4 inch thick topping slab that is 1/3 the weight or 50 psf and likewise a 6 inch slab would be 1/2 or 75 psf.
Taking the 4.5 inch square load and now place this on a 4 inch topping slab and even without reinforcing it will distribute the load out to an area of about 13 inches square or more. given that your design will likely end you up with joists on 12 inch centers, you are getting close to having this point load distributed over two joists or allowing you to cut the point load in half when doing the calculations on a single joist. If you reinforce the floor slab well which we always design for our elevated floor slabs then it is easy to take this reduction on a single joist and we have even been known to distribute it over 3 joists with an actual design concrete slab.
So your loads to design to are going to look like this:
Assume a single span joist.
Assume joist design is for Joints 12 inches O.C.
Joist Dead Load: 12 PLF (Pounds per Linear Foot) for the lumber.
Joist Dead Load: 50 PLF for the Concrete.
Then run two calculations for the following:
Joist Live of: 50 plf
or Joist Point Live Load: 1,000 lbs at center of the span
and choose the loading that requires the largest joists.
So now you have all the numbers you need to go to table or on line calculators etc. to design the floor joists. Now you know enough to be dangerous ad can be an engineer too, have fun. One little thing with this is that do not forget to check bearing lengths and or areas and hanger capacities or even supporting wall calculations.
Also as noted above water proofing is key and needs to be detailed well. We specify at minimum to put down a continuous layer of Ice and Water shield and run it up the walls to create a "pan" effect and drain that pan out the doors then pour the concrete over that. There are better products than just I & W shield such as some of the ones for foundation sealing that I would use on my place if I ever constructed such a garage.
Thanks and welcome to Breaktime.
I am also one of those engineers previously referred to though I am only licensed in one state though and like TopToolPro, not in any of the states mentioned. My main area of focus is consulting and not design, even though I do plenty of design by default because I only consult on designs that I have personally reviewed and researched. So, that brings us to why I am commenting on a 14-year-old thread. The first thing I wanted to do is give kudos to TopToolPro for a few reasons. He provided a very well thought out and practical design that not only meets code but one that he has personally prescribed and installed for customers. In other words, it worked, and the customer and inspector were both satisfied on multiple occasions. Additionally, he communicated his response in a manner that someone with basic building knowledge can apply his methods and achieve like results without going so far as to fully design it for them. Lastly, he commented on a 11 year old comment, I am sure for many of the reason I am commenting, but I would assume it was to pass along the information to anyone who might be trying to figure out how to do this and not the OP necessarily.
Back to why I am commenting. I have a friend who owns an architecture firm on the east coast and his specialty is mass timber construction. I have always admired the use of wood in floors, especially in mass timber construction, which some people in this thread have pointed out that wood floor used to be common in factories, barns and even garages.
So, lets get into it. My starting point would be "the what", "the how" and "the who".
"The what" we know. "Build a wood floor garage that meets required ascetics, structural integrity and life safety requirements."
"The how" Before we go any further, let me make the caveat that IF your intention is to just use the garage as a private storage facility for your vehicles, then our work is done and you can refer to TopTool Pro post as your final design but if your intention is to not only store but also to work on your vehicles and toys(which I think the vast majority of folks do) then let's keep going. Keep in mind that the IRC and the IBC are aggregates, meaning the international code council didn't sit around coming up with rules to stuff up your design. For the most part they use existing codes and aggregate them as they determine is best suited for each application. Both of these codes (IBC and IRC) recognize each other as ways to meet the same objective but also reference engineering codes like ASCE, ASME, ASABE. Trade councils, like the AWA and life safety codes like most NFPA standards (one exception being NFPA70 which is a whole other discussion). So, if you meet the requirements of these other codes or guidelines, then the IRC and IBC are satisfied. Back to "the how". The IRC only allows non-combustible material to be used for garage floors as TopToolPro noted. So lets back up to the 2024 IBC. Starting point, use designation: 312.2 - Private Garages being Group U and 311.2.2 Repair Garages Group S. Both refer to section 406. Reading through 406, the requirements are easily met up until we get to 406.2.4. and again, non-combustible floor, like concrete but there is an exception this time. Slip-resistant, nonabsorbent, interior floor finishes having a critical radiant flux not more than 0.45 W/cm 2 , as determined by NFPA 253 / ASTME64 , shall be permitted for repair garages. Repair garages also need to meet 406.8 which require mechanical ventilation, which I hope you will be installing regardless, sprinklers? nope, under 1000SF not required, gas monitoring is only needing if you are working on LPG or hydrogen vehicles, so not needed. But it does need to meet the requirements for use designations U and S per the international fire code, IFC. So we are left we 3 requirements, a floor coating that meets NFPA 253, mechanical ventilation and any IFC requirements for type U and S use types. So to have a wood garage floor you work on vehicles in, I would use 24 IBC section 2304.9.3 mechanically laminated decking as my go by. Keep in mind, dead load will be much higher(~20PSF) but joist spacing can be greater since your decking in essentially 1.5"OC. Use TopToolPro's recommendations for applying the dead loads and live loads for joist design and layout. Use a coating that meeting NFPA 253 / ASTME64 ( a quick google search ) MiraFlor CQ Clear is one I know of and as always follow the manufactures directions for application and thickness. A clear aggregate can be added for slip-resistance. For required slope for drainage, I would either field cut the laminations (dimensional lumber on edge) so start with 2x6's and field cut down as needed for drainage. Then plane and sand for final slope or install the laminations and plane and sand the entire slope. Use the smallest height dimension for your load calcs and the tallest for your dead load calcs to be conservative. For your dead load, you will want to calculate this and not use a standard like 12psf. Use the AWA design supplement for the weight of your lamination and joists, add it all up and divide by your building square footage. IBC 2304.9.3 has layout and fastener schedules. I would add the recommendation of gluing each lamination with a good polyurethane adhesive sealant. You can't take any credits in strength for this ( thanks trade groups) but it will help with shrinkage and vapor penetration. If this is being installed over a crawlspace then the back side of the decking should have rigid insulation like XPS attached / adhered and sealed against it as a thermal break then the joist cavity fully insulated and a vapor barrier installed on the underside of the joists. Assuming the crawl space meets code for size, access, vapor barrier and ventilation requirements. This completes “the how”.
Now onto “The who”. So who has to buy onto your design. First being the customer and if you or they want a wood garage floor like described in “the how” then this part is satisfied. Next, everybody else, so lets start with governing bodies. Zoning. This should be zoned as residential garage. Even though you have used what is typically commercial standards for the construction of the floor, the test should be if you are conducting business out of it, which if not should pose no issues with zoning provided the zoning allows for a garage in the first place. Next is code enforcement, city, township, county, parish, state. This is all jurisdictional and will be up to you see who's by off you need. In most places a city or county will have a building code inspector and if you need their buy off then it is in your best interest to involve them as early as possible and not be a know it all. They may not buy off at all or insist it meet older codes or locally derived codes but my experience is most inspectors are reasonable people and care much more about developers and contractors taking short cuts then actually trying to stuff up a DYI's or small local contractors design especially if they already have already done the codes research. So what if you live in a rural area with no codes? Why should I follow any codes? 2 reasons come to mind. I am not a fan of math and to actually do all the engineering calcs starting from scratch aka basic statics and strength of materials would be very intensive but if I can use a prescribed method where someone else has done the math then I have saved myself time and I know its been thoroughly vetted. 2nd reason, life safety. Your insurance company will likely be fine with your design from a structural standpoint since they can refer to “like” similar construction techniques, the only area they might question are life safety concerns. Which brings us to a little code requirement we haven't discussed in “the how” section, being that your repair garage will need to meet applicable standards under the international fire code IFC for your occupancy type. The IFC is mostly derived from multiple NFPA standards which are life safety Not building code standards even though your building inspector or insurance company may want confirmation from the authority having jurisdiction AHJ for life safety requirements under NPFA or the IFC and even if you live in a rural area with no zoning or code enforcement, that vast majority will still be in a fire district. Most if not all states have delegated enforcement of most life safety codes to the local level, especially for residential life safety which for a rural fire district would be the fire chief or for cities or counties would be the fire chiefs delegate, the fire inspector. Going through the requirements in the IFC for a type U / S repair garage under 1000SF, you will find most standards to not apply and the ones that do will be at the discretion of the AHJ. They will want to see a properly rated fire extinguisher, smoke and CO detectors, make sure there are proper means of egress and worse case may require signage. All this said, I doubt any rural fire chief would not immediately sign off once they knew your intended use and even the strictest might want to come inspect it and give you a lecture about proper use of fire extinguishers and to test you smoke detectors twice a year when the time changes. Good luck and happy building.