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Question DetailsAsked on 10/17/2017

I want a 2 car garage built as a basement w/roof deck. What kind of precautions to take and what kind of costs.

House is a walk out to back. On front want 2 car basement garage and additional workspace (approx. 30' x 25'). I want a deck for the roof so can walkout front door onto it. On front ground is 2 ' below front door. On left, land elevation is about 2 ' above back walkout but ultimately land slopes to a river 30' below about 80' distant. Would have to excavate basement garage( see above) and pad area (20'x20') and slope up to existing driveway 15' x 50' up 7' slope. Have plenty of room for dirt disposal. Want to keep water out of garage. How to do besides sloping ground away from ground. What would cost of excav., block walls,floor and deck roof be?

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Talk to your architect about this (because you will almost certainly need plans and specs to get a building permit, plus need them for bidders to bid to and the successful bidder to build to). First thing to check is Planning and Zoning requirements - especially clearance required from adjacent properties, wells, leach fields, etc.

Things you particularly need to pay attention to, in no particular order:

1) water control around it - roof runoff, surrounding ground runoff, etc - keep it away from the garage foundation in the first place with proper grading and you will avoid a lot of troubles. Also make sure the surface soil around the foundation is relatively impervious for 3-4 inches in depth (which can be under pervious surficial planting or landscaping) to stop infiltration into the ground along the foundation. Can also be done with long-life buried pond liner liek Hypalon or HDPE.

2) Unless in very dry climate like desert without any heavy rain periods, during garage construction, since this is essentially a daylight basement used as an attached deck tuckunder garage, have a complete 4-sided foundation subdrain put in around the outside of the strip footers, placed at least 4 and preferably 12 inches below the bottom of the garage slab, to intercept any water getting in around the foundation and conduct it by gravity flow to free-surface drainage downhill of the garage entrance - and in a place where it also will not drain in under the driveway.

3) go with full bitumastic waterproofing on the outside of the foundation - if you have a full perimeter french drain per above, a sprayed-on coating is commonly used. If garage will be heated, common to put closed cell insulation right on the fresh sprayed-on coating. Even if not needed for insulation, a thin layer of open-cell or foam drainage board is commonly used as a foundation-face drainage layer and to protect the waterproofing coating from damage from rocks in the backfill which can cause leaks in it.

4) my recommendation in most areas with a ready-mix plant nearby - go with monolithic poured foundation rather than block for underground garages - just so much more waterproof in the first place, and much less cracking tendency over time. And waterproof seal bwtween the strip footer and the interior slab. - very little added cost to install peel-and-stick bitumastic strip sealant on the cleaned strip footer before pouring the slab.

5) have the excavation for the slabslope slightly toward the downhill side - so the base material under the slab will be thicker at the back than at the front - so any water getting in under the slab drains out to the front, where it should be intercepted by the french drain.

6) Normal care needs to be taken about keeping car fumes out of house - but with subgrade garage like that some areas also require additional ventilation in the garage to prevent accumulation of any gas or chemical fumes, some do not worry about it and figure the garage door air leakage will provide enough ventilation. Can affect energy efficiency if garage is inside the "conditioned space" - not a problem if you have combustion (gas or oil) appliances in there like furnace or boiler/water heater because their air intake is considered enough ventilation for the garage too.

7) The deck is the biggest issue - I always recommend it be built in sections that are easily lifted out / removeable for repair, roof repair, cleaning underneath, etc. Best done raised on as few brackets or sleepers (pads like pavers) as possible and with clearance so you can hose in underneath to clear out leaves and dirt and such, especially if over a wood-framed roof rather concrete slab. Where feasible and economic, I recommend concrete deck with walk-on membrane roofing, with the concrete shaped to slope to drain to the edges to avoid ponding. Padded gravity deck supports sitting on top of the membrane (with extra sacrificial layer under the pads) areless likely to leak than through-roof support brackets.

You can alternatively go with an IRMA roof - membrane under gravel or pavers, though of course the roof of the garage has to be heftier to carry that weight. [Note - one common mistake by designers is combining design snow load and people load which is typically 40 psf for that sort of deck - so like in my area that would be 40 psf snow lopad PLUS 40 psf live load PLUS the dead load of the roof and deack at say 10 psf - getting into warehouse loadings. I havenever seen an area that requires max snow load AND people load - you use one or the other one the premise you are not going to have more than a person or two on the deck (maybe shoveling) when it has design snow load on it - so you use the greater of the two design live loads.

One other thing - consider environmental factors - cleaning in sreas with heavy leaves, moss/lichen control if in dark/damp area (especially under raised wood decks, which should be made of ground-contact rated timber or rot-free synthetics like metal in all but driest areas), and provisions for easy snow removal if in snowy area.

8) If decking over a roof on frame construction, still best (especially if this deck will not be used in icing conditions) to slope the roof for drainage even though the deck may be level - easily done, just takes a bit of planning in the supports. Provide for easy deck panel removal, and I recommend easily removeable railings mounted so as to not block drainage off the roof - prefearably without a raised curb. Use of a long-life membrane roofing like HDPE is my preferance in this case,wrapped over the edge of the roof and flashed out to drain (usually to gutter), with the railing mounted on standoffs so it does not block drainage. Can be tricky to detail but most architects can do it fine - easier with metal than wood railing, making it so the railing can be removed along with the decking as necessary for membrane replacement 20-30 years down the road.

9) Make sure you have good floor drain in the garage - and even with that, I recommend making the overall slab slope from swides to center and back to front so even if the floor drain plugs up, the water will (granted with pooling over the drain area) ultimately exit at approximately the center of the garage door (and provide a small drainage gap in any doorseal to allow that. Then should drain off the door sill to the approach slab, which should slope away from the garage at minimum 1/2% slope - preferably 1%.

10) Make sure all surface drainage near the garage is away from it - including providing positive surface drainage along foundation walls so any runoff coming along the sides of the garage (from sides or back) drains toward the front corners, then directly into drain channel(s) leading away from the garage and house - preferably NOT alongside the drive unless absolutely necessary because of topography - because that drainage (and french drain outflow) WILL damage the drive ultimately if it goes in under the driveway. Should drain the other way - from drive and approach slab to nearby but not immediately adjacent drain swales to appropriate surface runoff areas or spreading on the lawn.

11) French drain - be sure to provide cleanouts at each back corner and from the front also (usually into the outlet at front corners) with sweep 90's at the back corners to allow for easy entry of sewer draining jetting tool and sewer camera if needed due to drain blockage. I also generally recommend joining the the french drain continuous across the back of the garage, and across the front as well, so you have 360 degree french drain and ifone passage along the side or at the outlet beomes blocked the water can flow around the back or across the front to the other side to drain to the other drain outlet - never have just one way for the water to get to free surface.

12) And of course protect the outlets from car traffic - and make so the outlet is not an accumulation point for leaves and such. In cold areas, the outlets may have to be reverse gravel french drain construction themselves - like a leach field - so the water can escape and not freeze in the outlet area of the pipes, blocking the pipes.

13) As I read it - after the garage approach slab (which should slope away from the garage if at all possible), you have to go UPHILL with the drive - so the drive slopes toward the house? You are going to have to trap that water and get rid of it - may even take culverts (bare minimum 12" for security against blockage and for easy cleaning) buried alongside the drive and tying in approach slab and french drain drainage to carry it away to lower land - a civil engineer, working in coordination with the architect, can figure this out. Try to avoid a wetwell and sump pump situation if at all possible - too power dependent and even with high water level alarms are a major risk of flooding - because Murphy's Law of Failure Alarms says that at the time it is needed either the alarm will have failed unnoticed, or you will have just left for a few week vacation at the time the system fails and the alarm is dutifully alarming to - nobody.

14) One other thing I always recommend - place the garage slab at least 1 foot BELOW the adjacent house floor level, so if you do ever get a water infiltration or broken pipe situation the garage acts as a drainage path and water cannot move from garage into the house.


Cost - depends on topography and site specifics of course, but very roughly (assuming not in one of the few very high cost urban areas) commonly about $40-100/SF (complete except for permits if pricey) for the garage depending on local costs (assuming no bedrock or very large erratic stones are encountered in excavating), plus $15-25/SF for the deck. Drive/approach slab commonly about $5-10/SF assuming no major excavation needed - commonly $3-5/CY for additional significant excavation in normally diggable ground, usually around $30-40/LF plus or minus for culvert installation done concurrent with the rest of the excavation work. Plus any required relocation of utility runs which happen to run through the work area. Your architect can readily give you a conceptual cost estimate for the entire project before you commit to the job.

Answered 2 years ago by LCD

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